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Revision as of 04:41, 25 January 2009 editConstan69 (talk | contribs)140 edits Fixed map caption. This is not a map of actual skin colour (Tasmanian aborigines weren't the same colour as the Swiss).← Previous edit Latest revision as of 17:00, 28 December 2024 edit undo2a01:cb14:de0:b400:298b:9098:c45a:bba2 (talk) Time scale of skin color evolution: clarified correlation between UV and skin reflectanceTags: Mobile edit Mobile app edit Android app edit App select source 
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]<!--NOTE: "Coloured" does not mean the same thing as "colored" in this case; check the respective Misplaced Pages articles for further detail.--> ({{langx|af|Kleurlinge}} or {{lang|af|Bruinmense}}) family from ] showing some spectrum of human skin coloration]]
] skin color distribution in the world based on "multiple environmental factors".]]

'''Human skin color''' can range from almost black (due to very high concentrations of the dark brown pigment melanin) to nearly colorless (appearing reddish white due to the ] under the ]) in different ]. Skin ] is determined by the amount and type of '']'', the ] in the skin. Variation in skin color is largely due to genetics.
'''Human skin color''' ranges from the ] to the ] hues. Differences in ] color among individuals is caused by variation in ]ation, which is the result of ] (inherited from one's ]s), ], disorders, or some combination thereof. Differences across ]s ] through ] or ], because of social norms and differences in environment, as well as regulations of the biochemical effects of ] ] penetrating the skin.<ref name=jabl1>{{cite book|last=Muehlenbein|first=Michael|title=Human Evolutionary Biology|year=2010|publisher=Cambridge University Press|pages=192–213}}</ref>
As a general pattern people with ]s from ] regions (hence greater sunlight exposure) have darker skin than people with ancestors from ] regions. This is far from a hard and fast rule however, because many light skinned groups have managed to survive at the equator by way of social adaptation<ref>http://photocamel.com/forum/lighting-technique/61437-exposure-skin-tone.html</ref>. The same can be said of dark skinned groups living at subtropical latitudes. <ref>{{cite web | title = Why Do Europeans Have So Many Hair and Eye Colors? | url= http://cogweb.ucla.edu/ep/Frost_06.html | year = 2006 | author = Frost, Peter | publisher = University of California – Los Angeles | accessdate = 2007-10-15}}</ref><ref name=norton>]</ref>

The actual skin color of different humans is affected by many substances, although the single most important substance is the pigment ]. Melanin is produced within the skin in cells called ]s and it is the main determinant of the skin color of darker-skin humans. The skin color of people with light skin is determined mainly by the bluish-white connective tissue under the ] and by the ] circulating in the veins of the dermis. The red color underlying the skin becomes more visible, especially in the face, when, as consequence of ] or ], or the stimulation of the ] (], ]), ]s dilate.<ref name=Jabl06>{{cite book|last=Jablonski|first=N.G.|title=Skin: A Natural History|url=https://archive.org/details/skinnaturalhisto00jabl|url-access=registration|year=2006|publisher=University of California Press|location=Berkeley}}</ref> Color is not entirely uniform across an individual's skin; for example, the skin of the palm and the ] is lighter than most other skin, and this is especially noticeable in darker-skinned people.<ref>{{cite journal |last1=Milburn |first1=Peter B. |last2=Sian |first2=Corazon S. |last3=Silvers |first3=David N. |date=1982 |title=The color of the skin of the palms and soles as a possible clue to the pathogenesis of acral-lentiginous melanoma |journal=American Journal of Dermatopathology |volume=4 |issue=5 |pages=429–33 |doi=10.1097/00000372-198210000-00009 |pmid=7149195 }}</ref>

There is a direct correlation between the geographic distribution of ultraviolet radiation (UVR) and the distribution of indigenous skin pigmentation around the world. Areas that receive higher amounts of UVR, generally located closer to the equator, tend to have darker-skinned populations. Areas that are far from the tropics and closer to the poles have lower intensity of UVR, which is reflected in lighter-skinned populations.<ref name="webb06">{{cite journal|last=Webb|first=A.R.|title=Who, what, where, and when: influences on cutaneous vitamin D synthesis|journal=Progress in Biophysics and Molecular Biology|volume=92|issue=1|pages=17–25|pmid=16766240|year=2006|doi=10.1016/j.pbiomolbio.2006.02.004|doi-access=free}}</ref> By the time modern '']'' evolved, all humans were dark-skinned.<ref name="jabl12">{{cite book |last=Muehlenbein |first=Michael |title=Human Evolutionary Biology |publisher=Cambridge University Press |year=2010 |pages=192–213}}</ref><ref name="Jablonski2017">{{cite journal |last1=Jablonski |first1=Nina |author-link=Nina Jablonski |last2=Chaplin |first2=George |date=May 2017 |title=The colours of humanity: the evolution of pigmentation in the human lineage |journal=Philosophical Transactions of the Royal Society B |volume=372 |issue=1724 |pages=20160349 |doi=10.1098/rstb.2016.0349 |pmc=5444068 |pmid=28533464}}</ref> Some researchers suggest that human populations over the past 50,000 years have changed from dark-skinned to light-skinned and vice versa as they migrated to different UV zones,<ref name="Jablonski2011AN">{{cite journal|last=Jablonski|first=Nina G.|title=Why Human Skin Comes in Colors|journal=AnthroNotes|date=Spring 2011|volume=32|issue=1|url=http://www.personal.psu.edu/ngj2/blogs/nina_jablonski/publications/Jablonski.2011.Anthro.Notes.pdf|access-date=2013-07-20|url-status=live|archive-url=https://web.archive.org/web/20140225192352/http://www.personal.psu.edu/ngj2/blogs/nina_jablonski/publications/Jablonski.2011.Anthro.Notes.pdf|archive-date=2014-02-25}}</ref> and that such major changes in pigmentation may have happened in as little as 100 generations (≈2,500 years) through ]s.<ref name="Jablonski2011AN" /><ref name="Natgeo">{{cite magazine|title=The Human Family Tree Facts|url=http://channel.nationalgeographic.com/channel/a-night-of-exploration/articles/the-human-family-tree-facts/|magazine=National Geographic|access-date=2013-07-20|url-status=dead|archive-url=http://archive.wikiwix.com/cache/20130805052507/http://channel.nationalgeographic.com/channel/a-night-of-exploration/articles/the-human-family-tree-facts/|archive-date=2013-08-05}}</ref><ref name=LivCol /> Natural skin color can also darken as a result of ] due to exposure to ]. The leading theory is that skin color adapts to intense sunlight irradiation to provide partial protection against the ultraviolet fraction that produces damage and thus mutations in the ] of the skin cells.<ref name=Jablonski2017/><ref name="pmid20445093">{{cite journal |last1=Jablonski |first1=N. G. |last2=Chaplin |first2=G. |title=Colloquium Paper: Human skin pigmentation as an adaptation to UV radiation |journal=Proceedings of the National Academy of Sciences |volume=107 |pages=8962–8 |year=2010 |issue=Suppl 2 |doi=10.1073/pnas.0914628107 |pmid=20445093 |pmc=3024016 |bibcode=2010PNAS..107.8962J|doi-access=free }}</ref>

In some populations, ] are significantly lighter-skinned than ]. However, in other populations, notably those of ] descent, men are significantly lighter-skinned than women.<ref name="Frost 2007 pp. 779–780" />{{Failed verification|date=December 2024}} European women may have darker skin than European men due to the female sex hormone ], which darkens light skin.<ref name="scie_Rese" />{{Failed verification|date=December 2024}} Women from darker-skinned populations may have evolved to lighter skin than men so their bodies could absorb more vitamin D during pregnancy, which improves calcium absorption.<ref name="jablonski2000">{{cite journal |last1=Jablonski |first1=Nina |last2=Chaplin |first2=George |year=2000 |title=The evolution of human skin coloration |url=http://www.bgsu.edu/departments/chem/faculty/leontis/chem447/PDF_files/Jablonski_skin_color_2000.pdf |url-status=dead |journal=Journal of Human Evolution |volume=39 |issue=1 |pages=57–106 |doi=10.1006/jhev.2000.0403 |pmid=10896812 |archive-url=https://web.archive.org/web/20150105164235/http://www.bgsu.edu/departments/chem/faculty/leontis/chem447/PDF_files/Jablonski_skin_color_2000.pdf |archive-date=January 5, 2015}}</ref>

The social significance of differences in skin color has varied across cultures and over time, as demonstrated with regard to ] and ].


== Melanin and genes == == Melanin and genes ==
{{main|Melanin}} {{See also|Melanin}}
Melanin is produced by cells called ]s in a process called ]. Melanin is made within small membrane–bound packages called ]s. As they become full of melanin, they move into the slender arms of melanocytes, from where they are transferred to the ]s. Under normal conditions, melanosomes cover the upper part of the keratinocytes and protect them from genetic damage. One melanocyte supplies melanin to thirty-six keratinocytes according to signals from the keratinocytes. They also regulate melanin production and replication of melanocytes.<ref name=LivCol>{{cite book|last=Jablonski|first=Nina|title=Living Color|year=2012|publisher=University of California Press|location=Berkeley, Los Angeles, London|isbn=978-0-520-25153-3}}</ref> People have different skin colors mainly because their melanocytes produce different amount and kinds of melanin.


The genetic mechanism behind human skin color is mainly regulated by the ] ], which creates the color of the skin, eyes, and hair shades.<ref>{{cite journal|last=Sturm|first=R. A.|title=A golden age of human pigmentation genetics|journal=Trends in Genetics|year=2006|volume=22|issue=9|pages=464–469|doi=10.1016/j.tig.2006.06.010|pmid=16857289}}</ref><ref>{{cite journal|title=Human pigmentation genes: Identification, structure and consequences of polymorphic variation|journal=Gene|year=2001|volume=277 |issue=1–2|pages=49–62|pmid=11602344|doi=10.1016/s0378-1119(01)00694-1|last1=Sturm|first1=R. A. |last2=Teasdale |first2=R. D.|last3=Box|first3=N. F.}}</ref> Differences in skin color are also attributed to differences in size and distribution of melanosomes in the skin.<ref name="LivCol" /> Melanocytes produce two types of melanin. The most common form of ] melanin is ], a brown-black ] of ] ]s, and their reduced forms. Most are derived from the amino acid ]. Eumelanin is found in hair, ], and skin, and the hair colors gray, black, blond, and brown. In humans, it is more abundant in people with dark skin. ], a pink to red hue is found in particularly large quantities in ],<ref>{{cite journal|title=Diversity of human hair pigmentation as studied by chemical analysis of eumelanin and pheomelanin|date=Dec 2011|journal=J Eur Acad Dermatol Venereol|issue=12|pages=1369–1380|volume=25 |doi=10.1111/j.1468-3083.2011.04278.x|pmid=22077870|last1=Ito|first1=S.|last2=Wakamatsu|first2=K.|s2cid=5121042}}</ref> the lips, nipples, glans of the penis, and vagina.<ref>{{cite web |title=pheomelanin |website=MetaCyc |url=http://www.metacyc.org/META/NEW-IMAGE?type=COMPOUND&object=CPD-12380 |access-date=2012-02-17 |url-status=live |archive-url=https://web.archive.org/web/20150121030624/http://www.metacyc.org/META/NEW-IMAGE?type=COMPOUND&object=CPD-12380 |archive-date=2015-01-21}}</ref>
Melanin comes in two types: ] (red) and ] (very dark brown). Both amount and type are determined by four to six ]s which operate under ]. One copy of each of those genes is inherited from each parent. Each gene comes in several ]s, resulting in the great variety of different skin tones.


Both the amount and type of melanin produced is controlled by a number of ]s that operate under ].<ref>{{cite web|url=https://www.pbs.org/race/000_About/002_04-teachers-06.htm |title=The Genetics and Evolution of Skin Color: The Case of Desiree's Baby|date=2003|access-date=14 April 2015 |website=RACE—The Power of an Illusion|publisher=Public Broadcasting Service |last=Schneider |first=Patricia |url-status=live|archive-url= https://web.archive.org/web/20150506172623/http://www.pbs.org/race/000_About/002_04-teachers-06.htm |archive-date=6 May 2015}}</ref> One copy of each of the various genes is inherited from each parent. Each gene can come in several ]s, resulting in the great variety of human skin tones. Melanin controls the amount of ] (UV) radiation from the sun that penetrates the skin by absorption. While UV radiation can assist in the production of vitamin D, excessive exposure to UV can damage health.
] child, presenting a contrast in tones.]]
The ] of the different skin tones is thought to have occurred as follows:{{Who|date=June 2007}} the haired primate ancestors of humans, like modern ]s, had light skin under their hair. When Hominids evolved relative hairlessness (the most likely function of which was to facilitate perspiration), they evolved dark skin, which was needed to prevent low ] levels since they lived in sun-rich ]. (The skin cancer connection is probably of secondary importance, since skin cancer usually kills only after the reproductive age and therefore does not exert much evolutionary selection pressure.) When humans migrated to less sun-intensive regions in the north, low ] levels became a problem and light skin color re-emerged. Sexual selection and diet may have played a part in the evolution of skin tone diversity, as well<ref>http://pages.globetrotter.net/peter_frost61z/European-skin-color.htm</ref>.


== Evolution of skin color ==
The ] and ] are special cases: even though they live in an extremely sun-poor environment, they have retained their relatively dark skin. This can be explained by the fact that their traditional fish-based diet provides plenty of vitamin D<ref>http://www.snowwowl.com/peopleinuit5.html</ref>.
{{Further|Melanocortin 1 receptor}}


=== Time scale of skin color evolution ===
Brown skin is the likely ancestral (or original) skin color among modern humans (Harding et al 2000). This is due to modern humanity's common origin in equatorial Africa ~200,000 years ago (Tishkoff, 1996). Dark skin was crucial in this UV rich context given that a thick coat of UV protective body hair had long been selected against by this time (Rogers et al 2004) most likely in order to facilitate the evaporation of perspiration (ie the cooling of the body). This trait (dark skin) continues to be under strong selection in equatorial regions such as Africa, India, and New Guinea (Harding 2000 p 1355). Geneticists estimate that a relatively small group of humans left Africa ~60,000 years ago, and that the descendants of this group went on to populate the entire non-sub-Saharan African world. Those migrants that settled in non-African equatorial regions (such as the mentioned India, New Guinea, and/or Australia) retained most of the ancestral sequence at the MC1R locus (Harding 2000 p 1355), a gene strongly associated with determining skin color. Specifically, Harding et al (2000 p 1355) found that the haplotype sequences for Indians and New Guineans are virtually identical to those of continental sub-Saharan Africans (except for a small number of variants at silent sites).
] in '']'' links to the ] through ] heat dissipation required for activity in hot open environments<ref name="Ruxton Wilkinson 2011 pp. 20965–20969">{{cite journal |last1=Ruxton |first1=Graeme D. |last2=Wilkinson |first2=David M. |date=2011-12-12 |title=Avoidance of overheating and selection for both hair loss and bipedality in hominins |journal=Proceedings of the National Academy of Sciences |volume=108 |issue=52 |pages=20965–20969 |doi=10.1073/pnas.1113915108 |pmid=22160694 |bibcode=2011PNAS..10820965R |issn=0027-8424|doi-access=free |pmc=3248486 }}</ref> and ].<ref name="Ruxton Wilkinson 2011 pp. 169–175">{{cite journal |last1=Ruxton |first1=Graeme D. |last2=Wilkinson |first2=David M. |year=2011 |title=Thermoregulation and endurance running in extinct hominins: Wheeler's models revisited |journal=Journal of Human Evolution |volume=61 |issue=2 |pages=169–175 |doi=10.1016/j.jhevol.2011.02.012 |pmid=21489604 |issn=0047-2484}}</ref> Humans as primates have a particular need for this thermoregulation since unlike other mammals they lack a ] that allows precooling of blood to the brain, an organ extremely sensitive to changes in body temperature.<ref name="Bruner Mantini Musso De La Cuétara 2010 pp. 35–43">{{cite journal |last1=Bruner |first1=Emiliano |last2=Mantini |first2=Simone |last3=Musso |first3=Fabio |last4=De La Cuétara |first4=José Manuel |last5=Ripani |first5=Maurizio |last6=Sherkat |first6=Shahram |date=2010-11-30 |title=The evolution of the meningeal vascular system in the human genus: From brain shape to thermoregulation |journal=American Journal of Human Biology |volume=23 |issue=1 |pages=35–43 |doi=10.1002/ajhb.21123 |pmid=21120884 |s2cid=24603018 |issn=1042-0533}}</ref> Given endurance running and its needs for thermoregulation arose with '']'',<ref name="Bramble Lieberman 2004">{{cite journal |last1=Bramble |first1=Dennis M. |last2=Lieberman |first2=Daniel E. |year=2004 |title=Endurance running and the evolution of Homo |journal=Nature |volume=432 |issue=7015 |pages=345–352 |doi=10.1038/nature03052 |pmid=15549097 |bibcode=2004Natur.432..345B |s2cid=2470602 |issn=0028-0836|url=http://doc.rero.ch/record/15289/files/PAL_E2588.pdf }}</ref> this links hairlessness with the origin of ''H. erectus''<ref name="Ruxton Wilkinson 2011 pp. 169–175" /> about 2 million years ago.<ref name="Branda Eaton 1978 pp. 625–626">{{cite journal |last1=Branda |first1=Richard F. |last2=Eaton |first2=John W. |date=1978-08-18 |title=Skin Color and Nutrient Photolysis: An Evolutionary Hypothesis |journal=Science |volume=201 |issue=4356 |pages=625–626 |doi=10.1126/science.675247 |pmid=675247 |bibcode=1978Sci...201..625B |issn=0036-8075}}</ref><ref name="Herries Martin Leece Adams 2020 p.">{{cite journal |last1=Herries |first1=Andy I. R. |last2=Martin |first2=Jesse M. |last3=Leece |first3=A. B. |last4=Adams |first4=Justin W. |last5=Boschian |first5=Giovanni |last6=Joannes-Boyau |first6=Renaud |last7=Edwards |first7=Tara R. |last8=Mallett |first8=Tom |last9=Massey |first9=Jason |last10=Murszewski |first10=Ashleigh |last11=Neubauer |first11=Simon |last12=Pickering |first12=Robyn |last13=Strait |first13=David S. |last14=Armstrong |first14=Brian J. |last15=Baker |first15=Stephanie |date=2020-04-03 |title=Contemporaneity of ''Australopithecus'' , ''Paranthropus'' , and early ''Homo erectus'' in South Africa |journal=Science |volume=368 |issue=6486 |page= |doi=10.1126/science.aaw7293 |issn=0036-8075 |last16=Caruana |first16=Matthew V. |last17=Denham |first17=Tim |last18=Hellstrom |first18=John |last19=Moggi-Cecchi |first19=Jacopo |last20=Mokobane |first20=Simon |last21=Penzo-Kajewski |first21=Paul |last22=Rovinsky |first22=Douglass S. |last23=Schwartz |first23=Gary T. |last24=Stammers |first24=Rhiannon C. |last25=Wilson |first25=Coen |last26=Woodhead |first26=Jon |last27=Menter |first27=Colin|pmid=32241925 |doi-access=free }}</ref>


As ] gradually lost their ] between 1.2 and 4 million years ago, to allow for better cooling through sweating, their naked skin was exposed to sunlight. In the tropics, natural selection favoured dark-skinned human populations as high levels of skin pigmentation protected against the harmful effects of sunlight. Indigenous populations' ] (the amount of sunlight the skin reflects) and the actual UV radiation in a particular geographic area is highly inversely correlated, which supports this idea. Genetic evidence also supports this notion, demonstrating that around 1.2&nbsp;million years ago there was a strong evolutionary pressure which acted on the development of dark skin pigmentation in early members of the genus '']''.<ref name="jabl04b">{{cite journal |last=Nina |first=Jablonski |year=2004 |title=The evolution of human skin and skin color |journal=Annual Review of Anthropology |volume=33 |pages=585–623 |doi=10.1146/annurev.anthro.33.070203.143955 |s2cid=53481281}}</ref> Hairlessness exposes ] circulating ] and in the ] to degradation from ].<ref name="Rocha 2019 pp. 77–87">{{cite journal |last=Rocha |first=Jorge |date=2019-07-30 |title=The Evolutionary History of Human Skin Pigmentation |journal=Journal of Molecular Evolution LLC |volume=88 |issue=1 |pages=77–87 |doi=10.1007/s00239-019-09902-7 |pmid=31363820 |s2cid=198998060 |issn=0022-2844}}</ref> The effect of sunlight on folic acid levels has been crucial in the development of dark skin and favored the emergence of skin pigmentation in order to protect from ] ] due to the increased exposure to sunlight.<ref name="Jablonski2017" /><ref name="pmid20445093" /><ref name="jabl13">{{cite book |last=Muehlenbein |first=Michael |title=Human Evolutionary Biology |publisher=Cambridge University Press |year=2010 |pages=192–213}}</ref><ref name="pbs">{{cite web |last=Gina |first=Kirchweger |title=The Biology of Skin Color: Black and White |url=https://www.pbs.org/wgbh/evolution/library/07/3/text_pop/l_073_04.html |url-status=live |archive-url=https://web.archive.org/web/20130216070146/http://www.pbs.org/wgbh/evolution/library/07/3/text_pop/l_073_04.html |archive-date=16 February 2013 |access-date=10 December 2012 |work=Evolution Library |publisher=PBS}}</ref>
The retention of the ancestral trait at the equator is due to natural selection for melanin pigment production which serves to protect the body from harmful UV rays (Jablonski 2006). Notably, given that hair is a part of the skin, the retention is also analogous to that which occurred for ] prior to pre-Holocene admixture events among people who settled in India and Australia. However, certain evidence suggests that, unlike skin color, Afro hair ceased to be under strong selection once dark skin arose ~1 million years ago (Harding 2000) (rather, it remained as a vestigial trait among Africans, Andamanese, and Melanesians and changed to straight in the north for adaptive reasons--see ]). In fact, dark skin is so selectively advantageous at the equator that initially light skinned native Americans who migrated to Mexico and/or South America experienced renewed selective pressure towards the evolution of dark skin.


