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The aquatic ape hypothesis (AAH) proposes that several aspects of human evolution obtain a simple explanation if it is assumed that the ancestors of modern humans spent a period of time adapting to life in a wet environment. AAH emerged from the claim that many of the traits that set humans apart from other primates have parallels in aquatic mammals and the idea was first proposed by the marine biologist Alister Hardy in 1960. After Hardy, the most prominent proponent was Welsh screenwriter Elaine Morgan. The most active proponent of AAH has been Elaine Morgan, who has written several thoroughly researched books on the topic. The most up to date account of these ideas was recently published in an eBook to commemorate 50 years since Hardy's idea was first announced.

It appears uncontroversial that both H. neanderthalensis and early H. sapiens were better suited to aquatic environments than other great apes. However, AAH has not been accepted among the mainstream explanations of human evolution. Few paleoanthropologists have explicitly evaluated AAH in scientific journals, and the theory was extensively critiqued in a peer reviewed paper in 1997. It suggested that since AAH was originally proposed by a non-paleoanthropologist in a popular essay, attention was diverted away from the possible scientific merits of the hypothesis. Most attempts to explain the radical differences between humans and other apes focus on other causes than adaptation to a radically different environment. AAH has been criticized for possessing a variety of theoretical problems, for lacking evidentiary support, and because alternative explanations are available for many of the observations suggested to support it.

History

In a 1942 book, the German pathologist Max Westenhöfer published the idea of humans evolving in proximity to water with the statement "The postulation of an aquatic mode of life during an early stage of human evolution is a tenable hypothesis, for which further inquiry may produce additional supporting evidence."

In 1930 marine biologist Alister Hardy hypothesized that humans may have had ancestors more aquatic than previously imagined. Because it was outside his field and he was aware of the controversy it would cause, Hardy delayed reporting his hypothesis. After he had become a respected academic, Hardy finally voiced his thoughts in a speech to the British Sub-Aqua Club in Brighton on 5 March 1960, not expecting any attention, but it was reported in a national newspaper. This generated immediate controversy in the field of paleoanthropology. Consequently Hardy published the hypothesis in an article in New Scientist on 17 March 1960. He defined his idea:

My thesis is that a branch of this primitive ape-stock was forced by competition from life in the trees to feed on the sea-shores and to hunt for food, shell fish, sea-urchins etc., in the shallow waters off the coast. I suppose that they were forced into the water just as we have seen happen in so many other groups of terrestrial animals. I am imagining this happening in the warmer parts of the world, in the tropical seas where Man could stand being in the water for relatively long periods, that is, several hours at a stretch.

The idea received some interest after the article was published, but was generally ignored by the scientific community thereafter. In 1967, the hypothesis was briefly mentioned in The Naked Ape, a book by Desmond Morris in which can be found the first use of the term "aquatic ape". Writer Elaine Morgan read about the idea in Morris' book and was struck by its potential explanatory power, becoming its main promoter and publishing six books over the next 40 years. The context of the initial presentations of AAH (a popular essay and a political text) diverted attention away from the possible scientific merits of the hypothesis, and it has never been seriously scrutinized and discussed within the field of paleoanthropology. Most supporters of AAH come from other fields, such as paleontology and nutrition science.

The hypothesis

AAH suggests that many features that distinguish humans from their nearest evolutionary relatives emerged because the ancestors of humans underwent a period when they were adapting to an aquatic or semiaquatic way of life, but returned to terrestrial life before having become fully adapted to the aquatic environment. Depending on the individual describing the theory, these protohumans would have spent time wading, swimming or diving on the shores of fresh, brackish or saline waters and feeding on littoral resources. Various traits that have been proposed to indicate past adaptation to aquatic conditions and the return to land, but generally the evidence provided for the AAH is equally well accounted for by land-based adaptations without needing to posit an aquatic phase of human development. In addition, the AAH is contradictory in several places; the AAH theorizes humans developed some unique skin features due to adaptation to water, but other features emerged after leaving the habitat, and the specialization that is hypothesized for an aquatic life are uneven, with humans lacking many truly specialized features of aquatic species (such as head shape, repositioned nostrils and streamlining of the body). Parallels made by proponents of the AAH between humans and the proboscis monkey, which shows mainly behavioral adaptations to a water-based habitat, contradicts any claims of anatomical evidence for the hypothesis. Many species of modern primates demonstrate some sort of aquatic behaviors (such as swimming, wading or diving) and use of aquatic environments (for thermoregulation, display behavior, range, diet and predation) but many do not display the traits posited by AAH, suggesting the traits posted as evidence for the AAH may facilitate aquatic behavior rather than evolving as a result of it.