In 2017, a study showed that both dark and light pigmentation alleles arose before the origin of modern humans,<ref>{{Cite journal |last1=Crawford |first1=Nicholas G. |last2=Kelly |first2=Derek E. |last3=Hansen |first3=Matthew E. B. |last4=Beltrame |first4=Marcia H. |last5=Fan |first5=Shaohua |last6=Bowman |first6=Shanna L. |last7=Jewett |first7=Ethan |last8=Ranciaro |first8=Alessia |last9=Thompson |first9=Simon |last10=Lo |first10=Yancy |last11=Pfeifer |first11=Susanne P. |last12=Jensen |first12=Jeffrey D. |last13=Campbell |first13=Michael C. |last14=Beggs |first14=William |last15=Hormozdiari |first15=Farhad |date=2017-11-17 |title=Loci associated with skin pigmentation identified in African populations |journal=Science |language=en |volume=358 |issue=6365 |doi=10.1126/science.aan8433 |issn=0036-8075 |pmc=5759959 |pmid=29025994}}</ref> with the older version of the variants in many cases being associated with lighter skin.<ref>{{Cite web |date=2017-10-12 |title=Penn-led Study Identifies Genes Responsible for Diversity of Human Skin Colors |url=https://penntoday.upenn.edu/news/penn-led-study-identifies-genes-responsible-diversity-human-skin-colors |access-date=2024-04-16 |website=Penn Today |language=en}}</ref> The earliest primate ancestors of humans most likely had pale non-pigmented skin covered with dark black hair, like the ] and other great apes.<ref>{{Cite journal |last1=Jablonski |first1=N. G. |last2=Chaplin |first2=G. |date=2000 |title=The evolution of human skin coloration |url=https://pubmed.ncbi.nlm.nih.gov/10896812/ |journal=Journal of Human Evolution |volume=39 |issue=1 |pages=57–106 |doi=10.1006/jhev.2000.0403 |issn=0047-2484 |pmid=10896812}}</ref><ref>{{Cite news |last=Wade |first=Nicholas |date=2003-08-19 |title=Why Humans and Their Fur Parted Ways |url=https://www.nytimes.com/2003/08/19/science/why-humans-and-their-fur-parted-ways.html |access-date=2024-01-13 |work=The New York Times |language=en-US |issn=0362-4331}}</ref><ref>{{Cite journal |last1=Elias |first1=Peter M. |last2=Menon |first2=Gopinathan |last3=Wetzel |first3=Bruce K. |last4=Williams |first4=John (Jack) W. |date=2009 |title=Evidence that stress to the epidermal barrier influenced the development of pigmentation in humans |journal=Pigment Cell & Melanoma Research |volume=22 |issue=4 |pages=420–434 |doi=10.1111/j.1755-148X.2009.00588.x |issn=1755-1471 |pmc=2843517 |pmid=19508412}}</ref><ref>{{Cite journal |last=Greaves |first=Mel |date=2014-04-22 |title=Was skin cancer a selective force for black pigmentation in early hominin evolution? |journal=Proceedings of the Royal Society B: Biological Sciences |volume=281 |issue=1781 |pages=20132955 |doi=10.1098/rspb.2013.2955 |issn=0962-8452 |pmc=3953838 |pmid=24573849}}</ref><ref>{{Cite web |last=Alex |first=Bridget |date=2019-01-28 |title=Why Did Darker and Lighter Human Skin Colors Evolve? |url=https://www.discovermagazine.com/planet-earth/why-did-darker-and-lighter-human-skin-colors-evolve |access-date=2024-04-16 |website=Discover Magazine |language=en}}</ref>
According to (]), light skin observed in ] (with deep red and/or yellowish skin tones), non-Indian ], ] and North Africa (]) is due to independent ] in at least three ]. They concluded that light pigmentation is at least partially due to ], however Jablonski postulates that the predominant reason revolved around the facilitation of vitamin D production in northern Eurasia (see ]).


With the evolution of hairless skin, abundant sweat glands, and skin rich in melanin, early humans could walk, run, and forage for food for long periods of time under the hot sun without brain damage due to overheating, giving them an evolutionary advantage over other species.<ref name="LivCol" /> Research on the ] alleles using assumptions about past population size and an absence of population bottlenecks suggests the allele for dark skin present in modern Africans arose at least by 1.2 million years ago.<ref name="rogers2004b">{{Harvnb|Rogers|Iltis|Wooding|2004b|p=107}}.</ref>
=== Health related effects ===


This was the genotype inherited by ], but retained only by part of the extant populations, thus forming an aspect of ]. About 100,000–70,000 years ago, some anatomically modern humans (''Homo sapiens'') began to ] away from the tropics to the north where they were exposed to less intense sunlight. This was possibly in part due to the need for greater use of clothing to protect against the colder climate. Under these conditions there was less photodestruction of folate and so the evolutionary pressure working against the survival of lighter-skinned gene variants was reduced. In addition, lighter skin is able to generate more ] (cholecalciferol) than darker skin, so it would have represented a health benefit in reduced sunlight if there were limited sources of vitamin D.<ref name="jablonski2000" /> Hence the leading hypothesis for the evolution of human skin color proposes that:
Dark skin (melanin) protects against ] light; this light causes ]s in skin cells, which in turn may cause ]s. Light-skinned persons have about a tenfold greater risk of dying from skin cancer under equal ] exposure, with ]s having the greatest risk<ref>http://www.ariseandshine.com/skincare.aspx</ref>. Furthermore, dark skin prevents ] of ] rays from destroying the essential ], derived from ]. Folic acid (or folate) is needed for the synthesis of ] in ] and folate deficiency in pregnant women are associated with birth defects<ref>http://www.nichd.nih.gov/news/releases/miscarriage_risk.cfm</ref>.
# From the origin of hairlessness and exposure to UV-radiation to less than 100,000 years ago, ], including ], were dark-skinned.
# As some ''Homo sapiens'' populations began to migrate, the evolutionary constraint keeping skin dark decreased proportionally to the distance north a population migrated, resulting in a range of skin tones within northern populations, although the bulk of humans remained dark-skinned.
# At some point, some northern populations experienced positive selection for lighter skin due to the increased production of vitamin D from sunlight and the genes for darker skin disappeared from these populations.
# Subsequent migrations into different UV environments and admixture between populations have resulted in the varied range of skin pigmentations we see today.


The genetic mutations leading to light skin, though partially different among ]ns and ]ans,<ref name="Norton2006" /> suggest the two groups experienced a similar selective pressure after settlement in northern latitudes.<ref name="pmid19481954">{{cite journal |last1=Juzeniene |first1=Asta |last2=Setlow |first2=Richard |last3=Porojnicu |first3=Alina |last4=Steindal |first4=Arnfinn Hykkerud |last5=Moan |first5=Johan |title=Development of different human skin colors: A review highlighting photobiological and photobiophysical aspects |journal=Journal of Photochemistry and Photobiology B: Biology |volume=96 |pages=93–100 |year=2009 |doi=10.1016/j.jphotobiol.2009.04.009 |pmid=19481954 |issue=2}}</ref>
While dark skin better preserves vitamin B, it can also lead to ] at higher latitudes which in turn can cause fatal cancers affecting the colon, lung and prostate. Dark-skinned people are also at higher risk for ], ], ] and ].<ref></ref> An American study by the USDA found 87% of African Americans to be Vitamin D deficient.<ref>http://www.ars.usda.gov/IS/pr/2006/060525.htm</ref> To address this issue, some countries have programs to ensure ] of milk with ].


The theory is partially supported by a study into the ] gene which found that the allele associated with light skin in Europe "determined that 18,000 years had passed since the light-skin allele was fixed in Europeans" but may have originated as recently as 12,000–6,000 years ago "given the imprecision of method" ,<ref name="pmid17446367" /> which is in line with the earliest evidence of farming.<ref>{{cite book |first=Luc-Normand |last=Tellier |year=2009 |url=https://books.google.com/books?id=cXuCjDbxC1YC&pg=PA26 |title=Urban world history: an economic and geographical perspective |page=26 |isbn=978-2-7605-1588-8}}</ref> Paleolithic ] groups, as well as Early Holocene Western and central European hunter-gatherers (]) have been suggested to have been dark skinned based on DNA analysis,<ref>{{Cite journal |last=Posth |first=Cosimo |last2=Yu |first2=He |last3=Ghalichi |first3=Ayshin |last4=Rougier |first4=Hélène |last5=Crevecoeur |first5=Isabelle |last6=Huang |first6=Yilei |last7=Ringbauer |first7=Harald |last8=Rohrlach |first8=Adam B. |last9=Nägele |first9=Kathrin |last10=Villalba-Mouco |first10=Vanessa |last11=Radzeviciute |first11=Rita |last12=Ferraz |first12=Tiago |last13=Stoessel |first13=Alexander |last14=Tukhbatova |first14=Rezeda |last15=Drucker |first15=Dorothée G. |date=2023-03-02 |title=Palaeogenomics of Upper Palaeolithic to Neolithic European hunter-gatherers |url=https://www.nature.com/articles/s41586-023-05726-0 |journal=Nature |language=en |volume=615 |issue=7950 |pages=117–126 |doi=10.1038/s41586-023-05726-0 |issn=0028-0836 |pmc=9977688 |pmid=36859578}}</ref><ref name="Brace">{{citation |last1=Brace |first1=Selina |title=Ancient genomes indicate population replacement in Early Neolithic Britain |journal=Nature Ecology & Evolution |volume=3 |issue=5 |pages=765–771 |year=2019 |bibcode=2019NatEE...3..765B |doi=10.1038/s41559-019-0871-9 |pmc=6520225 |pmid=30988490 |last2=Diekmann |first2=Yoan |last3=Booth |first3=Thomas J. |last4=Faltyskova |first4=Zuzana |last5=Rohland |first5=Nadin |last6=Mallick |first6=Swapan |last7=Ferry |first7=Matthew |last8=Michel |first8=Megan |last9=Oppenheimer |first9=Jonas |first10=Nasreen |last10=Broomandkhoshbacht |first11=Kristin |last11=Stewardson |first12=Susan |last12=Walsh |first13=Manfred |last13=Kayser |first14=Rick |last14=Schulting |first15=Oliver E. |last15=Craig |first16=Alison |last16=Sheridan |first17=Mike Parker |last17=Pearson |first18=Chris |last18=Stringer |first19=David |last19=Reich |first20=Mark G. |last20=Thomas |first21=Ian |last21=Barnes}} (p.18-19): "This individual has light or blue/green eye colour, it is not light blue, there are elements of brown/yellow in the eye to give a proposed perceived green colour. Better coverage at the low sequenced marker would clarify this but blue/hazel cannot be ruled out. It is certainly not a brown eyed or clear blue-eyed individual... Skin pigmentation The following range for skin pigmentation prediction is possible for this individual with these parameters:... Intermediate 0.152 - 0.038 Dark-Black 0.848 - 0.962 Final prediction: Dark/Dark-to-Black skin If we omit the three missing alleles, our tool produces 0.752 and 0.248 probabilities for the intermediate and dark-black category respectively, changing the prediction ranges to 0.752- 0.038 and 0.248-0.962. However, note that this completely removes the locus from the prediction model; hence the prediction will not perform optimally (how the prediction model was made). It is therefore best to have some allele present to infer the most probable range for Cheddar Man and we derive the ranges above from the extreme allele constellations only. Explanation: The missing loci certainly impact on this prediction; however, utilizing the input of all ancestral alleles is the preferred option over the use of the derived alleles at these loci – hence 0.152 for intermediate and 0.848 for Dark-to-Black would be the most probable profile. That being said a broad range is present in both the intermediate and dark-black categories due to the missing loci. Also, this effect of skipping a skin pigmentation prediction category with regards probability values, tends to be observed more often in admixed individuals. What is important to note is the input of the dark-black prediction is significant on the intermediate category and therefore it is acceptable to propose a dark complexion individual over an intermediate/light prediction even though the intermediate range is present. It is unlikely that this individual has the darkest possible pigmentation, but it cannot be ruled out. Better sequencing coverage would clarify to what degree this individual has a dark complexion."</ref><ref>{{Cite journal |last=González-Fortes |first=Gloria |last2=Jones |first2=Eppie R. |last3=Lightfoot |first3=Emma |last4=Bonsall |first4=Clive |last5=Lazar |first5=Catalin |last6=Grandal-d’Anglade |first6=Aurora |last7=Garralda |first7=María Dolores |last8=Drak |first8=Labib |last9=Siska |first9=Veronika |last10=Simalcsik |first10=Angela |last11=Boroneanţ |first11=Adina |last12=Vidal Romaní |first12=Juan Ramón |last13=Vaqueiro Rodríguez |first13=Marcos |last14=Arias |first14=Pablo |last15=Pinhasi |first15=Ron |date=June 2017 |title=Paleogenomic Evidence for Multi-generational Mixing between Neolithic Farmers and Mesolithic Hunter-Gatherers in the Lower Danube Basin |url=https://linkinghub.elsevier.com/retrieve/pii/S0960982217305596 |journal=Current Biology |language=en |volume=27 |issue=12 |pages=1801–1810.e10 |doi=10.1016/j.cub.2017.05.023 |pmc=5483232 |pmid=28552360}}</ref> with a number of the most prominent light-skin tone gene variants found in modern Europeans being introduced by ] that migrated into Europe beginning around 9,000 years ago,<ref name=":0">{{Cite journal |last=Ju |first=Dan |last2=Mathieson |first2=Iain |date=2021-01-05 |title=The evolution of skin pigmentation-associated variation in West Eurasia |url=https://pnas.org/doi/full/10.1073/pnas.2009227118 |journal=Proceedings of the National Academy of Sciences |language=en |volume=118 |issue=1 |doi=10.1073/pnas.2009227118 |issn=0027-8424 |pmc=7817156 |pmid=33443182}}</ref> with ] for lighter skin intensifying from the ] period onwards.<ref name=":0" /><ref>{{Cite journal |last=Lazaridis |first=Iosif |last2=Alpaslan-Roodenberg |first2=Songül |last3=Acar |first3=Ayşe |last4=Açıkkol |first4=Ayşen |last5=Agelarakis |first5=Anagnostis |last6=Aghikyan |first6=Levon |last7=Akyüz |first7=Uğur |last8=Andreeva |first8=Desislava |last9=Andrijašević |first9=Gojko |last10=Antonović |first10=Dragana |last11=Armit |first11=Ian |last12=Atmaca |first12=Alper |last13=Avetisyan |first13=Pavel |last14=Aytek |first14=Ahmet İhsan |last15=Bacvarov |first15=Krum |date=2022-08-26 |title=A genetic probe into the ancient and medieval history of Southern Europe and West Asia |url=https://www.science.org/doi/10.1126/science.abq0755 |journal=Science |language=en |volume=377 |issue=6609 |pages=940–951 |doi=10.1126/science.abq0755 |issn=0036-8075 |pmc=10019558 |pmid=36007020}}</ref>
The advantage of light skin at high latitudes is that it allows more sun absorption, leading to increased production of vitamin D<sub>3</sub>, necessary for ] absorption and ]. The lighter skin of women at higher latitudes most likely results from the higher calcium needs of women during ] and ]. However, some have postulated that it may also derive from sexual selection<ref>http://answers.yahoo.com/question/index?qid=20080913055751AACeYIj</ref>.


Research by ] suggests that an estimated time of about 10,000 to 20,000 years is enough for human populations to achieve optimal skin pigmentation in a particular geographic area but that development of ideal skin coloration may happen faster if the evolutionary pressure is stronger, even in as little as 100 generations.<ref name="Jablonski2011AN" /> The length of time is also affected by cultural practices such as food intake, clothing, body coverings, and shelter usage which can alter the ways in which the environment affects populations.<ref name="LivCol" />]
] is a condition characterized by the absence of melanin, resulting in very light skin, eyes, and hair; it is caused by an inability to synthesize tyrosine, and has a genetic basis.


Population and admixture studies suggest a three-way model for the evolution of human skin color, with dark skin evolving in early ] in ] and light skin evolving only recently after modern humans had expanded ]. For the most part, the evolution of light skin has followed different genetic paths in Western and Eastern Eurasian populations; however, some mutations associated with lighter skin have estimated origin dates after humans spread out of Africa but before the divergence of the two lineages.<ref name="Belezal2012" />
=== Cultural effects ===
Sexual preference of ] in women by men has been found in certain cultures throughout the world. The effect has been discovered in ], where the ruling class was of darker complexion than the conquered natives.<ref name="Frost">] "Fair Women, Dark Men: The Forgotten Roots of Color Prejudice," (2005).</ref> Also, preference of lighter-skinned women by black men is reported both in sub-Saharan Africa and in the black diaspora.<ref>)</ref>
In his foreword to Peter Frost's 2005 ''Fair Women, Dark Men'', U. of Washington sociologist Pierre L. van den Berghe summarizes:
:"Although virtually all cultures express a marked preference for fair female skin, even those with little or no exposure to European imperialism, and even those whose members are heavily pigmented, many are indifferent to male pigmentation or even prefer men to be darker."<ref>see Steve Sailer, (2005)</ref>
A consequence of this is that, since higher-ranking men get to marry the more attractive women, the upper classes of a society generally tend to develop a lighter complexion than the lower classes by ] (see also ]).<ref name="Frost"/>


According to Crawford et al. (2017), most of the genetic variants associated with light and dark pigmentation in African populations appear to have originated more than 300,000 years ago.<ref>{{cite web |url=https://www.sciencedaily.com/releases/2017/10/171012143324.htm |date=2017 |title=Genes responsible for diversity of human skin colors identified |website=ScienceDaily}}</ref> African, South Asian and Australo-Melanesian populations also carry derived alleles for dark skin pigmentation that are not found in Europeans or East Asians.<ref name="science-skin pigmentation">{{cite journal |journal=Science |volume=358 |issue=6365 |date=2017 |title=Loci associated with skin pigmentation identified in African populations |last1=Crawford |first1=Nicholas |pages=eaan8433 |doi=10.1126/science.aan8433|pmid=29025994 |pmc=5759959 }}</ref> Huang et al. 2021 found the existence of "selective pressure on light pigmentation in the ancestral population of Europeans and East Asians", prior to their divergence from each other. Skin pigmentation was also found to be affected by directional selection towards darker skin among Africans, as well as lighter skin among Eurasians.<ref>{{cite journal |url=https://journals.biologists.com/bio/article/10/2/bio056523/222729/Dissecting-dynamics-and-differences-of-selective |journal=Biology Open |volume=10 |issue=2 |date=2021 |title=Dissecting dynamics and differences of selective pressures in the evolution of human pigmentation |last1=Huang |first1=Xin |doi=10.1242/bio.056523|pmid=33495209 |pmc=7888712 }}</ref> Crawford et al. (2017) similarly found evidence for selection towards light pigmentation prior to the divergence of West Eurasians and East Asians.<ref name="science-skin pigmentation" />
Differences in skin tone are the most readily perceptible phenotypical distinction of human populations, and hence has historically lent itself to ], often to the effect of darker skin being seen as being of lowest social value, and lighter skin of highest. However, according to classical scholar Frank Snowden,<ref>Snowden, Frank. Blacks in antiquity: Ethiopians in the Greco-Roman experience. Harvard University Press, 1970.</ref> the Egyptians and Greeks (et al.) assigned relatively neutral connotations to skin color variation because conquest rather than skin color was the major determinant of slave status.


=== Functional considerations ===
== Skin tone variability ==
Elias et. al. in 2010 showed a superior barrier function in darkly pigmented skin. Most protective functions of the skin, including the permeability barrier and the antimicrobial barrier, reside in the ] and the researchers surmise that the stratum corneum has undergone the most genetic change since the loss of human body hair. ] would have favored mutations that protect this essential barrier; one such protective adaptation is the pigmentation of ] ], because it improves barrier function as compared to non-pigmented skin.<ref name="Elias2010"/>
{{Mergefrom|Olive skin|date=September 2008}}


The authors argue that lack of significant differences between modern light-skinned and dark-skinned populations in vitamin D deficiency, early death from UV-induced cancers and birth defects — as well as instances of light and dark populations living side-by-side in areas with similar UV — suggest the standard model is insufficient to explain the strong selection drive for pigmented skin.<ref name="Elias2010">{{cite journal |last1=Elias |first1=PM |last2=Menon |first2=G |last3=Wetzel |first3=BK |last4=Williams |first4=J |journal=American Journal of Human Biology |pmid=20209486 |year=2010 |volume=22 |issue=4 |pages=526–537 |title=Barrier Requirements as the Evolutionary "Driver" of Epidermal Pigmentation in Humans |doi=10.1002/ajhb.21043 |pmc=3071612}}</ref> Jablonski rejects this theory on the grounds that the human ] is driven by UV-B exposure, not xeric stress, and that the positive selection for vitamin D production is "well-established".<ref name="Jablonski2017" />
The tone of human skin can vary from a dark brown to nearly a colorless pigmentation, which may appear reddish due to the blood in the skin. Europeans generally have lighter skin, hair, and eyes than any other group on Earth, although this is not always the case. For practical purposes, such as exposure time for ], six skin types are distinguished following Fitzpatrick (1975), listed in decreasing lightness:<ref>{{cite book
| last = Weller
| first = Richard
| authorlink =
| coauthors = John Hunter, John Savin, Mark Dahl
| title = Clinical Dermatology
| edition = 4th
| publisher = Blackwell Publishing
| year = 2008
| location = Malden, Massachusetts, USA
| pages = 268
| url =
| doi =
| id =
| isbn = 978-1-4051-4663-0}}; , </ref>
{| class="wikitable"
|-
!type !! also called !! tanning behavior !! hair and eye color !! ]
|-
| I || very light, also "Nordic" <ref name="nicknames">these are commonly encountered names for the types, e.g. , etc.</ref> || Often burns, rarely tans. || Tends to have freckles, red or blond hair, blue or green eyes. || 1-5
|-
| II || light, or light-skinned European<ref name="nicknames"/> || Usually burns, sometimes tans || Tends to have light hair, blue/green or brown eyes. || 6-10
|-
| III || light intermediate, or dark-skinned European or "average Caucasian"<ref name="nicknames"/> || Sometimes burns, usually tans. || Tends to have brown hair and eyes. || 11-15
|-
| IV || dark intermediate, also "Mediterranean" or "Olive"<ref name="nicknames"/> || Sometimes burns, often tans. || Tends to have dark brown eyes and hair. || 16-20
|-
| V || dark or "]" type || Naturally black-brown skin || Often has dark brown eyes and hair. || 21-28
|-
| VI || very dark, or "]" type || Naturally black-brown skin || Usually has black-brown eyes and hair. || 29-36
|-
|}


== Genetics ==
In attempting to discover the mechanisms that have generated such a wide variation in human skin tone, {{harvtxt|Jablonski|Chaplin|2000}} discovered that there is a high correlation between the tone of human skin of indigenous peoples and the average annual ] (UV) radiation available for skin exposure where the indigenous peoples live. Accordingly, Jablonski and Chaplin plotted the skin tone (W) of indigenous peoples who have stayed in the same geographical area for the last 500 years versus the annual UV available for skin exposure (AUV) for over 200 indigenous persons and found that skin tone lightness W is related to the annual UV available for skin exposure AUV according to<ref>{{harv|Jablonski|Chaplin|2000|p=67}}, formula coefficients have been rounded to one-figure accuracy</ref>
{{See also|Human genetic variation|Race and genetics}}
:<math>W = 70 - \frac{AUV}{10}</math>
To some extent, skin color is determined independently of eye and hair color, as can be seen from variation in skin coloration in human populations. For the evolution of human skin color, see section above.<ref name="Norton2006">{{cite journal |last1=Norton |first1=H. L. |last2=Kittles |first2=R. A. |last3=Parra |first3=E. |last4=McKeigue |first4=P. |last5=Mao |first5=X. |last6=Cheng |first6=K. |last7=Canfield |first7=V. A. |last8=Bradley |first8=D. G. |last9=McEvoy |first9=B. |last10=Shriver |first10=M. D. |title=Genetic Evidence for the Convergent Evolution of Light Skin in Europeans and East Asians |journal=Molecular Biology and Evolution |volume=24 |issue=3 |pages=710–22 |year=2006 |pmid=17182896 |doi=10.1093/molbev/msl203 |doi-access=free }}</ref><ref name=McEvoy2006>{{cite journal |journal=Human Molecular Genetics |year=2006 |volume=15 |issue=2 |pages=176–181 |doi=10.1093/hmg/ddl217 |title=The genetic architecture of normal variation in human pigmentation: an evolutionary perspective and model |last1=McEvoy |first1=B. |pmid=16987881 |doi-access=free }}</ref><ref name="Sturm2009">{{cite journal|journal=Human Molecular Genetics |year=2009 |volume=18 |issue=R1 |pages=9–17 |doi=10.1093/hmg/ddp003 |title=Molecular genetics of human pigmentation diversity|last1=Sturm |first1=R. A. |pmid=19297406|doi-access=free }}</ref><ref name=Belezal2012>{{cite journal |url=http://mbe.oxfordjournals.org/content/early/2012/08/25/molbev.mss207.short|title=The timing of pigmentation lightening in Europeans |journal=Molecular Biology and Evolution |year=2012 |doi=10.1093/molbev/mss207 |pmid=22923467 |pmc=3525146 |volume=30 |issue=1 |pages=24–35 |last2=Santos |first2=A. M. |last3=McEvoy |first3=B. |last4=Alves |first4=I. |last5=Martinho |first5=C. |last6=Cameron |first6=E. |last7=Shriver |first7=M. D. |last8=Parra |first8=E. J. |last9=Rocha |first9=J. |last1=Belezal |first1=Sandra |url-status=live |archive-url=https://web.archive.org/web/20120829050356/http://mbe.oxfordjournals.org/content/early/2012/08/25/molbev.mss207.short|archive-date=2012-08-29 }}</ref><ref name=Edwards2010 /><ref name=Loa2007 />
where the skin tone lightness W is measured as the percentage of light reflected from the upper inner arm at which location on humans there should be minimal tanning of human skin due to personal exposure to the sun; a lighter skinned human would reflect more light and would have a higher W number. Judging from the above linear fit to the empirical data, the theoretical lightness maximum of human skin would reflect only 70 per cent of incident light for a hypothetical indigenous human-like population that lived where there was zero annual UV available for skin exposure (AUV = 0 in the above formula). Jablonski and Chaplin evaluated average annual UV available for skin exposure AUV from satellite measurements that took into consideration the measured daily variation in the thickness of the ozone layer that blocked UV hitting the Earth, measured daily variation in opacity of cloud cover, and daily change in angle at which the sunlight containing UV radiation strikes the Earth and passes through different thicknesses of Earth's atmosphere at different latitudes for each of the different human indigenous peoples' home areas from 1979 to 1992.


For skin color, ] is very high, even though it can be modified by exposure to sunlight.
Jablonski and Chaplin proposed an explanation for the observed variation of untanned human skin with annual UV exposure. By Jablonski and Chaplin's explanation, there are two competing ] affecting human skin tone:
#the melanin that produces the darker tones of human skin serves as a light filter to protect against too much UV light getting under the human skin where too much UV causes sunburn and disrupts the synthesis of precursors necessary to make human DNA; versus
#humans need at least a minimum threshold of UV light to penetrate the epidermis to produce ], which is essential for building and maintaining the ]s of the ].


A recent systematic study found 169 genes involved in human skin coloration. Most of the genes were involved in ] biogenesis, ] transport, and ]. Notably, the function of these genes was verified in ] experiments using ] knockouts, showing that these genes are indeed involved in ] production.<ref>{{Cite journal |last1=Bajpai |first1=Vivek K. |last2=Swigut |first2=Tomek |last3=Mohammed |first3=Jaaved |last4=Naqvi |first4=Sahin |last5=Arreola |first5=Martin |last6=Tycko |first6=Josh |last7=Kim |first7=Tayne C. |last8=Pritchard |first8=Jonathan K. |last9=Bassik |first9=Michael C. |last10=Wysocka |first10=Joanna |date=2023-08-11 |title=A genome-wide genetic screen uncovers determinants of human pigmentation |journal=Science |language=en |volume=381 |issue=6658 |pages=eade6289 |doi=10.1126/science.ade6289 |pmid=37561850 |s2cid=260776374 |issn=0036-8075|pmc=10901463 }}</ref>
Jablonski and Chaplin note that when human indigenous peoples have migrated, they have carried with them a sufficient gene pool so that within a thousand years, the skin of their descendants living today has turned dark or turned light to adapt to fit the formula given above—with the notable exception of dark-skinned peoples moving north, such as to populate the seacoast of Greenland, to live where they have a year-round supply of food rich in vitamin D, such as fish, so that there was no necessity for their skin to lighten to let enough UV under their skin to synthesize the vitamin D that humans need for healthy bones.


=== Dark skin ===
In considering the tone of human skin in the long span of ], Jablonski and Chaplin note that there is no empirical evidence to suggest that the hominid ancestors six million years ago had a skin tone different from the skin tone of today's ]s&mdash;namely light-skinned under black hair. But as humans evolved to lose their ] a parallel evolution permitted human populations to turn their base skin tone dark or light to adjust to the competing demands of 1) increasing eumelanin to protect from UV that was too intense and 2) reducing eumelanin so that enough UV would penetrate to synthesize enough vitamin D. By this explanation, prior to ''Homo Sapien'' colonization of extra-African territories, humans had dark skin given that they lived for extended periods of time where the sunlight is intense. As some humans migrated north, over time they developed light skin.{{ref harv|Jablonski|Chaplin|2000}}
{{Main|Dark skin}}
All modern humans share a ] who lived around 200,000 years ago in Africa.<ref name="Soares P, Ermini L, Thomson N, et al. 2009 740–59">{{cite journal |title=Correcting for purifying selection: an improved human mitochondrial molecular clock |journal=Am. J. Hum. Genet. |volume=84 |issue=6 |pages=740–59 |date=June 2009 |pmid=19500773 |pmc=2694979 |doi=10.1016/j.ajhg.2009.05.001 |last1=Soares |first1=P |last2=Ermini |first2=L |last3=Thomson |first3=N |last4=Mormina |first4=M |last5=Rito |first5=T |last6=Röhl |first6=A |last7=Salas |first7=A |last8=Oppenheimer |first8=S |last9=MacAulay |first9=V |last10=Richards |first10=M. B.}}</ref> Comparisons between known skin pigmentation genes in ] and modern Africans show that dark skin evolved along with the loss of body hair about 1.2 million years ago and that this common ancestor had dark skin.<ref name=Rogers2004b>{{cite journal |title=Genetic Variation at the MC1R Locus and the Time since Loss of Human Body Hair |journal=Current Anthropology |volume=45 |issue=6 |pages=105–108 |year=2004b |doi=10.1086/381006 |jstor=381006|last1=Rogers |first1=Alan R. |last2=Iltis |first2=David |last3=Wooding |first3=Stephen|s2cid=224795768 }}</ref> Investigations into dark-skinned populations in ] and ] indicate that skin pigmentation in these populations is due to the preservation of this ancestral state and not due to new variations on a previously lightened population.<ref name=jablonski2000 /><ref name=Harding2000>{{cite journal |last1=Harding |first1=R |last2=Healy |first2=E |last3=Ray |first3=A |last4=Ellis |first4=N |last5=Flanagan |first5=N |last6=Todd |first6=C |last7=Dixon |first7=C |last8=Sajantila |first8=A |last9=Jackson |first9=I |last10=Birch-Machin |first10=Mark A. |last11=Rees |first11=Jonathan L. |title=Evidence for Variable Selective Pressures at MC1R |journal=The American Journal of Human Genetics |volume=66 |pages=1351–61 |year=2000 |doi=10.1086/302863 |pmid=10733465 |issue=4 |pmc=1288200}}</ref>


==== MC1R ====
== Genetics of skin color variation ==
]
Several genes have been invoked to explain variations of skin tones in humans, including ],<ref></ref> ASIP, MATP, TYR, and OCA2.<ref>http://backintyme.com/admixture/shriver01.pdf</ref> A recently discovered gene, ] has been shown to account for a substantial fraction of the difference in the average of 30 or so melanin units between Europeans and Africans.
The ] (MC1R) gene is primarily responsible for determining whether pheomelanin and eumelanin are produced in the human body. Research shows at least 10 differences in MC1R between African and chimpanzee samples and that the gene has probably undergone a strong ] (a ]) in early Hominins around 1.2 million years ago.<ref>{{cite journal|title=Genetic variation at the MC1R locus and the time since loss of human body hair|journal=Current Anthropology|year=2004a|volume=45|issue=1|pages=105–124|doi=10.1086/381006|last1=Rogers|first1=Alan R.|last2=Iltis|first2=David|last3=Wooding|first3=Stephen|s2cid=224795768 }}</ref> This is consistent with positive selection for the high-eumelanin ] seen in Africa and other environments with high UV exposure.<ref name=Rogers2004b/><ref name=Harding2000/>


=== Light skin ===
Wide variations in human skin tones have been correlated with mutations in another ]; the ] gene <ref>], pg.1351</ref>. The "MC1R" label for the gene stands for '''melanocortin 1 receptor''', where
{{Main|Light skin}}
* "melano" refers to black,
] based on genetics]]
* "]" refers to the hormone stimulant produced by the ] that stimulates cells to produce the ] that makes skin cells black,
For the most part, the evolution of light skin has followed different genetic paths in European and East Asian populations. Two genes, however, ] and ], have mutations associated with lighter skin that have high frequencies in both European and East Asian populations. They are thought to have originated after humans spread out of Africa but before the divergence of the European and Asian lineages around 30,000 years ago.<ref name=Belezal2012 /> Two subsequent ] found no significant correlation between these genes and skin color, and suggest that the earlier findings may have been the result of incorrect correction methods and small panel sizes, or that the genes have an effect too small to be detected by the larger studies.<ref>{{cite journal|title=Genetic Architecture of Skin and Eye Color in an African-European Admixed Population|date=March 2013|last1=Beleza|first1=Sandra|journal=PLOS Genetics|issue=3|pages=e1003372|volume=9|doi=10.1371/journal.pgen.1003372|pmid=23555287|pmc=3605137|last2=Johnson|first2=Nicholas A.|last3=Candille|first3=Sophie I.|last4=Absher|first4=Devin M.|last5=Coram|first5=Marc A.|last6=Lopes|first6=Jailson|last7=Campos|first7=Joana|last8=Araújo|first8=Isabel Inês|last9=Anderson|first9=Tovi M.|last10=Vilhjálmsson|first10=Bjarni J.|last11=Nordborg|first11=Magnus|last12=Correia e Silva|first12=António|last13=Shriver|first13=Mark D.|last14=Rocha|first14=Jorge|last15=Barsh|first15=Gregory S.|last16=Tang|first16=Hua |doi-access=free }}</ref><ref>{{cite journal|title=Genome-Wide Association Studies of Quantitatively Measured Skin, Hair, and Eye Pigmentation in Four European Populations|date=31 October 2012|last1=Candille|first1=Sophie I.|journal=PLOS ONE|issue=10|pages=e48294|volume=7|doi=10.1371/journal.pone.0048294|pmid=23118974|pmc=3485197|last2=Absher|first2=Devin M.|last3=Beleza|first3=Sandra|last4=Bauchet|first4=Marc|last5=McEvoy|first5=Brian|last6=Garrison|first6=Nanibaa' A.|last7=Li|first7=Jun Z.|last8=Myers|first8=Richard M.|last9=Barsh|first9=Gregory S.|last10=Tang|first10=Hua|last11=Shriver|first11=Mark D.|bibcode=2012PLoSO...748294C|doi-access=free}}</ref>
* the "1" in the MC1R gene name specifies the first family of melanocortin genes, and
* "]" indicates that the protein from the gene serves as a signal relay from outside the ] to inside the cell—to the place in the cell where the black melanin is synthesized.


==== KITLG ====
Accordingly, the MC1R gene specifies the amino acid sequence in the receptor protein that relays through the cell membrane the hormone signal from the pituitary gland to produce the melanin that makes human skin very dark. Many variations in the amino acid sequence of this receptor protein result in lighter or darker skin.
]
The ] (KITLG) gene is involved in the permanent survival, proliferation and migration of melanocytes.<ref name="pmid12753403">{{cite journal |last1=Wehrle-Haller |first1=Bernhard |title=The Role of Kit-Ligand in Melanocyte Development and Epidermal Homeostasis |journal=Pigment Cell Research |volume=16 |issue=3 |pages=287–96 |year=2003 |pmid=12753403 |doi=10.1034/j.1600-0749.2003.00055.x}}</ref> A mutation in this gene, ''A326G'' (rs642742<ref> {{webarchive|url= https://web.archive.org/web/20180328071821/https://www.ncbi.nlm.nih.gov/projects/SNP/snp_ref.cgi?rs=642742 |date=2018-03-28 }}. Ncbi.nlm.nih.gov (2008-12-30). Retrieved 2011-02-27.</ref>), has been positively associated with variations of skin color in African-Americans of mixed West African and European descent and is estimated to account for 15–20% of the melanin difference between African and European populations.<ref name=Miller2007>{{cite journal |last1=Miller |first1=Craig T. |last2=Beleza |first2=Sandra |last3=Pollen |first3=Alex A. |last4=Schluter |first4=Dolph |last5=Kittles |first5=Rick A. |last6=Shriver |first6=Mark D. |last7=Kingsley |first7=David M. |title=cis-Regulatory Changes in Kit Ligand Expression and Parallel Evolution of Pigmentation in Sticklebacks and Humans |journal=Cell |volume=131 |issue=6 |pages=1179–89 |year=2007 |pmid=18083106 |pmc=2900316 |doi=10.1016/j.cell.2007.10.055}}</ref> This allele shows signs of strong positive selection outside Africa<ref name=Loa2007>{{cite journal |title=Signatures of Positive Selection in Genes Associated with Human Skin Pigmentation as Revealed from Analyses of Single Nucleotide Polymorphisms |journal=Annals of Human Genetics |volume= 71|issue=3 |pages=354–369 |date=May 2007| doi=10.1111/j.1469-1809.2006.00341.x|last1=Lao |first1=O. |last2=De Gruijter |first2=J. M. |last3=Van Duijn |first3=K. |last4=Navarro |first4=A. |last5=Kayser |first5=M. |pmid=17233754|s2cid=20657917}}</ref><ref name=Pickrell2009>{{cite journal |title=Signals of recent positive selection in a worldwide sample of human populations |journal=Genome Research |year=2009 |volume=19 |pages=826–837 |doi=10.1101/gr.087577.108 |issue=5 |pmid=19307593 |pmc=2675971 |last1=Pickrell |first1=J. K. |last2=Coop |first2=G |last3=Novembre |first3=J |last4=Kudaravalli |first4=S |last5=Li |first5=J. Z. |last6=Absher |first6=D |last7=Srinivasan |first7=B. S. |last8=Barsh |first8=G. S. |last9=Myers |first9=R. M. |last10=Feldman |first10=M. W. |last11=Pritchard |first11=J. K. }}</ref> and occurs in over 80% of European and Asian samples, compared with less than 10% in African samples.<ref name=Miller2007 />


==== ASIP ====
The human MC1R gene consists of a string of 954 ]s, where each nucleotide is one of the four bases ] (A), ] (G), ] (T), or ] (C). But 261 of the nucleotides in the MC1R gene can change with no effect on the amino acid sequence in the receptor protein produced from the gene. For example, the nucleotide triplets GGT, GGC, GGA, and GGG are all ] and all produce the amino acid Glycine<ref>See </ref>, so a mutation in the third position in the triplet GGT is a "]" and has no effect on the amino acid produced from the triplet. (], pg.1355) analyzed the amino acid sequences in the receptor proteins from 106 individuals from Africa and 524 individuals from outside Africa to find why the tone of the sampled Africans' skin was dark.
] (ASIP) acts as an ], binding in place of ] and thus inhibiting eumelanin production. Studies have found two alleles in the vicinity of ASIP are associated with skin color variation in humans. One, rs2424984,<ref> {{webarchive|url= https://web.archive.org/web/20160501175840/http://www.ncbi.nlm.nih.gov/projects/SNP/snp_ref.cgi?rs=2424984 |date=2016-05-01 }}. Ncbi.nlm.nih.gov (2008-12-30). Retrieved 2011-02-27.</ref> has been identified as an indicator of skin reflectance in a forensics analysis of human phenotypes across Caucasian, African-American, South Asian, East Asian, Hispanic and Native American populations<ref name=Valenzuela>{{cite journal |last1=Valenzuela |pages=315–22 |first1=Robert K. |issue=2 |last2=Henderson |first2=Miquia S. |last3=Walsh |first3=Monica H. |last4=Garrison |first4=Nanibaa' A. |last5=Kelch |first5=Jessica T. |last6=Cohen-Barak |first6=Orit |last7=Erickson |first7=Drew T. |last8=John Meaney |first8=F. |last9=Bruce Walsh |first9=J. |last10=Cheng |first10=Keith C. |last11=Ito |first11=Shosuke |last12=Wakamatsu |first12=Kazumasa |last13=Frudakis |first13=Tony |last14=Thomas |first14=Matthew |last15=Brilliant |first15=Murray H. |volume=55 |title=Predicting Phenotype from Genotype: Normal Pigmentation |year=2010 |pmid=20158590 |doi=10.1111/j.1556-4029.2009.01317.x |journal=Journal of Forensic Sciences |url=http://www.pennstatehershey.org/c/document_library/get_file?uuid=7b0b4263-ec43-40b2-9b76-f129ad14d4e9&groupId=101483 |pmc=3626268 |url-status=live |archive-url=https://web.archive.org/web/20130613160844/http://www.pennstatehershey.org/c/document_library/get_file?uuid=7b0b4263-ec43-40b2-9b76-f129ad14d4e9&groupId=101483 |archive-date=2013-06-13 }}</ref> and is about three times more common in non-African populations than in Africa.<ref>. Hapmap.ncbi.nlm.nih.gov (2009-10-19). Retrieved 2011-02-27.</ref> The other allele, ''8188G'' (rs6058017<ref>{{cite web |url=https://www.ncbi.nlm.nih.gov/projects/SNP/snp_ref.cgi?rs=6058017 |title=Reference SNP (refSNP) Cluster Report: rs6058017 ** With Pathogenic allele ** |publisher=Ncbi.nlm.nih.gov |date=2006-01-28 |access-date=2015-02-03 |url-status=live |archive-url=https://web.archive.org/web/20150203174915/http://www.ncbi.nlm.nih.gov/projects/SNP/snp_ref.cgi?rs=6058017 |archive-date=2015-02-03 }}</ref>) is significantly associated with skin color variation in African-Americans and the ancestral version occurs in only 12% of European and 28% of East Asian samples compared with 80% of West African samples.<ref name=Bonilla2005>{{cite journal |title=The 8818G allele of the agouti signaling protein (ASIP) gene is ancestral and is associated with darker skin color in African Americans |journal=Human Genetics |year=2005 |volume=116 |issue=5 |pages=402–6 |doi=10.1007/s00439-004-1251-2|pmid=15726415 |last1=Bonilla |first1=C |last2=Boxill |first2=L. A. |last3=Donald |first3=S. A. |last4=Williams |first4=T |last5=Sylvester |first5=N |last6=Parra |first6=E. J. |last7=Dios |first7=S |last8=Norton |first8=H. L. |last9=Shriver |first9=M. D. |last10=Kittles |first10=R. A.|s2cid=12910408 }}</ref><ref>{{cite journal |url=http://journals.ohiolink.edu/ejc/article.cgi?issn=08935785&issue=v17i0002&article=185_pditfotaspgp |title=Population Differences in the Frequency of the Agouti Signaling Protein g.8818A>G Polymorphism |journal=Pigment Cell Research |volume=17 |issue=2 |pages=185–187 |year=2004 |pmid=15016309 |doi=10.1111/j.1600-0749.2004.00134.x|last1=Zeigler-Johnson |first1=C |last2=Panossian |first2=S |last3=Gueye |first3=S. M. |last4=Jalloh |first4=M |last5=Ofori-Adjei |first5=D |last6=Kanetsky |first6=P. A.}}</ref>


==== Europe ====
Harding found that there were zero differences among the Africans for the amino acid sequences in their receptor proteins, so the skin of each individual from Africa was dark. In contrast, among certain (European) non-African individuals, there were 18 different amino acid sites in which the receptor proteins differed, and each amino acid that differed from the African receptor protein resulted in skin lighter than the skin of the African (and other equatorial) individuals. Nonetheless, the variations in the 261 silent sites in the MC1R were similar between the Africans and non-Africans, so the basic mutation rates among the Africans and non-Africans were the same. Also, close examination of the haplotype variation among the non-Europeans (including East Asians) suggested that, among most non-European non-Africans, the most common variants were in the silent mutation positions (Harding et al 2000 p 1355). Thus, at least at this locus, most non-Europeans share the ancestral function. The fact that relatively light skinned east Asians varied little genetically from dark skinned Africans at this locus supports the fact that skin color is a complex trait determined by several genes. Thus light skin among east Asians occurs by way of a different genetic mechanism than that among Europeans.
A number of genes have been positively associated with the skin pigmentation difference between European and non-European populations. Mutations in SLC24A5 and SLC45A2 are believed to account for the bulk of this variation and show very strong signs of selection. A variation in TYR has also been identified as a contributor.


Research indicates the selection for the light-skin alleles of these genes in Europeans is comparatively recent, having occurred later than 20,000 years ago and perhaps as recently as 12,000 to 6,000 years ago.<ref name=Belezal2012 /> In the 1970s, ] suggested that the selective sweep that rendered light skin ubiquitous in Europe might be correlated with the ] and thus have taken place only around 6,000 years ago;<ref name="pmid17446367">{{cite journal |last1=Gibbons |first1=A. |doi=10.1126/science.316.5823.364a |title=European Skin Turned Pale Only Recently, Gene Suggests |year=2007 |pages=364a |volume=316 |journal=Science |issue=5823|pmid=17446367|s2cid=43290419 |url=http://galsatia.files.wordpress.com/2007/04/blanche_paleur.pdf |archive-url= https://web.archive.org/web/20101103171424/http://galsatia.files.wordpress.com//2007//04//blanche_paleur.pdf |archive-date=2010-11-03 |url-status=dead |access-date=2011-01-26}} "a suggestion made 30 years ago by Stanford University geneticist L. Luca Cavalli-Sforza … that the early immigrants to Europe … survived on ready-made sources of vitamin D in their diet. But when farming spread in the past 6,000 years, he argued, Europeans had fewer sources of vitamin D in their food and needed to absorb more sunlight to produce the vitamin in their skin."</ref> This scenario found support in a 2014 analysis of mesolithic (7,000 years old) ] DNA from ], Spain, which showed a version of these genes not corresponding with light skin color.<ref>{{cite journal |title= Derived immune and ancestral pigmentation alleles in a 7,000-year-old Mesolithic European|journal=Nature|volume=507|issue=7491|pages=225–228 |doi=10.1038/nature12960|pmid=24463515|quote=The La Braña individual carries ancestral alleles in several skin pigmentation genes, suggesting that the light skin of modern Europeans was not yet ubiquitous in Mesolithic time|year=2014|last1=Olalde|first1=Iñigo|last2=Allentoft|first2=Morten E.|last3=Sánchez-Quinto |first3=Federico|last4=Santpere|first4=Gabriel|last5=Chiang|first5=Charleston W. K.|last6=Degiorgio |first6=Michael|last7=Prado-Martinez|first7=Javier|last8=Rodríguez|first8=Juan Antonio|last9=Rasmussen |first9=Simon|last10=Quilez |first10=Javier|last11=Ramírez|first11=Oscar|last12=Marigorta|first12=Urko M. |last13=Fernández-Callejo|first13=Marcos|last14=Prada|first14=María Encina|last15=Encinas|first15=Julio Manuel Vidal|last16=Nielsen|first16=Rasmus |last17=Netea|first17=Mihai G.|last18=Novembre|first18=John |last19=Sturm|first19=Richard A.|last20=Sabeti|first20=Pardis|author20-link=Pardis Sabeti|last21=Marquès-Bonet|first21=Tomàs |last22=Navarro|first22=Arcadi|last23=Willerslev|first23=Eske |last24=Lalueza-Fox|first24=Carles |pmc=4269527|bibcode=2014Natur.507..225O}}</ref>
With regards to Europeans, the next question to ask would be: why were there zero differences and no divergences in the amino acid sequences of the receptor protein among the Africans (and other equatorial groups) while there were 18 differences among the populations in Ireland, England, and Sweden? (], pp.1359-1360) concluded that the intense sun in Africa created an evolutionary constraint that reduced severely the survival of progeny with any difference in the 693 sites of the MC1R gene that resulted in even one small change in the amino acid sequence of the receptor protein—because any variation from the African receptor protein produced significantly lighter skin that gave less protection from the intense African sun. In contrast, in Sweden, for example, the sun was so weak that no mutation in the receptor protein reduced the survival probability of progeny. Indeed, for the individuals from Ireland, England, and Sweden, the mutation variations among the 693 gene sites that caused changes in amino acid sequence was the same as the mutation variations in the 261 gene sites at which silent mutations still produced the same amino acid sequence. Thus, Harding concluded that the intense sun in Africa selectively killed off the progeny of individuals who had a mutation in the MC1R gene that made the skin lighter. However, the mutation rate toward lighter skin in the progeny of those African individuals who had moved North to areas with weaker sun was comparable to the mutation rate of the folks whose ancient ancestors grew up in Sweden. Hence, Harding concluded that the lightness of human skin was a direct result of random mutations in the MC1R gene that were non-lethal at the latitudes of Sweden. Even the mutations that produce red hair with little ability to tan were non-lethal in the northern latitudes.