Bipedalism is suggested as evidence for the AAH, as no other terrestrial mammal habitually walks on its hind legs with its spine held erect. However, all great apes do so when wading in water, as bipedal locomotion allows them to wade deeper while continuing to breathe. Maintaining an erect posture in water is also easier than on land, where problems related to maintaining balance and to physical strain on the back, knees, blood circulation and inner organs are substantial. However, the disadvantages cited for bipedalism within the AAH are often the result of comparing humans to medium, terrestrial quadrupeds, but the evolution of humans from ape ancestors never included a period of quadrupedal locomotion. Instead, human evolution features mainly brachiation, suspension and climbing as the primary method of transportation, with a gradual increase in bipedal locomotion over time. In addition, the elongated lower limbs of humans, which is explained as improving swimming speeds, appears only after the evolution of the Homo genus..

Humans lack the thick coat of most other primates, and Morgan claims that naked skin is commonly found in aquatic mammals and land-dwelling mammals that have aquatic ancestors as well as those that currently spend much of their time in wet conditions. What body hair humans do have follows the flow of water over the body when swimming. However, most aquatic mammals that are comparably sized to humans are not hairless, but have dense, insulating fur and swim very well, with fatty layers beneath the skin. Humans also vary greatly in the amount and texture of body hair, unlike actual aquatic species. Hairless skin is also only an advantage for fully aquatic mammals that dive, swim quickly or migrate long distances such as whales and dolphins, and only appears and is an advantage for extremely large aquatic mammals who would overheat with large amounts of body hair, who are fully aquatic and have evolved as an aquatic species for millions of years. The loss of body hair is also explainable through a lower parasite load, and maintenance through sexual selection. Furthermore, while shaving human swimmers to eliminate the little body hair that remains results in a minor decrease in drag, this cannot be extrapolated to a beneficial effect of loss of a full coat of fur, which has been shown to have superior drag reduction ability. While relative hairlessness and hair direction is cited as an adaptation to swimming and diving, there is no evidence of similar skeletal or soft tissue adaptations that are expected to accompany such adaptations.

Proponents of the AAH claim humans are unusual among terrestrial mammals in having a potentially very thick layer of subcutaneous fat attached to the skin and suggest that subcutaneous fat is universally present in aquatic mammals, for which it provides both insulation and buoyancy. However, the subcutaneous fat distribution in humans is more similar to a domesticated animal than an aquatic one, and is nearly identical to that of other primates. The subcutaneous fat of aquatic mammals and humans also seems to serve different uses – it forms the streamlined shape of seals, while in humans it is used for sexual selection. In addition, the distribution of fat and blood vessels in humans allows for improved thermoregulation, as hot blood from the body can bypass the fat to radiate heat through the skin.

Humans have a Descended larynx, which in most mammals is situated in the nasal cavity from which it temporarily descends for vocalizing or panting. In humans, unlike in the great apes and other primates, the larynx descends to the throat during infancy and stays there permanently. A descended larynx is also found in some aquatic mammals. It facilitates closing the air tract when diving and makes it possible to inhale a large quantity of air through the mouth when surfacing. However, the human larynx is not shaped like the larynxes of aquatic animals; it forms and descends as an infant begins to speak, making it easier to aspirate water and drown. Additionally, a descended larynx is not unique to aquatic animals, and permanently or temporarily descended larynxes are seen in dogs, pigs, goats, monkeys, big cats, deer, and young chimps. Mainstream anthropology explain the descended larynx as an adaptation to improve vocalizations by increasing the number of pronounceable vowels and improving the ability of humans to control their speech.