In 2015, researchers analysed for light skin genes in the DNA of 94 ancient skeletons ranging from 8,000 to 3,000 years old from Europe and Russia. They found c. 8,000-year-old hunter-gatherers in Spain, Luxembourg, and Hungary were dark skinned while similarly aged hunter gatherers in Sweden were light skinned (having predominately derived alleles of SLC24A5, SLC45A2 and also HERC2/OCA2). ] entering Europe at around the same time were intermediate, being nearly fixed for the derived SLC24A5 variant but only having the derived SLC45A2 allele in low frequencies. The SLC24A5 variant spread very rapidly throughout central and southern Europe from about 8,000 years ago, whereas the light skin variant of SLC45A2 spread throughout Europe after 5,800 years ago.<ref>{{cite journal |last=Gibbons |first=Ann |date=2 April 2015 |url=https://www.science.org/content/article/how-europeans-evolved-white-skin |title=How Europeans evolved white skin |doi=10.1126/science.aab2435 |archive-url=https://web.archive.org/web/20150414000323/http://news.sciencemag.org/archaeology/2015/04/how-europeans-evolved-white-skin |archive-date=2015-04-14 |journal=Science |access-date=13 April 2015 |url-status=live}}</ref><ref>{{cite bioRxiv |biorxiv=10.1101/016477|title=Eight thousand years of natural selection in Europe|year=2015 |last1=Mathieson |first1=I. |last2=Lazaridis |first2=I. |last3=Rohland |first3=N. |last4=Mallick |first4=S. |last5=Llamas |first5=B. |last6=Pickrell |first6=J. |last7=Meller |first7=H. |last8=Rojo Guerra |first8=M. A. |last9=Krause |first9=J. |last10=Anthony |first10=D. |last11=Brown |first11=D. |last12=Lalueza Fox |first12=C. |last13=Cooper |first13=A. |last14=Alt |first14=K. W. |last15=Haak |first15=W. |last16=Patterson |first16=N. |last17=Reich |first17=D.}}</ref>
{{harv|Rogers|Iltis|Wooding|2004}} examined Harding's data on the variation of MC1R nucleotide sequences for people of different ancestry to determine the most probable progression of the skin tone of human ancestors over the last five million years. Comparing the MC1R nucleotide sequences for chimpanzees and humans in various regions of the Earth, Rogers concluded that the common ancestors of all humans had light skin tone under dark hair—similar to the skin tone and hair color pattern of today's chimpanzees. That is 5 million years ago, the human ancestors' dark hair protected their light skin from the intense African sun so that there was no evolutionary constraint that killed off the progeny of those who had mutations in the MC1R nucleotide sequences that made their skin light. {{harv|Sweet|2002}} argues that based on cave paintings, Europeans may have been dark as recently as 13,000 years ago. The painters depicted themselves as having darker complexions than the animals they hunted.


Some authors have expressed caution regarding the skin pigmentation predictions. According to Ju et al. (2021), in a study addressing 40,000 years of modern human history, stated:<blockquote>"Relatively dark skin pigmentation in Early Upper Paleolithic Europe would be consistent with those populations being relatively poorly adapted to high-latitude conditions as a result of having recently migrated from lower latitudes. On the other hand, although we have shown that these populations carried few of the light pigmentation alleles that are segregating in present-day Europe, they may have carried different alleles that we cannot now detect."<ref>{{cite journal |journal=PNAS |volume=118 |issue=1 |date=2021 |title=The evolution of skin pigmentation-associated variation in West Eurasia |last1=Ju |first1=Dan |last2=Mathieson |first2=Ian|pages=e2009227118 |doi=10.1073/pnas.2009227118|pmid=33443182 |pmc=7817156 |bibcode=2021PNAS..11809227J |quote="Relatively dark skin pigmentation in Early Upper Paleolithic Europe would be consistent with those populations being relatively poorly adapted to high-latitude conditions as a result of having recently migrated from lower latitudes. On the other hand, although we have shown that these populations carried few of the light pigmentation alleles that are segregating in present-day Europe, they may have carried different alleles that we cannot now detect." |doi-access=free }}</ref></blockquote>
However, over 1.2 million years ago, judging from the numbers and spread of variations among human and chimpanzee MC1R nucleotide sequences, the human ancestors in Africa began to lose their hair and they came under increasing evolutionary pressures that killed off the progeny of individuals that retained the inherited lightness of their skin. Folate breakdown in sun-exposed skin is inhibited by the presence of melanin and is essential for human fetal development. It is likely that folate conservation played an important role in the selection of dark skin in the ancient African ancestors of modern humans. By 1.2 million years ago, all people having descendants today had exactly the receptor protein of today's Africans; their skin was dark, and the intense sun killed off the progeny with any lighter skin that resulted from mutational variation in the receptor protein {{harv|Rogers|Iltis|Wooding|2004|p=107}}.


===== SLC24A5 =====
However, the progeny of those humans who migrated North away from the intense African sun had another evolutionary constraint: vitamin D availability. Human requirements for vitamin D (cholecalciferol) are in part met through photoconversion of a precursor to vitamin D3. As humans migrated north from the equator, they were exposed to less intense sunlight, in part because of the need for greater use of clothing to protect against the colder climate. Thus, under these conditions, evolutionary pressures would tend to select for lighter-skinned humans as there was less photodestruction of folate and a greater need for photogeneration of cholecalciferol. Tracking back the statistical patterns in variations in DNA among all known people sampled who are alive on the Earth today, it appears that
] (SLC24A5) regulates calcium in melanocytes and is important in the process of ].<ref name="pmid18166528">{{cite journal |last1=Ginger |first1=R. S. |last2=Askew |first2=S. E. |last3=Ogborne |first3=R. M. |last4=Wilson |first4=S. |last5=Ferdinando |first5=D. |last6=Dadd |first6=T. |last7=Smith |first7=A. M. |last8=Kazi |first8=S. |last9=Szerencsei |first9=R. T. |last10=Winkfein |first10=R. J. |last11=Schnetkamp |first11=P. P. M. |last12=Green |first12=M. R. |title=SLC24A5 Encodes a trans-Golgi Network Protein with Potassium-dependent Sodium-Calcium Exchange Activity That Regulates Human Epidermal Melanogenesis |journal=Journal of Biological Chemistry |volume=283 |issue=9 |pages=5486–95 |year=2007 |pmid=18166528 |doi=10.1074/jbc.M707521200|doi-access=free }}</ref> The SLC24A5 gene's derived ''Ala111Thr'' allele (rs1426654<ref name="ncbi_rs14">{{Cite web |title=rs1426654 RefSNP Report |author= |work=ncbi.nlm.nih.gov |date= |access-date=15 February 2021 |url= https://www.ncbi.nlm.nih.gov/snp/rs1426654 |quote=}}</ref>) has been shown to be a major factor in light skin pigmentation and is common in Western ].<ref name=Valenzuela /> Recent studies have found that the variant represents as much as 25–40% of the average skin tone difference between Europeans and West Africans.<ref name=Norton2006 /><ref name="pmid16357253">{{cite journal |last1=Lamason |first1=R. L. |last2=Mohideen |first2=MA |last3=Mest |first3=JR |last4=Wong |first4=AC |last5=Norton |first5=HL |last6=Aros |first6=MC |last7=Jurynec |first7=MJ |last8=Mao |first8=X |last9=Humphreville |first9=VR |last10=Humbert |first10=J. E. |last11=Sinha |first11=S |last12=Moore |first12=J. L. |last13=Jagadeeswaran |first13=P |last14=Zhao |first14=W |last15=Ning |first15=G |last16=Makalowska |first16=I |last17=McKeigue |first17=P. M. |last18=O'Donnell |first18=D |last19=Kittles |first19=R |last20=Parra |first20=E. J. |last21=Mangini |first21=N. J. |last22=Grunwald |first22=D. J. |last23=Shriver |first23=M. D. |last24=Canfield |first24=V. A. |last25=Cheng |first25=K. C. |title=SLC24A5, a Putative Cation Exchanger, Affects Pigmentation in Zebrafish and Humans |journal=Science |volume=310 |issue=5755 |pages=1782–6 |year=2005 |pmid=16357253 |doi=10.1126/science.1116238 |bibcode=2005Sci...310.1782L |s2cid=2245002}}</ref> This derived allele is a reliable predictor of phenotype across a range of populations.<ref name=Lopez>{{cite journal |last1=López |first1=Saioa |title=The Interplay between Natural Selection and Susceptibility to Melanoma on Allele 374F of SLC45A2 Gene in a South European Population |journal=PLOS ONE |volume=9 |issue=8 |pages=e104367 |date=5 August 2014 |doi=10.1371/journal.pone.0104367 |pmid=25093503 |pmc=4122405 |bibcode=2014PLoSO...9j4367L |doi-access=free }}</ref><ref name=Stokowski2007 /> It has been the subject of recent selection in Western Eurasia, and is fixed in European populations.<ref name=Belezal2012 /><ref name=Soejima2006>{{cite journal |last1=Soejima |first1=Mikiko |last2=Koda |first2=Yoshiro |title=Population differences of two coding SNPs in pigmentation-related genes SLC24A5 and SLC45A2 |journal=International Journal of Legal Medicine |volume=121 |issue=1 |pages=36–9 |year=2006 |pmid=16847698 |doi=10.1007/s00414-006-0112-z|s2cid=11192076 }}</ref><ref name=Khai2012>{{cite journal |title=Skin Color Variation in Orang Asli Tribes of Peninsular Malaysia |journal=PLOS ONE |volume=7 |issue=8 |pages=e42752 |doi=10.1371/journal.pone.0042752 |year=2012 |pmid=22912732 |pmc=3418284 |bibcode=2012PLoSO...742752A |last1=Ang |first1=K. C. |last2=Ngu |first2=M. S. |last3=Reid |first3=K. P. |last4=Teh |first4=M. S. |last5=Aida |first5=Z. S. |last6=Koh |first6=D. X. |last7=Berg |first7=A |last8=Oppenheimer |first8=S |last9=Salleh |first9=H |last10=Clyde |first10=M. M. |last11=Md-Zain |first11=B. M. |last12=Canfield |first12=V. A. |last13=Cheng |first13=K. C. |doi-access=free }}</ref>


===== SLC45A2 =====
# From ~1.2 million years ago for at least ~1.35 million years, the ancestors of all people alive were as dark as today's Africans.
] (SLC45A2 or MATP) aids in the transport and processing of tyrosine, a precursor to melanin. It has also been shown to be one of the significant components of the skin color of modern Europeans through its ''Phe374Leu'' (]<ref>{{Cite web |title=rs16891982 RefSNP Report - dbSNP - NCBI |url=https://www.ncbi.nlm.nih.gov/snp/rs16891982 |access-date=2022-10-02 |website=www.ncbi.nlm.nih.gov}}</ref>) allele that has been directly correlated with skin color variation across a range of populations.<ref>{{cite journal |last1=Adhikari |first1=Kaustubh |title=A GWAS in Latin Americans highlights the convergent evolution of lighter skin pigmentation in Eurasia |journal=Nature Communications |volume=10 |issue=1 |pages=358 |date=21 January 2019 |doi=10.1038/s41467-018-08147-0 |pmid=30664655 |pmc=6341102 |bibcode=2019NatCo..10..358A }}</ref><ref>{{cite journal |last1=Deng |first1=Lian |title=Adaptation of human skin color in various populations |journal=Hereditas |volume=155 |pages=1 |date=15 June 2017 |doi=10.1186/s41065-017-0036-2 |pmid=28701907 |pmc=5502412 |doi-access=free }}</ref><ref name=Lopez/><ref name=Valenzuela /><ref name=Stokowski2007 /> This variation is ubiquitous in European populations but extremely rare elsewhere and shows strong signs of selection.<ref name=Soejima2006 /><ref name=Khai2012 /><ref>{{cite journal |title=Evidence for Recent Positive Selection at the Human AIM1 Locus in a European Population |journal=Molecular Biology and Evolution |date=January 2006 |volume=23 |issue=1 |pages=179–188 |doi=10.1093/molbev/msj018 |pmid=16162863 |last1=Soejima |first1=M |last2=Tachida |first2=H |last3=Ishida |first3=T |last4=Sano |first4=A |last5=Koda |first5=Y }}</ref>
# The descendants of any pre-historic people who migrate North from the equator will mutate to become light over time because the evolutionary constraint keeping Africans' skin dark decreases generally the further North a people migrates<ref>{{harv|Rogers|Iltis|Wooding|2004}}, {{harv|Jablonski|2004}}</ref>. This also occurs as a result of selection for light skin due to the need to produce vitamin D by way of the penetration of sunlight into the skin (the exception being if dietary sources of vitamin D are available--see the ]).
# The genetic mutations leading to light skin among East Asians are different from those of Europeans, suggesting that, following the migration out of Africa, the two groups became distinct populations that experienced a similar selective pressure due to settlement in northern latitudes.


==See also== ===== TYR =====
The ] encodes the enzyme tyrosinase, which is involved in the production of melanin from tyrosine. It has an allele, ''Ser192Tyr'' (rs1042602<ref> {{webarchive|url= https://web.archive.org/web/20160416220943/http://www.ncbi.nlm.nih.gov/projects/SNP/snp_ref.cgi?rs=1042602 |date=2016-04-16 }}. Ncbi.nlm.nih.gov (2008-12-30). Retrieved 2011-02-27.</ref>), found solely in 40–50% of Europeans<ref name="Norton2006" /><ref name=Belezal2012 /> and linked to light-colored skin in studies of South Asian<ref name=Stokowski2007>{{cite journal |last1=Stokowski |first1=R |last2=Pant |first2=P |last3=Dadd |first3=T |last4=Fereday |first4=A |last5=Hinds |first5=D |last6=Jarman |first6=C |last7=Filsell |first7=W |last8=Ginger |first8=R |last9=Green |first9=M |last10=Van Der Ouderaa |first10=Frans J. |last11=Cox |first11=David R. |title=A Genomewide Association Study of Skin Pigmentation in a South Asian Population |journal=The American Journal of Human Genetics |volume=81 |pages=1119–32 |year=2007 |doi=10.1086/522235 |pmid=17999355 |issue=6 |pmc=2276347 }}</ref> and African-American<ref name=Shriver2003>{{cite journal|url=http://dl.dropbox.com/u/38568440/admixture/shriver01.pdf |title=Skin pigmentation, biogeographical ancestry and admixture mapping |journal=Human Genetics |year=2003 |volume=112 |issue=4 |pages=387–399 |doi=10.1007/s00439-002-0896-y |pmid=12579416 |last1=Shriver|first1=MD |last2=Parra|first2=EJ |last3=Dios|first3=S |last4=Bonilla |first4=C |last5=Norton|first5=H |last6=Jovel|first6=C |last7=Pfaff |first7=C |last8=Jones|first8=C |last9=Massac|first9=A |last10=Cameron|first10=N |last11=Baron|first11=A |last12=Jackson|first12=T |last13=Argyropoulos|first13=G |last14=Jin|first14=L |last15=Hoggart|first15=CJ |last16=McKeigue|first16=PM |last17=Kittles|first17=RA |s2cid=7877572 |url-status=dead |archive-url= https://web.archive.org/web/20120415112141/http://dl.dropbox.com/u/38568440/admixture/shriver01.pdf |archive-date=2012-04-15}}</ref> populations.
* ]
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==Footnotes== ==== East Asia ====
A number of genes known to affect skin color have alleles that show signs of positive selection in East Asian populations. Of these, only OCA2 has been directly related to skin color measurements, while DCT, MC1R and ATRN are marked as ]s for future study.
{{reflist|2}}
<!-- <nowiki>


===== OCA2 =====
IMPORTANT:
]
Footnotes using {{ref}}/{{note}} templates (and their variants) are becoming obsolete:
] (OCA2) assists in the regulation of ] in melanocytes. The OCA2 gene's derived ''His615Arg'' (rs1800414<ref name="ncbi_rs18">{{Cite web |title=rs1800414 RefSNP Report |author= |work=ncbi.nlm.nih.gov |date= |access-date=15 February 2021 |url= https://www.ncbi.nlm.nih.gov/snp/rs1800414 |quote=}}</ref>) allele has been shown to account for about 8% of the skin tone difference between African and East Asian populations in studies of an East Asian population living in Toronto and a Chinese ] population. This variant is essentially restricted to East Asia, with highest frequencies in Eastern East Asia (49–63%), midrange frequencies in Southeast Asia, and the lowest frequencies in Western China and some Eastern European populations.<ref name="Edwards2010">{{cite journal |last1=Edwards |first1=Melissa |last2=Bigham |first2=Abigail |last3=Tan |first3=Jinze |last4=Li |first4=Shilin |last5=Gozdzik |first5=Agnes |last6=Ross |first6=Kendra |last7=Jin |first7=Li |last8=Parra |first8=Esteban J. |editor1-last=McVean |editor1-first=Gil |title=Association of the OCA2 Polymorphism His615Arg with Melanin Content in East Asian Populations: Further Evidence of Convergent Evolution of Skin Pigmentation |journal=PLOS Genetics |volume=6 |issue=3 |pages=e1000867 |year=2010 |pmid=20221248 |pmc=2832666 |doi=10.1371/journal.pgen.1000867 |doi-access=free }}</ref><ref>{{cite journal|title=A global view of the OCA2-HERC2 region and pigmentation |year=2012 |url=http://medicine.yale.edu/labs/kidd/www/511.pdf |last1=Donnelly |doi=10.1007/s00439-011-1110-x |journal=Human Genetics |volume=131 |issue=5 |pages=683–696 |first1=Michael P. |pmid=22065085 |pmc=3325407 |last2=Paschou |first2=Peristera |last3=Grigorenko |first3=Elena |last4=Gurwitz |first4=David |last5=Barta |first5=Csaba |last6=Lu |first6=Ru-Band |last7=Zhukova |first7=Olga V. |last8=Kim |first8=Jong-Jin |last9=Siniscalco |first9=Marcello |last10=New |first10=Maria |last11=Li |first11=Hui |last12=Kajuna |first12=Sylvester L. B. |last13=Manolopoulos |first13=Vangelis G. |last14=Speed |first14=William C. |last15=Pakstis |first15=Andrew J. |last16=Kidd |first16=Judith R. |last17=Kidd |first17=Kenneth K. |archive-url=https://web.archive.org/web/20131103050619/http://medicine.yale.edu/labs/kidd/www/511.pdf |archive-date=2013-11-03 |url-status=dead}}</ref>
A more up-to-date system uses MediaWiki software <ref>...</ref> and <references/> tags,
as explained in ].


===== Candidate genes =====
IF CONTINUING TO USE {{ref}}/{{note}} FOOTNOTES ON THIS PAGE:
A number of studies have found genes linked to human skin pigmentation that have alleles with statistically significant frequencies in Chinese and East Asian populations. While not linked to measurements of skin tone variation directly, ] (DCT or TYRP2 rs2031526<ref name="ncbi_rs20">{{Cite web |title=rs2031526 RefSNP Report |author= |work=ncbi.nlm.nih.gov |date= |access-date=15 February 2021 |url= https://www.ncbi.nlm.nih.gov/snp/rs2031526 |quote=}}</ref><ref>{{cite journal |title=Identifying genes underlying skin pigmentation differences among human populations|year=2006 |journal=Human Genetics|volume=120|issue=5|pages=613–621|doi=10.1007/s00439-006-0256-4 |pmid=16977434 |last1=Myles|first1=S |last2=Somel|first2=M |last3=Tang|first3=K |last4=Kelso|first4=J |last5=Stoneking|first5=M |s2cid=32371450}}</ref>), ] (MC1R) ''Arg163Gln'' (rs885479<ref> {{webarchive|url= https://web.archive.org/web/20160919022256/http://www.ncbi.nlm.nih.gov/projects/SNP/snp_ref.cgi?rs=885479 |date=2016-09-19 }}. Ncbi.nlm.nih.gov (2008-12-30). Retrieved 2011-02-27.</ref><ref>{{cite journal |last1=Shi |first1=Peng |last2=Lu |first2=Xue Mei |last3=Luo |first3=Huai Rong |last4=Xiang-Yu |first4=Jin-Gong |last5=Zhang |first5=Ya Ping |title=Melanocortin-1 receptor gene variants in four Chinese ethnic populations |journal=Cell Research |volume=11 |issue=1 |pages=81–4 |year=2001 |pmid=11305330 |doi=10.1038/sj.cr.7290070|doi-access=free }}</ref>) and ] (ATRN<ref name="Norton2006" />) have been indicated as potential contributors to the evolution of light skin in East Asian populations.
Footnotes in this article use names, not numbers. Please see ] for details.
How to add a *SEMI-AUTOMATICALLY NUMBERED* footnote:
1) Assign your footnote a unique name, for example TheSun_Dec9.
2) Add the macro {{ref|TheSun_Dec9}} to the body of the article, where you want the new footnote.
3) Take note of the name of the footnote that immediately precedes yours in the article body.
4) Add #{{Note|TheSun_Dec9}} to the list, immediately below the footnote you noted in step 3. No need to re-number anything!
5) Multiple footnotes to the same reference: see ] for a how-to.
NOTE: It is important to add footnotes in the right order in the list!


=== Tanning response ===
HARVARD REFERENCES
Tanning response in humans is controlled by a variety of genes. ''MC1R'' variants ''Arg151Sys'' (rs1805007<ref> {{webarchive|url= https://web.archive.org/web/20161108195136/https://www.ncbi.nlm.nih.gov/projects/SNP/snp_ref.cgi?rs=1805007 |date=2016-11-08 }}. Ncbi.nlm.nih.gov (2008-12-30).</ref>), ''Arg160Trp'' (rs1805008<ref> {{webarchive|url= https://web.archive.org/web/20161108140647/https://www.ncbi.nlm.nih.gov/projects/SNP/snp_ref.cgi?rs=1805008 |date=2016-11-08 }}. Ncbi.nlm.nih.gov (2008-12-30).</ref>), ''Asp294Sys'' (rs1805009<ref> {{webarchive|url= https://web.archive.org/web/20161108135958/https://www.ncbi.nlm.nih.gov/projects/SNP/snp_ref.cgi?rs=1805009 |date=2016-11-08 }}. Ncbi.nlm.nih.gov (2008-12-30).</ref>), ''Val60Leu'' (rs1805005<ref> {{webarchive|url= https://web.archive.org/web/20180328064754/https://www.ncbi.nlm.nih.gov/projects/SNP/snp_ref.cgi?rs=1805005 |date=2018-03-28 }}. Ncbi.nlm.nih.gov (2008-12-30).</ref>) and ''Val92Met'' (rs2228479<ref> {{webarchive|url= https://web.archive.org/web/20180328111055/https://www.ncbi.nlm.nih.gov/projects/SNP/snp_ref.cgi?rs=2228479 |date=2018-03-28 }}. Ncbi.nlm.nih.gov (2008-12-30).</ref>) have been associated with reduced tanning response in European and/or East Asian populations. These alleles show no signs of positive selection and only occur in relatively small numbers, reaching a peak in Europe with around 28% of the population having at least one allele of one of the variations.<ref name=Harding2000 /><ref>{{cite journal |title=Variants of the melanocyte-stimulating hormone receptor gene are associated with red hair and fair skin in humans|journal=Nature Genetics |year=1995 |volume=11 |issue=3 |pages=328–30 |doi=10.1038/ng1195-328|pmid=7581459 |last1=Valverde|first1=P|last2=Healy|first2=E|last3=Jackson|first3=I|last4=Rees|first4=J. L.|last5=Thody|first5=A. J.|s2cid=7980311 }}</ref> A study of self-reported tanning ability and skin type in American non-] ] found that ''SLC24A5 Phe374Leu'' is significantly associated with reduced tanning ability and also associated ''TYR Arg402Gln'' (rs1126809<ref>{{cite web |url=https://www.ncbi.nlm.nih.gov/SNP/snp_ref.cgi?rs=1126809 |title=Reference SNP (refSNP) Cluster Report: rs1126809 ** With Pathogenic allele ** |publisher=Ncbi.nlm.nih.gov |date=2006-01-28 |access-date=2015-02-03 |archive-url=https://web.archive.org/web/20150722102440/http://www.ncbi.nlm.nih.gov/SNP/snp_ref.cgi?rs=1126809 |archive-date=2015-07-22 |url-status=live}}</ref>), ''OCA2 Arg305Trp'' (rs1800401<ref>{{cite web |url=https://www.ncbi.nlm.nih.gov/projects/SNP/snp_ref.cgi?rs=1800401 |title=Reference SNP (refSNP) Cluster Report: rs1800401 ** With Pathogenic allele ** |publisher=Ncbi.nlm.nih.gov |date=2006-01-28 |access-date=2015-02-03 |url-status=live |archive-url=https://web.archive.org/web/20150203171523/http://www.ncbi.nlm.nih.gov/projects/SNP/snp_ref.cgi?rs=1800401 |archive-date=2015-02-03 }}</ref>) and a 2-] ] in ''ASIP'' (rs4911414<ref> {{webarchive|url= https://web.archive.org/web/20180328061736/https://www.ncbi.nlm.nih.gov/projects/SNP/snp_ref.cgi?rs=4911414 |date=2018-03-28 }}. Ncbi.nlm.nih.gov (2008-12-30). Retrieved 2011-02-27.</ref> and rs1015362<ref> {{webarchive|url= https://web.archive.org/web/20180328105513/https://www.ncbi.nlm.nih.gov/projects/SNP/snp_ref.cgi?rs=1015362 |date=2018-03-28 }}. Ncbi.nlm.nih.gov (2008-12-30). Retrieved 2011-02-27.</ref>) to skin type variation within a "fair/medium/olive" context.<ref name=Nan2009>{{cite journal |last1=Nan |first1=Hongmei |last2=Kraft |first2=Peter |last3=Hunter |first3=David J. |last4=Han |first4=Jiali |title=Genetic variants in pigmentation genes, pigmentary phenotypes, and risk of skin cancer in Caucasians |journal=International Journal of Cancer |volume=125 |pages=909–17 |year=2009 |doi=10.1002/ijc.24327 |pmid=19384953 |issue=4 |pmc=2700213}}</ref>
See:
* ]
* {{ref harvard}}/{{note label}} templates can be used as explained in ]


=== Albinism ===
</nowiki> -->
{{Main|Albinism in humans}}
] (OCA) is a lack of pigment in the eyes, skin and sometimes hair that occurs in a very small fraction of the population. The four known types of ''OCA'' are caused by mutations in the ''TYR'', ''OCA2'', '']'', and ''SLC45A2'' genes.<ref name="ncbi.nlm.nih.gov"> {{webarchive|url= https://web.archive.org/web/20100803161806/http://www.ncbi.nlm.nih.gov/omim/203100 |date=2010-08-03 }}, Johns Hopkins University</ref>


==References== == Age ==
In ]s, the parts of the body not covered with hair, like the face and the back of the hands, start out pale in infants and turn darker as the skin is exposed to more sun. All human babies are born pale, regardless of what their adult color will be. In humans, ] production does not peak until after puberty.<ref name=LivCol />
* {{anchor|Harding}} {{cite journal|first=Rosalind M.|last=Harding|coauthors=Eugene Healy, Amanda J. Ray, Nichola S. Ellis, Niamh Flanagan, Carol Todd, Craig Dixon, Antti Sajantila, Ian J. Jackson, Mark A. Birch-Machin, and Jonathan L. Rees|year=2000|title=Evidence for variable selective pressures at MC1R|journal=American Journal of Human Genetics|volume=66|pages=1351–1361|doi=10.1086/302863}}


The skin of children becomes darker as they go through ] and experience the effects of sex hormones.<ref>{{Cite web |title=Everything You Wanted to Know About Puberty (for Teens) - Nemours KidsHealth |url=https://kidshealth.org/en/teens/puberty.html |access-date=2022-05-24 |website=kidshealth.org}}</ref> This darkening is especially noticeable in the skin of the ]s, the ] of the nipples, the ] in females, and the ] in males. In some people, the ] become slightly darker during puberty. The interaction of genetic, hormonal, and environmental factors on skin coloration with age is still not adequately understood, but it is known that men are at their darkest baseline skin color around the age of 30, without considering the effects of tanning. Around the same age, women experience darkening of some areas of their skin.<ref name=LivCol />
* {{citation|first1=Nina G.|last1=Jablonski|first2=George|last2=Chaplin|year=2000|url=http://www.bgsu.edu/departments/chem/faculty/leontis/chem447/PDF_files/Jablonski_skin_color_2000.pdf |title=The evolution of human skin coloration|journal=Journal of Human Evolution|volume=39|pages=57–106 |doi=10.1006/jhev.2000.0403}}


Human skin color fades with age. Humans over the age of thirty experience a decrease in melanin-producing cells by about 10% to 20% per decade as melanocyte stem cells gradually die.<ref>{{Cite journal|title=Aging of the Hair Follicle Pigmentation System|journal = International Journal of Trichology|volume = 1|issue = 2|pages = 83–93|pmc = 2938584|year = 2009|last1 = Tobin|first1 = D. J.|pmid = 20927229|doi = 10.4103/0974-7753.58550 | doi-access=free }}</ref> The skin of face and hands has about twice the amount of pigment cells as unexposed areas of the body, as chronic exposure to the sun continues to stimulate melanocytes. The blotchy appearance of skin color in the face and hands of older people is due to the uneven distribution of pigment cells and to changes in the interaction between ]s and ]s.<ref name=LivCol />
* {{Citation|first=Nina G.|last=Jablonski|title=The evolution of human skin and skin color|journal=Annual Review of Anthropology|volume=33|pages=585–623|month=October|year=2004|doi=10.1146/annurev.anthro.33.070203.143955}}


== Sexual dimorphism ==
* {{anchor|Norton}} {{cite journal|first=Heather L.|last=Norton|coauthors=Rick A. Kittles, Esteban Parra, Paul McKeigue, Xianyun Mao, Keith Cheng, Victor A. Canfield, Daniel G. Bradley, Brian McEvoy and Mark D. Shriver|month=December 11|year=2006|url=http://mbe.oxfordjournals.org/cgi/reprint/msl203v1.pdf|title=Genetic Evidence for the Convergent Evolution of Light Skin in Europeans and East Asians|journal=]|format=PDF}}
It has been observed that ] are found to have lighter skin pigmentation than ] in some studied populations.<ref name=jablonski2000 /> However, in light skinned populations, namely those of ] descent, women have darker skin than men.<ref name="Frost 2007 pp. 779–780">{{cite journal | last=Frost | first=Peter | title=Human skin‐color sexual dimorphism: A test of the sexual selection hypothesis | journal=American Journal of Physical Anthropology | volume=133 | issue=1 | date=2007 | issn=0002-9483 | doi=10.1002/ajpa.20555 | pages=779–780}} p.779: "It is perhaps significant that this sex difference seems to disappear or even reverse itself when skin reflectance is close to the physiological maximum, notably in Dutch and Belgian subjects (Leguebe, 1961; van Rijn-Tournel, 1966; Rigters-Aris, 1973)...Even after adolescence, age-related noise continues to muddy the data. Human skin may change more slowly during adulthood, but it does change. In particular, there is some evidence that skin color becomes less sexually dimorphic after 40, with some papers finding that women become nonsignificantly darker than men (Chamla and De´moulin, 1978; Mori and Tokuhashi, 1956; Rigters-Aris, 1973)."</ref><ref name="Candille">{{Cite journal|doi = 10.1371/journal.pone.0048294|title = Genome-Wide Association Studies of Quantitatively Measured Skin, Hair, and Eye Pigmentation in Four European Populations|year = 2012|last1 = Candille|first1 = Sophie I.|last2 = Absher|first2 = Devin M.|last3 = Beleza|first3 = Sandra|last4 = Bauchet|first4 = Marc|last5 = McEvoy|first5 = Brian|last6 = Garrison|first6 = Nanibaa' A.|last7 = Li|first7 = Jun Z.|last8 = Myers|first8 = Richard M.|last9 = Barsh|first9 = Gregory S.|last10 = Tang|first10 = Hua|last11 = Shriver|first11 = Mark D.|journal = PLOS ONE|volume = 7|issue = 10|pages = e48294|pmid = 23118974|pmc = 3485197|bibcode = 2012PLoSO...748294C|doi-access = free}}</ref>


It is unknown why skin color is sexually dimorphic in some populations. White women may have darker skin than white men because the female sex hormone ] is known to increase skin pigmentation in lighter-skinned populations.<ref name="scie_Rese">{{Cite web |title=Researchers discover genetic causes of higher melanoma risk in men |author= |work=ScienceDaily |date=21 July 2016 |access-date=15 February 2021 |url= https://www.sciencedaily.com/releases/2016/07/160721072753.htm |quote=}} "Meanwhile, several studies have shown that female hormones promote the production of melanin, the pigment that protects the skin from the sun. Indeed, "estrogen could be the reason why women have a darker skin tone, even when the genotypes of both sexes are the same, meaning that their risk of skin cancer is lower. So much so that skin cancer is much more prevalent in men," explains Bárbara Hernando, fellow researcher at the Melanogen research group and coauthor of the study."</ref> White women's skin is significantly darker than White men's, to the extent that White women have a much lower rate of skin cancer than White men.<ref name="scie_Rese" />
* {{citation|last1=Rogers|first1=Alan R.|first2=David|last2=Iltis|first3=Stephen|last3=Wooding|year=2004|title=Genetic variation at the MC1R locus and the time since loss of human body hair|journal=Current Anthropology|volume=45|issue=1|pages=105–108|doi=10.1086/381006}}


In populations where women have lighter skin than men, it has been hypothesized that the requirement for high amounts of calcium during ] and ] may be related to the dimorphism. Breastfeeding newborns, whose skeletons are growing, require high amounts of calcium intake from the mother's milk (about 4 times more than during prenatal development),<ref>{{cite journal|pmid=18689394 |last=Kovacs|first=Christine|title=Vitamin D in pregnancy and lactation: maternal, fetal, and neonatal outcomes from human and animal studies|journal=American Journal of Clinical Nutrition|year=2008|volume=88|issue=2|pages=520S–528S|doi=10.1093/ajcn/88.2.520S|doi-access=free}}</ref> part of which comes from reserves in the mother's skeleton.<ref name="LivCol" />
* {{citation|year=2002|first=Frank W.|last=Sweet|url=http://www.backintyme.com/Essay021215.htm|title=The Paleo-Etiology of Human Skin Tone}}
** Proposes that the advent of agriculture and a grain diet low in vitamin D gave Northern Europeans their very pale skin.
** Argues that skin tone is regulated by five genes and suggests Native Americans lost some genes in passage through the Arctic, preventing them from evolving very dark skin in equatorial America.
** Gives some history of global skin tone maps, noting that Biasutti map is out of date.


Adequate vitamin D resources are needed to absorb calcium from the diet, and it has been shown that deficiencies of vitamin D and calcium increase the likelihood of various birth defects such as ] and ]. Natural selection may have led to females with lighter skin than males in some indigenous populations because women must get enough vitamin D and calcium to support the development of fetus and nursing infants and to maintain their own health.<ref name="LivCol" /> However, some authors have cast doubt on the theory that vitamin D synthesis is related to the sexual dimorphism of human skin color in these populations.<ref name="Madrigal Kelly 2007 pp. 470–482">{{cite journal | last=Madrigal | first=Lorena | last2=Kelly | first2=William | title=Human skin‐color sexual dimorphism: A test of the sexual selection hypothesis | journal=American Journal of Physical Anthropology | volume=132 | issue=3 | date=2007 | issn=0002-9483 | doi=10.1002/ajpa.20453 | pages=470–482}} "Although Jablonski (2004) and Jablonski and Chaplin (2000) argued that pro- duction of vitamin D is the main evolutionary explanation for the origin of light skin in areas of solar radiation, others, such as Aoki (2002), were less convinced, still pointing to the lack of paleopathological evidence of rick- ets (Webb and Holick, 1988)..."</ref>
{{Nofootnotes|date=February 2008}} <!-- remainder uncited; not the best template to indicate :( -->


The sexes also differ in how they change their skin color with age.<ref name="Frost 2007 pp. 779–780" /> Women can change pigmentation in certain parts of their body, such as the ] and ] during the menstrual cycle and pregnancy. Between 50 and 70% of pregnant women will develop the "]", which refers to the browning and yellowing of the cheeks, upper lips, and forehead that occurs during pregnancy.<ref name="LivCol" /> This is caused by increases in the female sex hormone ], and it can develop in women who take birth control pills or participate in hormone replacement therapy.<ref>{{cite journal|last1=Costin|first1=G. E.|title=Human skin pigmentation: Melanocytes modulate skin color in response to stress|journal=FASEB Journal|year=2007 |volume=21|issue=4|pages=976–994|pmid=17242160|last2=Hearing|first2=V. J. |doi=10.1096/fj.06-6649rev |doi-access=free |s2cid=10713500}}</ref>
* Jablonski, Nina G., and George Chaplin (2002). "Skin deep." ''Scientific American'' '''287''' (4) (October): 74-82.
*Lamason RL, Mohideen MA, Mest JR, Wong AC, Norton HL, Aros MC, Jurynec MJ, Mao X, Humphreville VR, Humbert JE, Sinha S, Moore JL, Jagadeeswaran P, Zhao W, Ning G, Makalowska I, McKeigue PM, O'donnell D, Kittles R, Parra EJ, Mangini NJ, Grunwald DJ, Shriver MD, Canfield VA, Cheng KC (2005). SLC24A5, a putative cation exchanger, affects pigmentation in zebrafish and humans. Science 310 (5755): 1782-6. PMID 16357253
* Rees, J.L., and N. Flanagan (1999). "Pigmentation, melanocortins, and red hair." ''Q. J. Med." '''92''': 125-131.
* Robins, A.H. 1991. ''.'' ].


== Disorders of pigmentation ==
Millington GWM. (2006) Proopiomelanocortin (POMC): the cutaneous roles of its melanocortin products and receptors. Clin Exp Dermatol 31: 407-412.
Uneven pigmentation of some sort affects most people, regardless of bioethnic background or skin color. Skin may either appear lighter, or darker than normal, or lack pigmentation at all; there may be blotchy, uneven areas, patches of brown to gray discoloration or freckling. Apart from blood-related conditions such as ], ], or ], skin pigmentation disorders generally occur because the body produces either too much or too little melanin.


=== Depigmentation ===
Millington GWM, Levell NJ. (2007) From genesis to gene-sequencing: historical progress in the understanding of skin color. Intl J Dermatol 46: 103-105.
{{Main|Depigmentation}}


==Further reading== ==== Albinism ====
Some types of ] affect only the skin and hair, while other types affect the skin, hair and eyes, and in rare cases only the eyes. All of them are caused by different genetic mutations. Albinism is a recessively inherited trait in humans where both pigmented parents may be carriers of the gene and pass it down to their children. Each child has a 25% chance of being albino and a 75% chance of having normally pigmented skin.<ref>Cummings (2011). ''Human Heridity Principles and Issues'. Cengage 9th edition. Retrieved 2014-02-16.</ref> One common type of albinism is ] or OCA, which has many subtypes caused by different genetic mutations.
* Nicholas Wade (] 2003), "" ''New York Times (Science Times)''. Summary of clues to the saga in which humans evolved to lose their hair and had to adjust, including turning from light skin to dark skin, together with an estimation of the time at which humans invented clothing.
Albinism is a serious problem in areas of high sunlight intensity, leading to extreme sun sensitivity, ], and eye damage.<ref name="LivCol" />
* SLC24A5 gene controls up to 38% of the tonal range in people with mixed European and West African ancestry

Albinism is more common in some parts of the world than in others, but it is estimated that 1 in 70 humans carry the gene for OCA.
The most severe type of albinism is OCA1A, which is characterized by complete, lifelong loss of melanin production, other forms of OCA1B, OCA2, OCA3, OCA4, show some form of melanin accumulation and are less severe.<ref name=LivCol /> The four known types of ''OCA'' are caused by mutations in the ''TYR'', ''OCA2'', '']'', and ''SLC45A2'' genes.<ref name="ncbi.nlm.nih.gov" />

Albinos often face social and cultural challenges (even threats), as the condition is often a source of ridicule, racism, fear, and violence. Many cultures around the world have developed beliefs regarding people with albinism. ] in ] by ]s, who use the body parts of albinos as ingredients in rituals and potions, as they are thought to possess magical power.<ref>BBC (27 July 2008). "Tanzania Albinos Targeted Again". BBC News (online edition). Retrieved 2010-01-03.</ref>

==== Vitiligo ====
], ], has vitiligo]]
] is a condition that causes depigmentation of sections of skin. It occurs when melanocytes die or are unable to function. The cause of vitiligo is unknown, but research suggests that it may arise from ], genetic, ], neural, or viral causes.<ref name="halder">{{cite journal |last1=Halder |first1=RM |last2=Chappell |first2=JL |title=Vitiligo update |journal=Seminars in Cutaneous Medicine and Surgery |volume=28 |issue=2 |pages=86–92 |year=2009 |pmid=19608058 |doi=10.1016/j.sder.2009.04.008}}</ref> The incidence worldwide is less than 1%.<ref name=pmid7977362>{{cite journal |title=Genetic epidemiology of vitiligo: multilocus recessivity cross-validated |journal=American Journal of Human Genetics |volume=55 |issue=5 |pages=981–90 |year=1994 |pmid=7977362 |pmc=1918341 |last1=Nath |first1=S. K. |last2=Majumder |first2=P. P. |last3=Nordlund |first3=J. J.}}</ref> Individuals affected by vitiligo sometimes suffer psychological discomfort because of their appearance.<ref name=LivCol />

=== Hyperpigmentation ===
Increased melanin production, also known as ], can be a few different phenomena:
* ] describes the darkening of the skin.
* ] describes skin discolorations caused by hormones. These hormonal changes are usually the result of pregnancy, birth control pills or estrogen replacement therapy.
* ], also known as "liver spots" or "senile freckles", refers to darkened spots on the skin caused by aging and the sun. These spots are quite common in adults with a long history of unprotected sun exposure.

Aside from sun exposure and hormones, hyperpigmentation can be caused by skin damage, such as remnants of blemishes, wounds or rashes.<ref>Cutis, August 2005, pp 19–23</ref> This is especially true for those with darker skin tones.

The most typical cause of darkened areas of skin, brown spots or areas of discoloration is unprotected sun exposure. Once incorrectly referred to as ]s, these pigment problems are not connected with the liver.

On lighter to medium skin tones, solar lentigenes emerge as small- to medium-sized brown patches of freckling that can grow and accumulate over time on areas of the body that receive the most unprotected sun exposure, such as the back of the hands, forearms, chest, and face. For those with darker skin colors, these discolorations can appear as patches or areas of ashen-gray skin.

== Exposure to the sun ==
{{See also|Sun tanning}}
].]]
] in the skin protects the body by absorbing solar radiation. In general, the more melanin there is in the skin the more solar radiation can be absorbed. Excessive solar radiation causes ] and ] to the skin and the body naturally combats and seeks to repair the damage and protect the skin by creating and releasing further melanin into the skin's cells. With the production of the melanin, the skin color darkens, but can also cause sunburn. The tanning process can also be created by artificial UV radiation.

There are two different mechanisms involved. Firstly, the UVA-radiation creates oxidative stress, which in turn oxidizes existing melanin and leads to rapid darkening of the melanin, also known as IPD (immediate pigment darkening). Secondly, there is an increase in production of melanin known as ].<ref name="Agar2005">{{cite journal |title=Melanogenesis: a photoprotective response to DNA damage? |journal=Mutation Research/Fundamental and Molecular Mechanisms of Mutagenesis |volume=571 |issue=1–2 |pages=121–132 |year=2005 |pmid=15748643 |doi=10.1016/j.mrfmmm.2004.11.016|last1=Agar |first1=N |last2=Young |first2=A. R.}}</ref> Melanogenesis leads to delayed tanning and first becomes visible about 72 hours after exposure. The tan that is created by an increased melanogenesis lasts much longer than the one that is caused by oxidation of existing melanin. Tanning involves not just the increased melanin production in response to UV radiation but the thickening of the top layer of the epidermis, the stratum corneum.<ref name="LivCol" />

A person's natural skin color affects their reaction to exposure to the sun. Generally, those who start out with darker skin color and more melanin have better abilities to tan. Individuals with very light skin and albinos have no ability to tan.<ref>{{cite book|last=Fitzpatrick|first=T. B.|title=Fitzpatrick's Dermatology in General Medicine|year=2003|publisher=McGraw-Hill|location=New York|pages=819–825|chapter=Normal skin color and general considerations of pigmentary disorders}}</ref> The biggest differences resulting from sun exposure are visible in individuals who start out with moderately pigmented brown skin: the change is dramatically visible as tan lines, where parts of the skin which tanned are delineated from unexposed skin.<ref name="LivCol" />

Modern lifestyles and mobility have created mismatch between skin color and environment for many individuals. Vitamin D deficiencies and UVR overexposure are concerns for many. It is important for these people individually to adjust their diet and lifestyle according to their skin color, the environment they live in, and the time of year.<ref name="LivCol" /> For practical purposes, such as exposure time for sun tanning, six skin types are distinguished following Fitzpatrick (1975), listed in order of decreasing lightness:

=== Fitzpatrick scale ===
{{Main|Fitzpatrick scale}}
The following list shows the six categories of the Fitzpatrick scale in relation to the 36 categories of the older ]:<ref>{{cite book|first=Nina|last=Jablonski|editor-first1=Michael P. |editor-last1=Muehlenbein|title=Human Evolutionary Biology|date=2010|publisher=Cambridge University Press|isbn=978-0-521-87948-4|page=177|url=https://books.google.com/books?id=1VXX1jkhPH8C |access-date=24 May 2016}}</ref><ref>{{cite web|title=Fitzpatrick Skin Type |url=http://www.arpansa.gov.au/pubs/RadiationProtection/FitzpatrickSkinType.pdf |publisher=Australian Radiation Protection and Nuclear Safety Agency |access-date=24 May 2016 |url-status=dead |archive-url= https://web.archive.org/web/20160331085655/http://www.arpansa.gov.au///pubs/RadiationProtection/FitzpatrickSkinType.pdf |archive-date=31 March 2016}}</ref>
{| class="wikitable"
|-
!Type !! Also called !! Sunburning !! Tanning behavior !! Von Luschan's chromatic scale
|-
| '''I''' || Light, pale white|| Always || Never || 0–6
|-
| '''II''' || White, fair || Usually || Minimally || 7–13
|-
| '''III''' || Medium white to light brown || Sometimes || Uniformly || 14–20
|-
| '''IV''' || Olive, moderate brown || Rarely || Easily || 21–27
|-
| '''V''' || Brown, dark brown || Very rarely || Very easily || 28–34
|-
| '''VI''' || Very dark brown to black || Never || Rarely || 35–36
|}

Dark skin with large concentrations of melanin protects against ultraviolet light and skin cancers; light-skinned people have about a tenfold greater risk of dying from skin cancer, compared with dark-skinned persons, under equal sunlight exposure. Furthermore, ] rays from sunlight are believed to interact with folic acid in ways that may damage health.<ref>{{cite journal |last1=Vorobey |first1=P |last2=Steindal |first2=AE |last3=Off |first3=MK |last4=Vorobey |first4=A |last5=Moan |first5=J |title=Influence of human serum albumin on photodegradation of folic acid in solution |journal=Photochemistry and Photobiology |volume=82 |issue=3 |pages=817–22 |year=2006 |pmid=16454580 |doi=10.1562/2005-11-23-RA-739|s2cid=36351333 }}</ref> In a number of traditional societies the sun was avoided as much as possible, especially around noon when the ultraviolet radiation in sunlight is at its most intense. Midday was a time when people stayed in the shade and had the main meal followed by a nap, a practice similar to the modern ].