Like all diving mammals, humans are able to decide when they inhale and how much air they take in at a time. In other terrestrial mammals, breathing is a reflex that cannot be controlled at will. However, the position, evolutionary timing of changes, and size of the nerve openings in the vertebrae suggest that breath control in humans improved because of the increased complexity and use of speech rather than an aquatic phase of evolution. In addition, breath control is thought to be preceded by bipedalism, which frees the muscles around the upper torso from locomotion and allows breathing rates to occur independent of locomotion. Voluntary speech is thought to be a sufficient evolutionary pressure to explain breath control, independent of other explanations. The vocalizations of dolphins and other aquatic species are not thought to be comparable to humans. In addition, certain birds have speech and breath control comparable to humans, without a phase of aquatic adaptation.

In the great apes, nostrils point directly forward while humans have a hooded nose in which the nostrils point downward, which prevents water from entering when swimming or diving. Among primates, a nose with downward pointing nostrils is also found in proboscis monkeys, which live in seasonally inundated forests and both swim and dive frequently. Aquatic animals, such as seals and hippopotamus, can actively close their nostrils when diving. Unlike the nostrils of the great apes, the nostrils of humans can be completely closed by pressing on the surrounding soft tissue (some persons can even close their nostrils by covering them with the upper lip). However, the shape of the human nose is extremely variable within the species, and believed to be related to climatic adaptations and the warming and moistening of air before it enters the respiratory tract, not to prevent water entry while swimming. In addition, the muscles surrounding the nose show no evidence of having been previously more developed, but are part of a complex of muscles that are specially developed in humans to show emotion and aid in communication.

Modern humans are inefficient swimmers, with shapes that are not well suited to rapid travel through water. Swimming is also a learned trait, and though newborns are able to propel themselves inefficiently through water, they are unable to lift their faces to breathe.

Proponents of the AAH claim that the brain requires for its normal development and functioning such materials that are in short supply in savanna food chains but abundant in marine ones. These include iodine and some essential fatty acids. However, a broad terrestrial diet ensures sufficient access to required essential fatty acids without a high consumption of seafood and the "best" fats found in fish are from cold water fish that did not occupy the same coastal environments as humans. In addition, the requirements of these fats are minimal, with no evidence that extra fats would result in an evolutionary pressure towards a larger brain. Humans without access to shoreline foods also develop normal brains. The encephalization of early Homo species predated the appearance of humans exploiting waterside food sources by over 400,000 years, driven by the consumption of hunted or scavenged animal brains supplying large amounts of scarce nutrients including docosahexaenoic acid.

In hairy mammals, the sebaceous glands excrete an oily substance that serves to make the hairs water-repellent. Humans only have vestigial body hair, but the sebaceous glands on the head and upper torso are especially active and proponents suggest they have been adapted for the waterproofing of the skin. However, many aquatic animals have rudimentary or no sebaceous glands. In humans, sebaceous glands become active during puberty, with men having far more than women, while women have much better scent receptors. This suggests the glands are sexually dimorphic for sexual selection rather than waterproofing. In seals that use sebaceous glands for waterproofing, the glands are active from birth and are secreted by hard, keratinized skin that is very different from human skin.

The mammalian diving reflex, which occurs when the head is immersed in cold water, is suggested as evidence for the AAH. However, the reflex is exhibited by terrestrial mammals as well as aquatic ones, and humans have not been compared to other living hominoids; there is not enough information on for this reflex for it to be used to support the AAH.

Vestigial webbing between the fingers (syndactylism) is also presented as evidence, but claims for this are based on the purported "rareness" of birth defects "adding" features normally thought absent from an evolutionary order. Interdigital webbing is not the "addition" of new tissue, it results from the failure to eliminate skin cells connecting the fingers, a process common to all tetrapods.

If the human and chimpanzee lineages split about 6 million years ago (Miocene), the period of aquatic adaptations would have been after that time. A younger estimate of nearly 2 million years ago has been proposed (Pliocene or early Pleistocene). A possible site where the aquatic adaptations could have evolved are the Danakil Alps in northern Ethiopia, which used to be an island when sea levels were higher. Another suggestion is that they evolved when protohumans migrated along the southern Asian coastline during a previous ice age when sea levels were considerably lower. Practically all the well-known hominin fossil sites were wetter when the fossils were laid down than they are now, and in most cases the associated fossils indicate the presence of lacustrine, fluvial or swamp conditions.