== Geographic variation ==
Approximately 10% of the variance in skin color occurs within regions, and approximately 90% occurs between regions.<ref>{{cite journal|title=Apportionment of global human genetic diversity based on craniometrics and skin color |year=2002|last=Relethford|first=J. H.|journal=American Journal of Physical Anthropology|issue=4 |volume=118|pages=393–8|pmid=12124919|doi=10.1002/ajpa.10079|citeseerx=10.1.1.473.5972|s2cid=8717358 }}</ref> Because skin color has been under strong selective pressure, similar skin colors can result from convergent adaptation rather than from genetic relatedness; populations with similar pigmentation may be genetically no more similar than other widely separated groups. Furthermore, in some parts of the world where people from different regions have mixed extensively, the connection between skin color and ancestry has substantially weakened.<ref>{{cite journal|title=Implications of correlations between skin color and genetic ancestry for biomedical research|journal=Nature Genetics|volume=36 |issue=11|year=2004 |pages=S54–S60 |doi=10.1038/ng1440|pmid=15508005|last1=Parra|first1=E. J.|last2=Kittles|first2=R. A. |last3=Shriver|first3=M. D. |s2cid=13712615|doi-access=free}}</ref> In Brazil, for example, skin color is not closely associated with the percentage of recent African ancestors a person has, as estimated from an analysis of genetic variants differing in frequency among continent groups.<ref>{{cite journal|title=Color and genomic ancestry in Brazilians |year=2003|journal=Proceedings of the National Academy of Sciences |issue=1|volume=100|pages=177–182 |doi=10.1073/pnas.0126614100|pmc=140919|pmid=12509516 |last1=Parra|first1=F. C.|last2=Amado|first2=R. C. |last3=Lambertucci|first3=J. R.|last4=Rocha|first4=J. |last5=Antunes|first5=C. M.|last6=Pena|first6=S. D. J. |bibcode=2003PNAS..100..177P |doi-access=free}}</ref>

In general, people living close to the equator are highly darkly pigmented, and those living near the poles are generally very lightly pigmented. The rest of humanity shows a high degree of skin color variation between these two extremes, generally correlating with UV exposure. The main exception to this rule is in the New World, where people have only lived for about 10,000 to 15,000 years and show a less pronounced degree of skin pigmentation.<ref name="LivCol" />

In recent times, humans have become increasingly mobile as a consequence of improved technology, domestication, environmental change, strong curiosity, and risk-taking. Migrations over the last 4000 years, and especially the last 400 years, have been the fastest in human history and have led to many people settling in places far away from their ancestral homelands. This means that skin colors today are not as confined to geographical location as they were previously.<ref name="LivCol" />

== Social status, colorism and racism ==
{{Main|Discrimination based on skin color}}
]]]
According to classical scholar Frank Snowden, skin color did not determine ] in ancient ], ] or ]. These ancient civilizations viewed relations between the major power and the subordinate state as more significant in a person's status than their skin colors.<ref name="Snowden1970">{{cite book|last=Snowden|first=Frank M. |author-link=Frank M. Snowden Jr.|title=Blacks in Antiquity: Ethiopians in the Greco-Roman Experience|url=https://books.google.com/books?id=37MTRCr9oAUC|year=1970|publisher=Harvard University Press|isbn=978-0-674-07626-6}}</ref>{{Page needed|date=February 2021}}

Nevertheless, some social groups favor specific skin coloring. The preferred skin tone varies by culture and has varied over time. A number of indigenous African groups, such as the ], associated pale skin with being cursed or caused by evil spirits associated with witchcraft. They would abandon their children born with conditions such as albinism and showed a sexual preference for darker skin.<ref> {{webarchive|url= https://web.archive.org/web/20110622125145/http://allafrica.com/stories/200909101054.html |date=2011-06-22 }}, Pambazuka News, 10 September 2009</ref>

Many cultures have historically favored lighter skin for women. Before the ], inhabitants of the continent of Europe preferred pale skin, which they interpreted as a sign of high social status. The poorer classes worked outdoors and got darker skin from exposure to the sun, while the upper class stayed indoors and had light skin. Hence light skin became associated with wealth and high position.<ref name=DrKarl>{{cite web|first= Karl|last= Kruszelnicki|title= News in Science: Skin Colour 1|date= March 1, 2001|publisher= abc.net.au|access-date= May 25, 2014|url= http://www.abc.net.au/science/articles/2001/03/01/249992.htm?site=science/greatmomentsinscience |archive-url= https://web.archive.org/web/20131228122132/http://www.abc.net.au/science/articles/2001/03/01/249992.htm?site=science%2Fgreatmomentsinscience|archive-date= December 28, 2013 |url-status=live}}</ref> Women would put lead-based cosmetics on their skin to whiten their skin tone artificially.<ref name=autogenerated1>{{cite journal |last1=Agredano |title=Accessibility to air travel correlates strongly with increasing melanoma incidence |journal= Melanoma Research |date=February 2006 |volume=16 |issue=1 |pmid=16432460 |pages=77–81 |doi= 10.1097/01.cmr.0000195696.50390.23 |s2cid=6462810}}</ref> However, when not strictly monitored, these cosmetics caused ]. Other methods also aimed at achieving a light-skinned appearance, including the use of ] to whiten skin, and powders. Women would wear full-length clothes when outdoors, and would use gloves and parasols to provide shade from the sun.

Colonization and enslavement as carried out by European countries became involved with ] and ], associated with the belief that people with dark skin were uncivilized, inferior, and should be subordinate to lighter-skinned invaders. This belief exists to an extent in modern times as well.<ref>{{cite web|first= Ron |last= Hall |title= The Psychogenesis of Color Based Racism: Implications of Projection for Dark-Skinned Puertorriqueños|access-date= 2012-09-25 |url= http://www.jsri.msu.edu/RandS/research/irr/rr21.pdf|archive-date= January 6, 2011|archive-url= https://web.archive.org/web/20110106160600/http://www.jsri.msu.edu/RandS/research/irr/rr21.pdf |work=Julian Samora Research Institute, Michigan State University |year=1997}}</ref> Institutionalized ] in North America led people to perceive lighter-skinned African-Americans as more intelligent, cooperative, and beautiful.<ref>{{cite web |url= http://northbysouth.kenyon.edu/2000/Beauty/goodlooks.htm |title=What Are "Good Looks"? |publisher=Kenyon College |access-date=2014-03-23 |url-status=live |archive-url= https://web.archive.org/web/20141013060552/http://northbysouth.kenyon.edu/2000/Beauty/goodlooks.htm |archive-date=2014-10-13}}</ref> Such lighter-skinned individuals had a greater likelihood of working as ]s and of receiving preferential treatment from plantation owners and from overseers. For example, they had a chance to get an education.<ref>{{cite news|url= http://www.sptimes.com/2003/08/31/Columns/The_paper_bag_test.shtml |title=The Paper Bag Test |newspaper=St. Petersburg Times |date=2003-08-31 |access-date=2014-03-23 |url-status=live |archive-url= https://web.archive.org/web/20130806113724/http://www.sptimes.com/2003/08/31/Columns/The_paper_bag_test.shtml |archive-date=2013-08-06}}</ref> The preference for fair skin remained prominent until the end of the ], but ] about worth and beauty persisted in the last half of the 20th century and continue in the present day. African-American journalist ] wrote that, "To be both prettiest and black was impossible,"<ref name=tws6nov14 /> and elaborated:

<blockquote>We learn as girls that in ways both subtle and obvious, personal and political, our value as females is largely determined by how we look. ... For black women, the domination of physical aspects of beauty in women's definition and value render us invisible, partially erased, or obsessed, sometimes for a lifetime, since most of us lack the major talismans of Western beauty. Black women find themselves involved in a lifelong effort to self-define in a culture that provides them no positive reflection.<ref name=tws6nov14>{{cite news|first1=Jill |last1=Nelson |title=Straight, No Chaser—How I Became a Grown-Up Black Woman— WHO'S THE FAIREST OF THEM ALL? |quote=As a girl and young woman, hair, body, and color were society's trinity in determining female beauty and identity... We learn as girls that in ways both subtle and obvious, personal and political, our value as females is largely determined by how we look. |work=] |year=1997 |url= http://comminfo.rutgers.edu/professional-development/childlit/books/Straight.PDF |access-date=2009-11-06 |url-status=live |archive-url= https://web.archive.org/web/20110510212534/http://comminfo.rutgers.edu/professional-development/childlit/books/Straight.PDF |archive-date=2011-05-10}}</ref></blockquote>

A preference for fair or lighter skin continues in some countries, including Latin American countries where whites form a minority.<ref>{{cite news|url= http://www.post-gazette.com/lifestyle/2006/12/26/Documentary-studies-renew-debate-about-skin-color-s-impact/stories/200612260092|title= Documentary, Studies Renew Debate About Skin Color's Impact|work= Pittsburgh Post-Gazette|date= 2006-12-26|access-date= 2014-03-23|url-status= live|archive-url= https://web.archive.org/web/20140130034050/http://www.post-gazette.com/lifestyle/2006/12/26/Documentary-studies-renew-debate-about-skin-color-s-impact/stories/200612260092|archive-date= 2014-01-30}}</ref> In Brazil, a dark-skinned person is more likely to experience discrimination.<ref>{{cite news|url= http://www.miamiherald.com/multimedia/news/afrolatin/part5/index.html|title= Racism Takes Many Hues|work= Miami Herald|date= 2007-08-24|access-date= 2014-03-23|url-status= live|archive-url= https://web.archive.org/web/20130223175545/http://www.miamiherald.com/multimedia/news/afrolatin/part5/index.html|archive-date= 2013-02-23}}</ref> Many actors and actresses in Latin America have European features—], ], and pale skin.<ref>{{cite news |url= http://www.newsweek.com/id/58525?tid=relatedcl |title= Y Tu Black Mama Tambien |access-date= 2008-05-02 |last= Quinonez |first= Ernesto |date= 2003-06-19 |url-status= live |archive-url= https://web.archive.org/web/20081027210105/http://www.newsweek.com/id/58525?tid=relatedcl |archive-date= 2008-10-27 }}; {{cite news |url= https://www.washingtonpost.com/ac2/wp-dyn?pagename=article&node=&contentId=A19009-2000Aug1&notFound=true |title= The Blond, Blue-Eyed Face of Spanish TV |newspaper= Washington Post |date= 2000-08-03 |access-date= 2012-08-18 |first= Michael A. |last= Fletcher }}; {{cite web |url= http://latinola.com/story.php?story=9009 |title= Blonde, Blue-Eyed Euro-Cute Latinos on Spanish TV |publisher= Latinola.com |date= 2010-10-24 |access-date= 2012-08-18 |url-status= live |archive-url= https://web.archive.org/web/20170902161618/http://latinola.com/story.php?story=9009 |archive-date= 2017-09-02 }}; {{cite web |url= http://www.vidadeoro.com/2010/10/latinos-not-reflected-on-spanish-tv.html |title= Latinos Not Reflected on Spanish TV |publisher= Vidadeoro.com |date= 2010-10-25 |access-date= 2012-08-18 |url-status= live |archive-url= https://web.archive.org/web/20170909234626/http://www.vidadeoro.com/2010/10/latinos-not-reflected-on-spanish-tv.html |archive-date= 2017-09-09 }}; {{cite web |url= http://www.bellaonline.com/articles/art40221.asp |title= What are Telenovelas?—Hispanic Culture |publisher= Bellaonline.com |access-date= 2012-08-18 |url-status= live |archive-url= https://web.archive.org/web/20170622150848/http://www.bellaonline.com/articles/art40221.asp |archive-date= 2017-06-22 }}; {{cite web |first= Michael A. |last= Fletcher |url= http://articles.sun-sentinel.com/2000-08-06/news/0008060066_1_spanish-latino-leaders-caste |title= Racial Bias Charged On Spanish-Language TV |work= Sun-Sentinel |date= 2000-08-06 |access-date= 2012-08-18 |url-status= dead |archive-url= http://archive.wikiwix.com/cache/20110913041543/http://articles.sun-sentinel.com/2000-08-06/news/0008060066_1_spanish-latino-leaders-caste |archive-date= 2011-09-13 }}; {{cite web |url= http://www.blackelectorate.com/articles.asp?ID=281 |title= Black Electorate |publisher= Black Electorate |date= 2 January 2001 |access-date= 2012-08-18 |url-status= live |archive-url= https://web.archive.org/web/20170620174141/http://www.blackelectorate.com/articles.asp?ID=281 |archive-date= 20 June 2017 }}</ref><ref name="skintone">{{cite news|url= http://www.boston.com/news/globe/living/articles/2004/08/19/pride_or_prejudice/?page=2|title= Pride or Prejudice?|work= Boston Globe|date= 2004-08-19|first1= Vanessa E.|last1= Jones|access-date= 2014-03-23|url-status= live|archive-url= https://web.archive.org/web/20110512042101/http://www.boston.com/news/globe/living/articles/2004/08/19/pride_or_prejudice/?page=2|archive-date= 2011-05-12}}</ref> A light-skinned person is more privileged and has a higher social status;<ref name="skintone" /> a person with light skin is considered more beautiful<ref name="skintone" /> and lighter skin suggests that the person has more wealth.<ref name="skintone" /> Many languages popularly use specific words to describe distinct skin tones - from (for example) "''jincha''", Puerto Rican slang for "glass of milk" to "''morena''", literally "brown".<ref name="skintone" />

Significant exceptions to a preference for lighter skin started to appear in Western culture in the mid-20th century.<ref name="Singer">{{cite book|last1= Singer |first1= Merrill |first2= Hans |last2= Beyer |author-link= Merrill Singer|title= Killer Commodities: Public Health and the Corporate Production of Harm|url= https://books.google.com/books?id=64xpa-rhHLgC&q=sun+tan+lower+class&pg=PA151|access-date= 2009-09-11 |date= 28 July 2008|publisher= ]|isbn= 978-0-7591-0979-7|page= 151|quote= Harris investigated the history of the parasol... everywhere ordinary people were forbidden to protect themselves with such devices "pallid skin became a marker of upper-class status". At the beginning of the 20th Century, in the United States, lighter-skinned people avoided the sun... Tanned skin was considered lower class.}}</ref> However, a 2010 study found a preference for lighter-skinned women in New Zealand and California.<ref>{{Cite journal|date=June 2010|title=Human Physique and Sexual Attractiveness in Men and Women: A New Zealand–U.S. Comparative Study|journal=Archives of Sexual Behavior|volume=39|issue=3|pages=798–806|quote=men expressed preferences for lighter skinned female figures in New Zealand and California|doi=10.1007/s10508-008-9441-y|pmid=19139985|last1=Dixson|first1=Barnaby J.|last2=Dixson|first2=Alan F.|last3=Bishop|first3=Phil J.|last4=Parish|first4=Amy|s2cid=33112678}}</ref> Though sun-tanned skin was once associated with the sun-exposed manual labor of the lower class, the associations became dramatically reversed during this time—a change usually credited to the trendsetting Frenchwoman ] (1883–1971) presenting tanned skin as fashionable, healthy, and luxurious.<ref name="Koskoff2007">{{cite book|last= Koskoff|first= Sharon |title= Art Deco of the Palm Beaches|url= https://books.google.com/books?id=RHdeo6uvlfYC&q=%22Coco%20Chanel%22%20%22Josephine%20Baker%22%20tan&pg=PA2|access-date= 2009-09-11 |date= 28 May 2007|publisher= ]|isbn= 978-0-7385-4415-1|page= 2|quote= In 1920s France, the caramel-skinned entertainer Josephine Baker became a Parisian idol. Concurrently, fashion designer Coco Chanel was "bronzed" while cruising on a yacht. A winter tan became a symbol of the leisure class and showed you could afford to travel to exotic climates.}}</ref>

{{As of | 2017}}, though an overall preference for lighter skin remains prevalent in the United States, many within the country regard tanned skin as both more attractive and healthier than pale or very dark skin.<ref name="pediatrics.aappublications.org">{{cite journal|last1= Geller|first1= A. C.|last2= Colditz|first2= G.|last3= Oliveria|first3= S.|last4= Emmons|first4= K.|last5= Jorgensen|first5= C.|last6= Aweh|first6= G. N.|last7= Frazier|first7= A. L.|title= Use of Sunscreen, Sunburning Rates, and Tanning Bed Use Among More Than 10 000 US Children and Adolescents|journal= Pediatrics|date= 1 June 2002|volume= 109|issue= 6|pages= 1009–1014|doi= 10.1542/peds.109.6.1009|pmid= 12042536}}</ref><ref name="interscience.wiley">{{cite journal|url= http://www3.interscience.wiley.com/journal/119318887/abstract?CRETRY=1&SRETRY=0 |archive-url= https://archive.today/20130105060850/http://www3.interscience.wiley.com/journal/119318887/abstract?CRETRY=1&SRETRY=0 |url-status= dead |archive-date= 2013-01-05 |title= Effects of Suntan on Judgements of Healthiness and Attractiveness by Adolescents |journal= Journal of Applied Social Psychology |volume= 22 |issue= 2 |pages= 157–172 |date= 2006-07-31 |doi= 10.1111/j.1559-1816.1992.tb01527.x|last1= Broadstock |first1= Marita |last2= Borland |first2= Ron |last3= Gason |first3= Robyn }}</ref><ref name="interscience.wiley.com">{{cite journal|doi= 10.1111/j.1559-1816.1993.tb01039.x|url= http://www3.interscience.wiley.com/journal/119292742/abstract |archive-url= https://archive.today/20130105084812/http://www3.interscience.wiley.com/journal/119292742/abstract |url-status= dead |archive-date= 2013-01-05 |title= The Social Psychology of Tanning and Sunscreen Use: Self-Presentational Motives as a Predictor of Health Risk |journal= Journal of Applied Social Psychology |volume= 23 |issue= 17 |pages= 1390–1406 |date= 2006-07-31|last1= Leary |first1= Mark R. |last2= Jones |first2= Jody L.}}</ref> Western mass media and popular culture continued{{when|date=November 2017}} to reinforce negative stereotypes about dark skin,<ref>{{cite web |first= Steven |last= Balkaran |title= Mass Media and Racism |year= 1999 |url= http://www.yale.edu/ypq/articles/oct99/oct99b.html |url-status= dead |archive-url= https://web.archive.org/web/20111124045408/http://www.yale.edu/ypq/articles/oct99/oct99b.html |archive-date= 2011-11-24 }}</ref> but in some circles pale skin has become associated with indoor office-work while tanned skin has become associated with increased leisure time, sportiness and good health that comes with wealth and higher social status.<ref name=DrKarl /> Studies have also emerged indicating that the degree of tanning is directly related to how attractive a young woman is.<ref name="Pp">{{cite journal |last1= Leary |first1= Mark R. |last2= Jones |first2= Jody L. |title= The Social Psychology of Tanning and Sunscreen Use: Self-Presentational Motives as a Predictor of Health Risk |journal= Journal of Applied Social Psychology |volume= 23 |pages= 1390–406 |year= 1993 |doi= 10.1111/j.1559-1816.1993.tb01039.x |issue= 17}}
</ref><ref>
{{cite journal
|title = Fact or wishful thinking? Biased expectations in "I think I look better when I'm tanned"
|journal = American Journal of Health Behavior
|volume = 32
|issue = 3
|year = 2008
|pages = 243–52
|url = https://www.ethicshare.org/node/425236
|pmid = 18067464
|last1 = Banerjee
|first1 = S. C.
|last2 = Campo
|first2 = S
|last3 = Greene
|first3 = K
|url-status = live
|archive-url = https://web.archive.org/web/20140324020512/https://www.ethicshare.org/node/425236
|archive-date = 2014-03-24
|doi = 10.5993/AJHB.32.3.2
}}
</ref>

=== Skin whitening ===
{{Main|Skin whitening}}

] soaps]]
In ], society regards fair skin as more attractive and associates dark skin with lower class status. This results in a massive market for ] creams.<ref>{{cite news |last= Sidner |first= Sara |title= Skin whitener advertisements labeled racist |publisher= CNN |date= 9 September 2009 |url= http://www.cnn.com/2009/WORLD/asiapcf/09/09/india.skin/index.html |quote= 'We always have a complex towards a white skin, towards foreign skin or foreign hair,' Jawed Habib says. Habib should know. He owns a chain of 140 salons located in India and across the world. 'We Indian people, we Asian people are more darker, so we want to look more fair.' … A marketing study found sales for skin whitening creams have jumped more than 100 percent in rural India and sales for male grooming products are increasing 20 percent annually. |access-date= 2009-09-11 |url-status= live |archive-url= https://web.archive.org/web/20090912195424/http://www.cnn.com/2009/WORLD/asiapcf/09/09/india.skin/index.html |archive-date= 12 September 2009 }}</ref> Fairer skin-tones also correlate to higher ] in the Hindu social order—although the system is not based on skin tone.<ref>{{cite news |title= Caste: Racism in all but name? |url= http://articles.timesofindia.indiatimes.com/2009-04-26/special-report/28005708_1_caste-based-discrimination-caste-discrimination-dalit-human-rights |archive-url= https://web.archive.org/web/20130522205451/http://articles.timesofindia.indiatimes.com/2009-04-26/special-report/28005708_1_caste-based-discrimination-caste-discrimination-dalit-human-rights |url-status= dead |archive-date= 22 May 2013 |location= New Delhi |first= Shobhan |last= Saxena |date= 26 April 2009 |work= ] |access-date= 2012-09-20}}</ref> Actors and actresses in Indian cinema tend to have light skin tones, and Indian cinematographers have used graphics and intense lighting to achieve more "desirable" skin tones.<ref>{{cite news |title= Has skin whitening in India gone too far? |url= https://www.bbc.co.uk/news/magazine-18268914 |work= BBC News |location= London |first= Rajini |last= Vaidyanathan |date= 5 June 2012 |access-date= 2012-09-20 |url-status= live |archive-url= https://web.archive.org/web/20120906041844/http://www.bbc.co.uk/news/magazine-18268914 |archive-date= 6 September 2012 }}</ref> Fair skin tones are advertised as an asset in Indian marketing.<ref>{{cite news |title= In India's Huge Marketplace, Advertisers Find Fair Skin Sells |url= https://www.washingtonpost.com/wp-dyn/content/article/2008/01/26/AR2008012601057.html |newspaper= Washington Post |location= Washington DC |first= Rama |last= Lakshmi |date= 27 January 2008 |access-date= 2012-09-20 |url-status= live |archive-url= https://web.archive.org/web/20131010072059/http://www.washingtonpost.com/wp-dyn/content/article/2008/01/26/AR2008012601057.html |archive-date= 10 October 2013 }}</ref>

In 2013, 77% of ]n women, 52% of ]ese women, and 25% of ]an women were using lightening products.<ref name=":20">{{Cite news |last1=Fihlani |first1=Pumza |date=January 2013 |title=Africa: Where black is not really beautiful |work=BBC News |url=https://www.bbc.com/news/world-africa-20444798}}</ref><ref name=":21">{{Cite journal |last1=Jacobs |first1=Meagan |last2=Levine |first2=Susan |last3=Abney |first3=Kate |last4=Davids |first4=Lester |year=2016 |title=Fifty shades of African lightness: A bio-psychosocial review of the global phenomenon of skin lightening practices |journal=Journal of Public Health in Africa |volume=7 |issue=2 |page=552 |doi=10.4081/jphia.2016.552 |pmc=5345401 |pmid=28299156}}</ref> In 2020, '']'' reported that in Ghana, "When You Are Light-Skinned, You Earn More", and that "ome pregnant women take tablets in the hopes that it will lead their child to be born with fair skin. Some apply bleaching lotion to their babies, in the hopes that it will improve their child's chances."<ref name="BackhausOkunmwendia">{{cite web |surname=Backhaus |given=Anne |date=2020-06-16 |title=Skin Bleaching in Ghana: "When You Are Light-Skinned, You Earn More" |url=https://www.spiegel.de/international/world/skin-bleaching-in-ghana-when-you-are-light-skinned-you-earn-more-a-3a46c628-23b2-4d05-9d32-6cb6deeb4a5a-amp |url-status=live |archive-url=https://web.archive.org/web/20201001234949/https://www.spiegel.de/international/world/skin-bleaching-in-ghana-when-you-are-light-skinned-you-earn-more-a-3a46c628-23b2-4d05-9d32-6cb6deeb4a5a-amp |archive-date=2020-10-01 |website=] |publisher=] |surname2=Okunmwendia |given2=Ella |department=Global Societies}}</ref>

Skin-whitening products have remained popular over time, often due to historical beliefs and perceptions about fair skin. Sales of skin-whitening products across the world grew from $40 billion to $43 billion in 2008.<ref name="Northwestern University">{{cite news|url= http://news.medill.northwestern.edu/chicago/news.aspx?id=161243 |title= Bleaching Creams: Fade to Beautiful? |publisher= Northwestern University |date= 2010-03-10 |url-status= dead |access-date= 2014-03-23 |archive-url= https://web.archive.org/web/20110720014201/http://news.medill.northwestern.edu/chicago/news.aspx?id=161243 |archive-date= July 20, 2011 }}</ref> In ] and ]n countries, people have traditionally seen light skin as more attractive, and a preference for lighter skin remains prevalent. In ancient China and Japan, for example, pale skin can be traced back to ancient drawings depicting women and goddesses with fair skin tones.{{citation needed|date=November 2017}} In ancient China, Japan, and Southeast Asia, pale skin was seen as a sign of wealth. Thus skin-whitening cosmetic products are popular in East Asia.<ref>{{cite news|url= http://edition.cnn.com/2002/WORLD/asiapcf/east/05/13/asia.whitening/|title= Skin Deep: Dying to be White|publisher= CNN|date= 2002-05-15|access-date= 2014-03-23|url-status= live|archive-url= https://web.archive.org/web/20100408132814/http://edition.cnn.com/2002/WORLD/asiapcf/east/05/13/asia.whitening/|archive-date= 2010-04-08}}</ref>

In 2010, four out of ten women surveyed in ], ], the ] and ] used a skin-whitening cream, and more than 60 companies globally compete for Asia's estimated $18 billion market.<ref> {{webarchive|url= https://web.archive.org/web/20100726034953/http://www.pri.org/world/asia/skin-whitening-big-business-asia.html |date=2010-07-26 }}. Pri.Org. Retrieved 2011-02-27.</ref> Changes in regulations in the cosmetic industry led to skin-care companies introducing harm-free skin lighteners. In ], the ] have a reputation for their white-painted faces, and the appeal of the {{Nihongo|''bihaku''|美白}}, or "beautiful white", ideal leads many Japanese women to avoid any form of tanning.<ref>{{cite news |title= Japanese girls choose whiter shade of pale |url= https://www.theguardian.com/japan/story/0,7369,1185335,00.html |work= Guardian Unlimited |location= London |first= Nicole |last= Mowbray |date= 4 April 2004 |access-date= 2010-05-24}}</ref>

There are exceptions to this, with Japanese fashion trends such as ] emphasizing tanned skin. Skin whitening is also not uncommon in ],<ref>{{cite news|url= http://news.bbc.co.uk/2/hi/africa/718359.stm|title= The Heavy Cost of Light Skin|work= BBC News|date= 2000-04-18|access-date= 2014-03-23|url-status= live|archive-url= https://web.archive.org/web/20140323231534/http://news.bbc.co.uk/2/hi/africa/718359.stm|archive-date= 2014-03-23}}</ref><ref> {{webarchive|url= https://web.archive.org/web/20100910013826/http://www.scienceinafrica.co.za/2004/march/skinlightening.htm |date=2010-09-10 }}. Scienceinafrica.co.za. Retrieved 2011-02-27.</ref> and several research projects have suggested a general preference for lighter skin in the African-American community.<ref> {{webarchive|url= https://web.archive.org/web/20140330120654/http://www.questia.com/library/1G1-143164170/color-counts-it-is-evident-that-differing-color |date=2014-03-30 }}. Retrieved 2012-09-25.</ref> In contrast, one study on men of the Bikosso tribe in Cameroon found no preference for attractiveness of females based on lighter skin color, bringing into question the universality of earlier studies that had exclusively focused on skin-color preferences among non-African populations.<ref>{{Cite journal|doi= 10.1007/s10508-006-9093-8|pmid= 17136587|title= Human Physique and Sexual Attractiveness: Sexual Preferences of Men and Women in Bakossiland, Cameroon|journal= Archives of Sexual Behavior|volume= 36|issue= 3|pages= 369–75|year= 2006|last1= Dixson|first1= Barnaby J.|last2= Dixson|first2= Alan F.|last3= Morgan|first3= Bethan|last4= Anderson|first4= Matthew J.|s2cid= 40115821}}</ref>

== See also ==
{{div col|colwidth=18em}}
* ]
* ]
* ]
* ]
* ]
* ]
* ]
* ]
* ]
* ]
* ]
{{div col end}}

== References ==
{{Reflist}}

== Further reading ==
{{refbegin|2|indent=yes}}
* {{cite journal |ref={{harvid|Harding et al|2000}} |last1=Harding |first1=R |last2=Healy |first2=E |last3=Ray |first3=A |last4=Ellis |first4=N |last5=Flanagan |first5=N |last6=Todd |first6=C |last7=Dixon |first7=C |last8=Sajantila |first8=A |last9=Jackson |first9=I |year=2000 |title=Evidence for Variable Selective Pressures at MC1R |journal=The American Journal of Human Genetics |volume=66 |issue=4 |pages=1351–61 |doi=10.1086/302863 |pmc=1288200 |pmid=10733465 |last10=Birch-Machin |first10=MA |last11=Rees |first11=JL}}
* {{cite journal |last1=Holick |first1=Michael F. |year=2004 |title=Sunlight and vitamin D for bone health and prevention of autoimmune diseases, cancers, and cardiovascular disease |journal=The American Journal of Clinical Nutrition |volume=80 |issue=6 Suppl |pages=1678S–88S |pmid=15585788 |doi=10.1093/ajcn/80.6.1678S |doi-access=free }}
* {{cite book |last=Jablonski |first=Nina G. |url=https://books.google.com/books?id=Jw7loAEACAAJ |title=Living Color: The Biological and Social Meaning of Skin Color |date=10 January 2014 |publisher=University of California Press |isbn=978-0-520-28386-2 |jstor=10.1525/j.ctt1pn64b}}
* {{cite journal |last1=Jablonski |first1=Nina G. |year=2004 |title=The Evolution of Human Skin and Skin Color |journal=Annual Review of Anthropology |volume=33 |pages=585–623 |doi=10.1146/annurev.anthro.33.070203.143955}}
* {{cite journal |last1=Jablonski |first1=Nina G. |last2=Chaplin |first2=George |year=2002 |title=Skin Deep |journal=Scientific American |volume=287 |issue=4|pages=74–82 |doi=10.1038/scientificamerican1002-74 |pmid=12271527|bibcode=2002SciAm.287d..74J }}
* {{cite journal |ref={{harvid|Lamason et al|2005}} |last1=Lamason |first1=RL |last2=Mohideen |first2=MA |last3=Mest |first3=JR |last4=Wong |first4=AC |last5=Norton |first5=HL |last6=Aros |first6=MC |last7=Jurynec |first7=MJ |last8=Mao |first8=X |last9=Humphreville |first9=VR |year=2005 |title=SLC24A5, a Putative Cation Exchanger, Affects Pigmentation in Zebrafish and Humans |journal=Science |volume=310 |issue=5755 |pages=1782–6 |bibcode=2005Sci...310.1782L|doi=10.1126/science.1116238 |pmid=16357253 |last10=Humbert |first10=J. E. |last11=Sinha |first11=S |last12=Moore |first12=J. L. |last13=Jagadeeswaran |first13=P |last14=Zhao |first14=W |last15=Ning |first15=G |last16=Makalowska |first16=I |last17=McKeigue |first17=P. M. |last18=O'Donnell |first18=D |last19=Kittles |first19=R |last20=Parra |first20=E. J. |last21=Mangini |first21=N. J. |last22=Grunwald |first22=D. J. |last23=Shriver |first23=M. D. |last24=Canfield |first24=V. A. |last25=Cheng |first25=K. C. |s2cid=2245002 }}
* {{cite journal |last1=Millington |first1=G. W. M. |year=2006 |title=Proopiomelanocortin (POMC): the cutaneous roles of its melanocortin products and receptors|journal=Clinical and Experimental Dermatology |volume=31 |issue=3 |pages=407–412 |doi=10.1111/j.1365-2230.2006.02128.x |pmid=16681590|s2cid=25213876 }}
* {{cite journal |last1=Millington |first1=George W. M. |last2=Levell |first2=Nick J. |year=2007 |title=From genesis to gene sequencing: historical progress in the understanding of skin color |journal=International Journal of Dermatology |volume=46 |issue=1 |pages=103–5 |doi=10.1111/j.1365-4632.2006.03068.x |pmid=17214732|s2cid=6699141 }}
* {{cite journal|last1=Rees |first1=J.L. |last2=Flanagan |first2=N |year=1999 |title=Pigmentation, melanocortins and red hair |journal=QJM |volume=92 |issue=3 |pages=125–31 |doi=10.1093/qjmed/92.3.125 |pmid=10326071 |doi-access=free}}
* {{cite journal|last1=Relethford |first1=JH |year=2000 |title=Human skin color diversity is highest in sub-Saharan African populations |journal=Human Biology; an International Record of Research |volume=72 |issue=5 |pages=773–80 |pmid=11126724}}
* {{cite book|last=Robins |first=AH |title=Biological Perspectives on Human Pigmentation |publisher=] |year=1991 |isbn=0-521-36514-7}}
* {{cite news |last=Wade |first=Nicholas |date=August 19, 2003 |title=Why Humans and Their Fur Parted Ways |work=The New York Times |page=F1 |url=https://www.nytimes.com/2003/08/19/science/why-humans-and-their-fur-parted-ways.html}} Summary of clues to the saga in which humans evolved to lose their hair and had to adjust, including turning from light skin to dark skin, together with an estimation of the time when humans invented clothing.
* {{cite book|last1=Walters|first1=KA|last2=Roberts|first2=MS |title=Dermatologic, Cosmeceutic, and Cosmetic Development: Therapeutic and Novel Approaches |publisher=Informa Healthcare|year=2008|isbn=978-0-8493-7589-7 |location=New York}}
* {{cite book|ref={{harvid|Weller et al|2008}} |last1=Weller|first1=R|last2=Hunter|first2=J |last3=Savin |first3=J |last4=Dahl|first4=M|title=Clinical Dermatology|publisher=Blackwell Publishing|year=2008 |isbn=978-1-4051-4663-0 |edition=4th |location=Malden, MA |pages=268}}
{{refend}}


== External links == == External links ==
{{Commons category|Human skin color}}
* Examples of a face tracking in videos using a non parametric skin color model.
* , ]. SLC24A5 gene controls up to 38% of the tonal range in people with mixed European and West African ancestry
*
* —PBS
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{{Human Evolution}}
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{{Skin colors}}
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Latest revision as of 17:00, 28 December 2024

"Skin pigmentation" redirects here. For animal skin pigmentation, see Biological pigment.

Extended Coloured (Afrikaans: Kleurlinge or Bruinmense) family from South Africa showing some spectrum of human skin coloration

Human skin color ranges from the darkest brown to the lightest hues. Differences in skin color among individuals is caused by variation in pigmentation, which is the result of genetics (inherited from one's biological parents), exposure to the sun, disorders, or some combination thereof. Differences across populations evolved through natural selection or sexual selection, because of social norms and differences in environment, as well as regulations of the biochemical effects of ultraviolet radiation penetrating the skin.

The actual skin color of different humans is affected by many substances, although the single most important substance is the pigment melanin. Melanin is produced within the skin in cells called melanocytes and it is the main determinant of the skin color of darker-skin humans. The skin color of people with light skin is determined mainly by the bluish-white connective tissue under the dermis and by the hemoglobin circulating in the veins of the dermis. The red color underlying the skin becomes more visible, especially in the face, when, as consequence of physical exercise or sexual arousal, or the stimulation of the nervous system (anger, embarrassment), arterioles dilate. Color is not entirely uniform across an individual's skin; for example, the skin of the palm and the sole is lighter than most other skin, and this is especially noticeable in darker-skinned people.

There is a direct correlation between the geographic distribution of ultraviolet radiation (UVR) and the distribution of indigenous skin pigmentation around the world. Areas that receive higher amounts of UVR, generally located closer to the equator, tend to have darker-skinned populations. Areas that are far from the tropics and closer to the poles have lower intensity of UVR, which is reflected in lighter-skinned populations. By the time modern Homo sapiens evolved, all humans were dark-skinned. Some researchers suggest that human populations over the past 50,000 years have changed from dark-skinned to light-skinned and vice versa as they migrated to different UV zones, and that such major changes in pigmentation may have happened in as little as 100 generations (≈2,500 years) through selective sweeps. Natural skin color can also darken as a result of tanning due to exposure to sunlight. The leading theory is that skin color adapts to intense sunlight irradiation to provide partial protection against the ultraviolet fraction that produces damage and thus mutations in the DNA of the skin cells.

In some populations, women are significantly lighter-skinned than men. However, in other populations, notably those of European descent, men are significantly lighter-skinned than women. European women may have darker skin than European men due to the female sex hormone estrogen, which darkens light skin. Women from darker-skinned populations may have evolved to lighter skin than men so their bodies could absorb more vitamin D during pregnancy, which improves calcium absorption.

The social significance of differences in skin color has varied across cultures and over time, as demonstrated with regard to social status and discrimination.

Melanin and genes

See also: Melanin

Melanin is produced by cells called melanocytes in a process called melanogenesis. Melanin is made within small membrane–bound packages called melanosomes. As they become full of melanin, they move into the slender arms of melanocytes, from where they are transferred to the keratinocytes. Under normal conditions, melanosomes cover the upper part of the keratinocytes and protect them from genetic damage. One melanocyte supplies melanin to thirty-six keratinocytes according to signals from the keratinocytes. They also regulate melanin production and replication of melanocytes. People have different skin colors mainly because their melanocytes produce different amount and kinds of melanin.

The genetic mechanism behind human skin color is mainly regulated by the enzyme tyrosinase, which creates the color of the skin, eyes, and hair shades. Differences in skin color are also attributed to differences in size and distribution of melanosomes in the skin. Melanocytes produce two types of melanin. The most common form of biological melanin is eumelanin, a brown-black polymer of dihydroxyindole carboxylic acids, and their reduced forms. Most are derived from the amino acid tyrosine. Eumelanin is found in hair, areola, and skin, and the hair colors gray, black, blond, and brown. In humans, it is more abundant in people with dark skin. Pheomelanin, a pink to red hue is found in particularly large quantities in red hair, the lips, nipples, glans of the penis, and vagina.

Both the amount and type of melanin produced is controlled by a number of genes that operate under incomplete dominance. One copy of each of the various genes is inherited from each parent. Each gene can come in several alleles, resulting in the great variety of human skin tones. Melanin controls the amount of ultraviolet (UV) radiation from the sun that penetrates the skin by absorption. While UV radiation can assist in the production of vitamin D, excessive exposure to UV can damage health.

Evolution of skin color

Further information: Melanocortin 1 receptor

Time scale of skin color evolution

Loss of body hair in Homo links to the thermoregulation through perspiration heat dissipation required for activity in hot open environments and endurance running. Humans as primates have a particular need for this thermoregulation since unlike other mammals they lack a carotid rete that allows precooling of blood to the brain, an organ extremely sensitive to changes in body temperature. Given endurance running and its needs for thermoregulation arose with H. erectus, this links hairlessness with the origin of H. erectus about 2 million years ago.

As hominids gradually lost their fur between 1.2 and 4 million years ago, to allow for better cooling through sweating, their naked skin was exposed to sunlight. In the tropics, natural selection favoured dark-skinned human populations as high levels of skin pigmentation protected against the harmful effects of sunlight. Indigenous populations' skin reflectance (the amount of sunlight the skin reflects) and the actual UV radiation in a particular geographic area is highly inversely correlated, which supports this idea. Genetic evidence also supports this notion, demonstrating that around 1.2 million years ago there was a strong evolutionary pressure which acted on the development of dark skin pigmentation in early members of the genus Homo. Hairlessness exposes folate circulating subcutaneously and in the dermis to degradation from UV-radiation. The effect of sunlight on folic acid levels has been crucial in the development of dark skin and favored the emergence of skin pigmentation in order to protect from folate depletion due to the increased exposure to sunlight.

In 2017, a study showed that both dark and light pigmentation alleles arose before the origin of modern humans, with the older version of the variants in many cases being associated with lighter skin. The earliest primate ancestors of humans most likely had pale non-pigmented skin covered with dark black hair, like the chimpanzee and other great apes.

With the evolution of hairless skin, abundant sweat glands, and skin rich in melanin, early humans could walk, run, and forage for food for long periods of time under the hot sun without brain damage due to overheating, giving them an evolutionary advantage over other species. Research on the MC1R alleles using assumptions about past population size and an absence of population bottlenecks suggests the allele for dark skin present in modern Africans arose at least by 1.2 million years ago.

This was the genotype inherited by anatomically modern humans, but retained only by part of the extant populations, thus forming an aspect of human genetic variation. About 100,000–70,000 years ago, some anatomically modern humans (Homo sapiens) began to migrate away from the tropics to the north where they were exposed to less intense sunlight. This was possibly in part due to the need for greater use of clothing to protect against the colder climate. Under these conditions there was less photodestruction of folate and so the evolutionary pressure working against the survival of lighter-skinned gene variants was reduced. In addition, lighter skin is able to generate more vitamin D (cholecalciferol) than darker skin, so it would have represented a health benefit in reduced sunlight if there were limited sources of vitamin D. Hence the leading hypothesis for the evolution of human skin color proposes that:

  1. From the origin of hairlessness and exposure to UV-radiation to less than 100,000 years ago, archaic humans, including archaic Homo sapiens, were dark-skinned.
  2. As some Homo sapiens populations began to migrate, the evolutionary constraint keeping skin dark decreased proportionally to the distance north a population migrated, resulting in a range of skin tones within northern populations, although the bulk of humans remained dark-skinned.
  3. At some point, some northern populations experienced positive selection for lighter skin due to the increased production of vitamin D from sunlight and the genes for darker skin disappeared from these populations.
  4. Subsequent migrations into different UV environments and admixture between populations have resulted in the varied range of skin pigmentations we see today.

The genetic mutations leading to light skin, though partially different among East Asians and Western Europeans, suggest the two groups experienced a similar selective pressure after settlement in northern latitudes.

The theory is partially supported by a study into the SLC24A5 gene which found that the allele associated with light skin in Europe "determined that 18,000 years had passed since the light-skin allele was fixed in Europeans" but may have originated as recently as 12,000–6,000 years ago "given the imprecision of method" , which is in line with the earliest evidence of farming. Paleolithic Cro-Magnon groups, as well as Early Holocene Western and central European hunter-gatherers (Western Hunter Gatherers) have been suggested to have been dark skinned based on DNA analysis, with a number of the most prominent light-skin tone gene variants found in modern Europeans being introduced by Anatolian Neolithic Farmers that migrated into Europe beginning around 9,000 years ago, with selection pressure for lighter skin intensifying from the Neolithic period onwards.

Research by Nina Jablonski suggests that an estimated time of about 10,000 to 20,000 years is enough for human populations to achieve optimal skin pigmentation in a particular geographic area but that development of ideal skin coloration may happen faster if the evolutionary pressure is stronger, even in as little as 100 generations. The length of time is also affected by cultural practices such as food intake, clothing, body coverings, and shelter usage which can alter the ways in which the environment affects populations.

Evolutionary model of human pigmentation in three continental populations. The colors of the branches roughly indicate the generalized skin pigmentation level of these populations.

Population and admixture studies suggest a three-way model for the evolution of human skin color, with dark skin evolving in early Homo sapiens in Africa and light skin evolving only recently after modern humans had expanded out of Africa. For the most part, the evolution of light skin has followed different genetic paths in Western and Eastern Eurasian populations; however, some mutations associated with lighter skin have estimated origin dates after humans spread out of Africa but before the divergence of the two lineages.

According to Crawford et al. (2017), most of the genetic variants associated with light and dark pigmentation in African populations appear to have originated more than 300,000 years ago. African, South Asian and Australo-Melanesian populations also carry derived alleles for dark skin pigmentation that are not found in Europeans or East Asians. Huang et al. 2021 found the existence of "selective pressure on light pigmentation in the ancestral population of Europeans and East Asians", prior to their divergence from each other. Skin pigmentation was also found to be affected by directional selection towards darker skin among Africans, as well as lighter skin among Eurasians. Crawford et al. (2017) similarly found evidence for selection towards light pigmentation prior to the divergence of West Eurasians and East Asians.

Functional considerations

Elias et. al. in 2010 showed a superior barrier function in darkly pigmented skin. Most protective functions of the skin, including the permeability barrier and the antimicrobial barrier, reside in the stratum corneum and the researchers surmise that the stratum corneum has undergone the most genetic change since the loss of human body hair. Natural selection would have favored mutations that protect this essential barrier; one such protective adaptation is the pigmentation of interfollicular epidermis, because it improves barrier function as compared to non-pigmented skin.

The authors argue that lack of significant differences between modern light-skinned and dark-skinned populations in vitamin D deficiency, early death from UV-induced cancers and birth defects — as well as instances of light and dark populations living side-by-side in areas with similar UV — suggest the standard model is insufficient to explain the strong selection drive for pigmented skin. Jablonski rejects this theory on the grounds that the human tanning response is driven by UV-B exposure, not xeric stress, and that the positive selection for vitamin D production is "well-established".

Genetics

See also: Human genetic variation and Race and genetics

To some extent, skin color is determined independently of eye and hair color, as can be seen from variation in skin coloration in human populations. For the evolution of human skin color, see section above.

For skin color, heritability is very high, even though it can be modified by exposure to sunlight.

A recent systematic study found 169 genes involved in human skin coloration. Most of the genes were involved in melanosome biogenesis, endosomal transport, and gene regulation. Notably, the function of these genes was verified in tissue culture experiments using CRISPR-Cas9 knockouts, showing that these genes are indeed involved in melanin production.

Dark skin

Main article: Dark skin

All modern humans share a common ancestor who lived around 200,000 years ago in Africa. Comparisons between known skin pigmentation genes in chimpanzees and modern Africans show that dark skin evolved along with the loss of body hair about 1.2 million years ago and that this common ancestor had dark skin. Investigations into dark-skinned populations in South Asia and Melanesia indicate that skin pigmentation in these populations is due to the preservation of this ancestral state and not due to new variations on a previously lightened population.

MC1R

MC1R (rs885479)

The melanocortin 1 receptor (MC1R) gene is primarily responsible for determining whether pheomelanin and eumelanin are produced in the human body. Research shows at least 10 differences in MC1R between African and chimpanzee samples and that the gene has probably undergone a strong positive selection (a selective sweep) in early Hominins around 1.2 million years ago. This is consistent with positive selection for the high-eumelanin phenotype seen in Africa and other environments with high UV exposure.

Light skin

Main article: Light skin
History of human skin pigmentation in Eurasia based on genetics

For the most part, the evolution of light skin has followed different genetic paths in European and East Asian populations. Two genes, however, KITLG and ASIP, have mutations associated with lighter skin that have high frequencies in both European and East Asian populations. They are thought to have originated after humans spread out of Africa but before the divergence of the European and Asian lineages around 30,000 years ago. Two subsequent genome-wide association studies found no significant correlation between these genes and skin color, and suggest that the earlier findings may have been the result of incorrect correction methods and small panel sizes, or that the genes have an effect too small to be detected by the larger studies.

KITLG

KITLG (rs1881227)

The KIT ligand (KITLG) gene is involved in the permanent survival, proliferation and migration of melanocytes. A mutation in this gene, A326G (rs642742), has been positively associated with variations of skin color in African-Americans of mixed West African and European descent and is estimated to account for 15–20% of the melanin difference between African and European populations. This allele shows signs of strong positive selection outside Africa and occurs in over 80% of European and Asian samples, compared with less than 10% in African samples.

ASIP

Agouti signalling peptide (ASIP) acts as an inverse agonist, binding in place of alpha-MSH and thus inhibiting eumelanin production. Studies have found two alleles in the vicinity of ASIP are associated with skin color variation in humans. One, rs2424984, has been identified as an indicator of skin reflectance in a forensics analysis of human phenotypes across Caucasian, African-American, South Asian, East Asian, Hispanic and Native American populations and is about three times more common in non-African populations than in Africa. The other allele, 8188G (rs6058017) is significantly associated with skin color variation in African-Americans and the ancestral version occurs in only 12% of European and 28% of East Asian samples compared with 80% of West African samples.

Europe

A number of genes have been positively associated with the skin pigmentation difference between European and non-European populations. Mutations in SLC24A5 and SLC45A2 are believed to account for the bulk of this variation and show very strong signs of selection. A variation in TYR has also been identified as a contributor.

Research indicates the selection for the light-skin alleles of these genes in Europeans is comparatively recent, having occurred later than 20,000 years ago and perhaps as recently as 12,000 to 6,000 years ago. In the 1970s, Luca Cavalli-Sforza suggested that the selective sweep that rendered light skin ubiquitous in Europe might be correlated with the advent of farming and thus have taken place only around 6,000 years ago; This scenario found support in a 2014 analysis of mesolithic (7,000 years old) hunter-gatherer DNA from La Braña, Spain, which showed a version of these genes not corresponding with light skin color.

In 2015, researchers analysed for light skin genes in the DNA of 94 ancient skeletons ranging from 8,000 to 3,000 years old from Europe and Russia. They found c. 8,000-year-old hunter-gatherers in Spain, Luxembourg, and Hungary were dark skinned while similarly aged hunter gatherers in Sweden were light skinned (having predominately derived alleles of SLC24A5, SLC45A2 and also HERC2/OCA2). Neolithic farmers entering Europe at around the same time were intermediate, being nearly fixed for the derived SLC24A5 variant but only having the derived SLC45A2 allele in low frequencies. The SLC24A5 variant spread very rapidly throughout central and southern Europe from about 8,000 years ago, whereas the light skin variant of SLC45A2 spread throughout Europe after 5,800 years ago.

Some authors have expressed caution regarding the skin pigmentation predictions. According to Ju et al. (2021), in a study addressing 40,000 years of modern human history, stated:

"Relatively dark skin pigmentation in Early Upper Paleolithic Europe would be consistent with those populations being relatively poorly adapted to high-latitude conditions as a result of having recently migrated from lower latitudes. On the other hand, although we have shown that these populations carried few of the light pigmentation alleles that are segregating in present-day Europe, they may have carried different alleles that we cannot now detect."

SLC24A5

Solute carrier family 24 member 5 (SLC24A5) regulates calcium in melanocytes and is important in the process of melanogenesis. The SLC24A5 gene's derived Ala111Thr allele (rs1426654) has been shown to be a major factor in light skin pigmentation and is common in Western Eurasia. Recent studies have found that the variant represents as much as 25–40% of the average skin tone difference between Europeans and West Africans. This derived allele is a reliable predictor of phenotype across a range of populations. It has been the subject of recent selection in Western Eurasia, and is fixed in European populations.

SLC45A2

Solute carrier family 45 member 2 (SLC45A2 or MATP) aids in the transport and processing of tyrosine, a precursor to melanin. It has also been shown to be one of the significant components of the skin color of modern Europeans through its Phe374Leu (rs16891982) allele that has been directly correlated with skin color variation across a range of populations. This variation is ubiquitous in European populations but extremely rare elsewhere and shows strong signs of selection.

TYR

The TYR gene encodes the enzyme tyrosinase, which is involved in the production of melanin from tyrosine. It has an allele, Ser192Tyr (rs1042602), found solely in 40–50% of Europeans and linked to light-colored skin in studies of South Asian and African-American populations.

East Asia

A number of genes known to affect skin color have alleles that show signs of positive selection in East Asian populations. Of these, only OCA2 has been directly related to skin color measurements, while DCT, MC1R and ATRN are marked as candidate genes for future study.

OCA2
OCA2 (rs12913832)

Oculocutaneous albinism II (OCA2) assists in the regulation of pH in melanocytes. The OCA2 gene's derived His615Arg (rs1800414) allele has been shown to account for about 8% of the skin tone difference between African and East Asian populations in studies of an East Asian population living in Toronto and a Chinese Han population. This variant is essentially restricted to East Asia, with highest frequencies in Eastern East Asia (49–63%), midrange frequencies in Southeast Asia, and the lowest frequencies in Western China and some Eastern European populations.

Candidate genes

A number of studies have found genes linked to human skin pigmentation that have alleles with statistically significant frequencies in Chinese and East Asian populations. While not linked to measurements of skin tone variation directly, dopachrome tautomerase (DCT or TYRP2 rs2031526), melanocortin 1 receptor (MC1R) Arg163Gln (rs885479) and attractin (ATRN) have been indicated as potential contributors to the evolution of light skin in East Asian populations.

Tanning response

Tanning response in humans is controlled by a variety of genes. MC1R variants Arg151Sys (rs1805007), Arg160Trp (rs1805008), Asp294Sys (rs1805009), Val60Leu (rs1805005) and Val92Met (rs2228479) have been associated with reduced tanning response in European and/or East Asian populations. These alleles show no signs of positive selection and only occur in relatively small numbers, reaching a peak in Europe with around 28% of the population having at least one allele of one of the variations. A study of self-reported tanning ability and skin type in American non-Hispanic Caucasians found that SLC24A5 Phe374Leu is significantly associated with reduced tanning ability and also associated TYR Arg402Gln (rs1126809), OCA2 Arg305Trp (rs1800401) and a 2-SNP haplotype in ASIP (rs4911414 and rs1015362) to skin type variation within a "fair/medium/olive" context.

Albinism

Main article: Albinism in humans

Oculocutaneous albinism (OCA) is a lack of pigment in the eyes, skin and sometimes hair that occurs in a very small fraction of the population. The four known types of OCA are caused by mutations in the TYR, OCA2, TYRP1, and SLC45A2 genes.

Age

In hominids, the parts of the body not covered with hair, like the face and the back of the hands, start out pale in infants and turn darker as the skin is exposed to more sun. All human babies are born pale, regardless of what their adult color will be. In humans, melanin production does not peak until after puberty.

The skin of children becomes darker as they go through puberty and experience the effects of sex hormones. This darkening is especially noticeable in the skin of the nipples, the areola of the nipples, the labia majora in females, and the scrotum in males. In some people, the armpits become slightly darker during puberty. The interaction of genetic, hormonal, and environmental factors on skin coloration with age is still not adequately understood, but it is known that men are at their darkest baseline skin color around the age of 30, without considering the effects of tanning. Around the same age, women experience darkening of some areas of their skin.

Human skin color fades with age. Humans over the age of thirty experience a decrease in melanin-producing cells by about 10% to 20% per decade as melanocyte stem cells gradually die. The skin of face and hands has about twice the amount of pigment cells as unexposed areas of the body, as chronic exposure to the sun continues to stimulate melanocytes. The blotchy appearance of skin color in the face and hands of older people is due to the uneven distribution of pigment cells and to changes in the interaction between melanocytes and keratinocytes.

Sexual dimorphism

It has been observed that females are found to have lighter skin pigmentation than males in some studied populations. However, in light skinned populations, namely those of European descent, women have darker skin than men.

It is unknown why skin color is sexually dimorphic in some populations. White women may have darker skin than white men because the female sex hormone estrogen is known to increase skin pigmentation in lighter-skinned populations. White women's skin is significantly darker than White men's, to the extent that White women have a much lower rate of skin cancer than White men.

In populations where women have lighter skin than men, it has been hypothesized that the requirement for high amounts of calcium during pregnancy and lactation may be related to the dimorphism. Breastfeeding newborns, whose skeletons are growing, require high amounts of calcium intake from the mother's milk (about 4 times more than during prenatal development), part of which comes from reserves in the mother's skeleton.

Adequate vitamin D resources are needed to absorb calcium from the diet, and it has been shown that deficiencies of vitamin D and calcium increase the likelihood of various birth defects such as spina bifida and rickets. Natural selection may have led to females with lighter skin than males in some indigenous populations because women must get enough vitamin D and calcium to support the development of fetus and nursing infants and to maintain their own health. However, some authors have cast doubt on the theory that vitamin D synthesis is related to the sexual dimorphism of human skin color in these populations.

The sexes also differ in how they change their skin color with age. Women can change pigmentation in certain parts of their body, such as the areola and nipples during the menstrual cycle and pregnancy. Between 50 and 70% of pregnant women will develop the "mask of pregnancy", which refers to the browning and yellowing of the cheeks, upper lips, and forehead that occurs during pregnancy. This is caused by increases in the female sex hormone estrogen, and it can develop in women who take birth control pills or participate in hormone replacement therapy.

Disorders of pigmentation

Uneven pigmentation of some sort affects most people, regardless of bioethnic background or skin color. Skin may either appear lighter, or darker than normal, or lack pigmentation at all; there may be blotchy, uneven areas, patches of brown to gray discoloration or freckling. Apart from blood-related conditions such as jaundice, carotenosis, or argyria, skin pigmentation disorders generally occur because the body produces either too much or too little melanin.

Depigmentation

Main article: Depigmentation

Albinism

Some types of albinism affect only the skin and hair, while other types affect the skin, hair and eyes, and in rare cases only the eyes. All of them are caused by different genetic mutations. Albinism is a recessively inherited trait in humans where both pigmented parents may be carriers of the gene and pass it down to their children. Each child has a 25% chance of being albino and a 75% chance of having normally pigmented skin. One common type of albinism is oculocutaneous albinism or OCA, which has many subtypes caused by different genetic mutations. Albinism is a serious problem in areas of high sunlight intensity, leading to extreme sun sensitivity, skin cancer, and eye damage.

Albinism is more common in some parts of the world than in others, but it is estimated that 1 in 70 humans carry the gene for OCA. The most severe type of albinism is OCA1A, which is characterized by complete, lifelong loss of melanin production, other forms of OCA1B, OCA2, OCA3, OCA4, show some form of melanin accumulation and are less severe. The four known types of OCA are caused by mutations in the TYR, OCA2, TYRP1, and SLC45A2 genes.

Albinos often face social and cultural challenges (even threats), as the condition is often a source of ridicule, racism, fear, and violence. Many cultures around the world have developed beliefs regarding people with albinism. Albinos are persecuted in Tanzania by witchdoctors, who use the body parts of albinos as ingredients in rituals and potions, as they are thought to possess magical power.

Vitiligo

Former Chief Justice of India, P. Sathasivam, has vitiligo

Vitiligo is a condition that causes depigmentation of sections of skin. It occurs when melanocytes die or are unable to function. The cause of vitiligo is unknown, but research suggests that it may arise from autoimmune, genetic, oxidative stress, neural, or viral causes. The incidence worldwide is less than 1%. Individuals affected by vitiligo sometimes suffer psychological discomfort because of their appearance.

Hyperpigmentation

Increased melanin production, also known as hyperpigmentation, can be a few different phenomena:

  • Melasma describes the darkening of the skin.
  • Chloasma describes skin discolorations caused by hormones. These hormonal changes are usually the result of pregnancy, birth control pills or estrogen replacement therapy.
  • Solar lentigo, also known as "liver spots" or "senile freckles", refers to darkened spots on the skin caused by aging and the sun. These spots are quite common in adults with a long history of unprotected sun exposure.

Aside from sun exposure and hormones, hyperpigmentation can be caused by skin damage, such as remnants of blemishes, wounds or rashes. This is especially true for those with darker skin tones.

The most typical cause of darkened areas of skin, brown spots or areas of discoloration is unprotected sun exposure. Once incorrectly referred to as liver spots, these pigment problems are not connected with the liver.

On lighter to medium skin tones, solar lentigenes emerge as small- to medium-sized brown patches of freckling that can grow and accumulate over time on areas of the body that receive the most unprotected sun exposure, such as the back of the hands, forearms, chest, and face. For those with darker skin colors, these discolorations can appear as patches or areas of ashen-gray skin.

Exposure to the sun

See also: Sun tanning
A suntanned arm showing darker skin where it has been exposed. This pattern of tanning is often called a farmer's tan.

Melanin in the skin protects the body by absorbing solar radiation. In general, the more melanin there is in the skin the more solar radiation can be absorbed. Excessive solar radiation causes direct and indirect DNA damage to the skin and the body naturally combats and seeks to repair the damage and protect the skin by creating and releasing further melanin into the skin's cells. With the production of the melanin, the skin color darkens, but can also cause sunburn. The tanning process can also be created by artificial UV radiation.

There are two different mechanisms involved. Firstly, the UVA-radiation creates oxidative stress, which in turn oxidizes existing melanin and leads to rapid darkening of the melanin, also known as IPD (immediate pigment darkening). Secondly, there is an increase in production of melanin known as melanogenesis. Melanogenesis leads to delayed tanning and first becomes visible about 72 hours after exposure. The tan that is created by an increased melanogenesis lasts much longer than the one that is caused by oxidation of existing melanin. Tanning involves not just the increased melanin production in response to UV radiation but the thickening of the top layer of the epidermis, the stratum corneum.

A person's natural skin color affects their reaction to exposure to the sun. Generally, those who start out with darker skin color and more melanin have better abilities to tan. Individuals with very light skin and albinos have no ability to tan. The biggest differences resulting from sun exposure are visible in individuals who start out with moderately pigmented brown skin: the change is dramatically visible as tan lines, where parts of the skin which tanned are delineated from unexposed skin.

Modern lifestyles and mobility have created mismatch between skin color and environment for many individuals. Vitamin D deficiencies and UVR overexposure are concerns for many. It is important for these people individually to adjust their diet and lifestyle according to their skin color, the environment they live in, and the time of year. For practical purposes, such as exposure time for sun tanning, six skin types are distinguished following Fitzpatrick (1975), listed in order of decreasing lightness:

Fitzpatrick scale

Main article: Fitzpatrick scale

The following list shows the six categories of the Fitzpatrick scale in relation to the 36 categories of the older von Luschan scale:

Type Also called Sunburning Tanning behavior Von Luschan's chromatic scale
I Light, pale white Always Never 0–6
II White, fair Usually Minimally 7–13
III Medium white to light brown Sometimes Uniformly 14–20
IV Olive, moderate brown Rarely Easily 21–27
V Brown, dark brown Very rarely Very easily 28–34
VI Very dark brown to black Never Rarely 35–36

Dark skin with large concentrations of melanin protects against ultraviolet light and skin cancers; light-skinned people have about a tenfold greater risk of dying from skin cancer, compared with dark-skinned persons, under equal sunlight exposure. Furthermore, UV-A rays from sunlight are believed to interact with folic acid in ways that may damage health. In a number of traditional societies the sun was avoided as much as possible, especially around noon when the ultraviolet radiation in sunlight is at its most intense. Midday was a time when people stayed in the shade and had the main meal followed by a nap, a practice similar to the modern siesta.

Geographic variation

Approximately 10% of the variance in skin color occurs within regions, and approximately 90% occurs between regions. Because skin color has been under strong selective pressure, similar skin colors can result from convergent adaptation rather than from genetic relatedness; populations with similar pigmentation may be genetically no more similar than other widely separated groups. Furthermore, in some parts of the world where people from different regions have mixed extensively, the connection between skin color and ancestry has substantially weakened. In Brazil, for example, skin color is not closely associated with the percentage of recent African ancestors a person has, as estimated from an analysis of genetic variants differing in frequency among continent groups.

In general, people living close to the equator are highly darkly pigmented, and those living near the poles are generally very lightly pigmented. The rest of humanity shows a high degree of skin color variation between these two extremes, generally correlating with UV exposure. The main exception to this rule is in the New World, where people have only lived for about 10,000 to 15,000 years and show a less pronounced degree of skin pigmentation.

In recent times, humans have become increasingly mobile as a consequence of improved technology, domestication, environmental change, strong curiosity, and risk-taking. Migrations over the last 4000 years, and especially the last 400 years, have been the fastest in human history and have led to many people settling in places far away from their ancestral homelands. This means that skin colors today are not as confined to geographical location as they were previously.

Social status, colorism and racism

Main article: Discrimination based on skin color
Skin colors according to von Luschan's chromatic scale

According to classical scholar Frank Snowden, skin color did not determine social status in ancient Egypt, Greece or Rome. These ancient civilizations viewed relations between the major power and the subordinate state as more significant in a person's status than their skin colors.

Nevertheless, some social groups favor specific skin coloring. The preferred skin tone varies by culture and has varied over time. A number of indigenous African groups, such as the Maasai, associated pale skin with being cursed or caused by evil spirits associated with witchcraft. They would abandon their children born with conditions such as albinism and showed a sexual preference for darker skin.

Many cultures have historically favored lighter skin for women. Before the Industrial Revolution, inhabitants of the continent of Europe preferred pale skin, which they interpreted as a sign of high social status. The poorer classes worked outdoors and got darker skin from exposure to the sun, while the upper class stayed indoors and had light skin. Hence light skin became associated with wealth and high position. Women would put lead-based cosmetics on their skin to whiten their skin tone artificially. However, when not strictly monitored, these cosmetics caused lead poisoning. Other methods also aimed at achieving a light-skinned appearance, including the use of arsenic to whiten skin, and powders. Women would wear full-length clothes when outdoors, and would use gloves and parasols to provide shade from the sun.

Colonization and enslavement as carried out by European countries became involved with colorism and racism, associated with the belief that people with dark skin were uncivilized, inferior, and should be subordinate to lighter-skinned invaders. This belief exists to an extent in modern times as well. Institutionalized slavery in North America led people to perceive lighter-skinned African-Americans as more intelligent, cooperative, and beautiful. Such lighter-skinned individuals had a greater likelihood of working as house slaves and of receiving preferential treatment from plantation owners and from overseers. For example, they had a chance to get an education. The preference for fair skin remained prominent until the end of the Gilded Age, but racial stereotypes about worth and beauty persisted in the last half of the 20th century and continue in the present day. African-American journalist Jill Nelson wrote that, "To be both prettiest and black was impossible," and elaborated:

We learn as girls that in ways both subtle and obvious, personal and political, our value as females is largely determined by how we look. ... For black women, the domination of physical aspects of beauty in women's definition and value render us invisible, partially erased, or obsessed, sometimes for a lifetime, since most of us lack the major talismans of Western beauty. Black women find themselves involved in a lifelong effort to self-define in a culture that provides them no positive reflection.

A preference for fair or lighter skin continues in some countries, including Latin American countries where whites form a minority. In Brazil, a dark-skinned person is more likely to experience discrimination. Many actors and actresses in Latin America have European features—blond hair, blue eyes, and pale skin. A light-skinned person is more privileged and has a higher social status; a person with light skin is considered more beautiful and lighter skin suggests that the person has more wealth. Many languages popularly use specific words to describe distinct skin tones - from (for example) "jincha", Puerto Rican slang for "glass of milk" to "morena", literally "brown".

Significant exceptions to a preference for lighter skin started to appear in Western culture in the mid-20th century. However, a 2010 study found a preference for lighter-skinned women in New Zealand and California. Though sun-tanned skin was once associated with the sun-exposed manual labor of the lower class, the associations became dramatically reversed during this time—a change usually credited to the trendsetting Frenchwoman Coco Chanel (1883–1971) presenting tanned skin as fashionable, healthy, and luxurious.

As of 2017, though an overall preference for lighter skin remains prevalent in the United States, many within the country regard tanned skin as both more attractive and healthier than pale or very dark skin. Western mass media and popular culture continued to reinforce negative stereotypes about dark skin, but in some circles pale skin has become associated with indoor office-work while tanned skin has become associated with increased leisure time, sportiness and good health that comes with wealth and higher social status. Studies have also emerged indicating that the degree of tanning is directly related to how attractive a young woman is.

Skin whitening

Main article: Skin whitening
Skin lightening soaps

In South Asia, society regards fair skin as more attractive and associates dark skin with lower class status. This results in a massive market for skin-whitening creams. Fairer skin-tones also correlate to higher caste-status in the Hindu social order—although the system is not based on skin tone. Actors and actresses in Indian cinema tend to have light skin tones, and Indian cinematographers have used graphics and intense lighting to achieve more "desirable" skin tones. Fair skin tones are advertised as an asset in Indian marketing.

In 2013, 77% of Nigerian women, 52% of Senegalese women, and 25% of Malian women were using lightening products. In 2020, Der Spiegel reported that in Ghana, "When You Are Light-Skinned, You Earn More", and that "ome pregnant women take tablets in the hopes that it will lead their child to be born with fair skin. Some apply bleaching lotion to their babies, in the hopes that it will improve their child's chances."

Skin-whitening products have remained popular over time, often due to historical beliefs and perceptions about fair skin. Sales of skin-whitening products across the world grew from $40 billion to $43 billion in 2008. In South and East Asian countries, people have traditionally seen light skin as more attractive, and a preference for lighter skin remains prevalent. In ancient China and Japan, for example, pale skin can be traced back to ancient drawings depicting women and goddesses with fair skin tones. In ancient China, Japan, and Southeast Asia, pale skin was seen as a sign of wealth. Thus skin-whitening cosmetic products are popular in East Asia.

In 2010, four out of ten women surveyed in Hong Kong, Malaysia, the Philippines and South Korea used a skin-whitening cream, and more than 60 companies globally compete for Asia's estimated $18 billion market. Changes in regulations in the cosmetic industry led to skin-care companies introducing harm-free skin lighteners. In Japan, the geisha have a reputation for their white-painted faces, and the appeal of the bihaku (美白), or "beautiful white", ideal leads many Japanese women to avoid any form of tanning.

There are exceptions to this, with Japanese fashion trends such as ganguro emphasizing tanned skin. Skin whitening is also not uncommon in Africa, and several research projects have suggested a general preference for lighter skin in the African-American community. In contrast, one study on men of the Bikosso tribe in Cameroon found no preference for attractiveness of females based on lighter skin color, bringing into question the universality of earlier studies that had exclusively focused on skin-color preferences among non-African populations.

See also

References

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