Several theoretical problems have been found with the AAH, and some claims made by the AAH have been challenged as having explanations aside from a period of aquatic adaptation. Review of the individual claims used as evidence for the AAH generally does not support the hypothesis overall, and most of these traits have an explanation within conventional theories of human evolution. Other authors have suggested that wading and other interactions with watery environments may have provided a less extreme but still present role in human evolution.

Theoretical considerations

The AAH has been criticized for containing multiple inconsistencies and lacking evidence from the fossil record to support its claims. It is also described as lacking parsimony, despite purporting to be a simple theory uniting many of the unique anatomical features of humans.

Though describing the hypothesis as plausible, Henry Gee went on to criticize it for being untestable, as most of the evolutionary adaptations described by Morgan would not have fossilized. Gee also stated that, while purely aquatic mammals such as whales show strong skeletal evidence of adaptation to water, humans and human fossils lack such adaptations; that there are many hypothetical and equally plausible scenarios explaining the unique characteristics of human adaptation without involving an aquatic phase of evolution; and that proponents are basing arguments about past adaptations on present physiology, when humans are not significantly aquatic. There is ultimately only circumstantial evidence to suggest, and no solid evidence to support the AAH. ScienceBlogs author Greg Laden has described the AAH as a "human evolution theory of everything" that attempts to explain all anatomical and physiological features of humans and is correct in some areas only by chance. Laden also states that the AAH was proposed when knowledge of human evolutionary history was unclear, while more recent research has found that many human traits have emerged at different times over millions of years, rather than simultaneously due to a single evolutionary pressure.

Habitat

Morgan presented the AAH as an alternative to the "savanna model", which uses very vague descriptive statements portraying protohumans as moving out from forested environments and into a hot dry savanna. However, this idea has been called a caricature of the actual environments in which protohumans are thought to have evolved, and presents a false dichotomy, as more recent theories propose a tree or forest-based habitat providing the driving forces for adaptation, and a straw man of the actual theories and arguments used in the study of paleoanthropology. Morgan has pointed out that scientists are uncertain regarding the reasons for the development of hairlessness, bipedalism, brain size and speech, however as anthropologist John H. Langdon points out, science legitimately admits ignorance when it is unclear and a lack of "final answers" does not legitimize a competing theory by default.

The belief that wading into shallow water would help proto-humans avoid dry-land predation discounts the risks presented by aquatic animals such as crocodiles and hippopotamuses that present a current risk to Africans living near bodies of water, and that protohumans lacked the fangs, claws or size to defend themselves from these threats.

The susceptibility of humans to waterborne parasites have been suggested as evidence against the AAH, though the presence of certain parasites that appear to co-exist with humans has also been presented as evidence for the AAH.

Reception

The AAH has received little serious scrutiny from mainstream paleoanthropologists and has been met with significant skepticism. The AAH is thought by some anthropologists to be accepted readily by popular audiences, students and non-specialist scholars because of its simplicity. In 1987 a symposium was held in Valkenburg, the Netherlands, titled "Aquatic Ape: Fact or fiction?", which published its proceedings in 1991. The chief editor summarized the results of the symposium as failing to support the idea that human ancestors were aquatic, but there is also some evidence that they may have swum and fed in inland lakes and rivers, with the result that modern humans can enjoy brief periods of time spent in the water. The results of the conference were reported in the anthropological press as having rejected the hypothesis. A review of Morgan's book The Scars of Evolution stated that it did not address the central questions of anthropology – how the human and chimpanzee gene lines diverged – which was why it was ignored by the scholarly community. The review also stated that Morgan ignored the fossil record and skirted the absence of evidence that australopithecine underwent any adaptations to water, making the hypothesis impossible to validate from fossils.

Morgan has claimed the AAH was rejected for a variety of reasons unrelated to its explanatory power: old academics were protecting their careers, sexism on the part of male researchers, and her status as a non-academic intruding on academic debates. Despite modifications to the hypothesis and occasional forays into scientific conferences, the AAH has neither been accepted as a mainstream theory nor managed to venture a genuine challenge to orthodox theories of human evolution. However, anthropologist Colin Groves has stated that Morgan's theories are sophisticated enough that they should be taken seriously as a possible explanation for bipedalism.

Morgan's critics have claimed that the appeal of AAH can be explained in several ways:

1. The hypothesis appears to offer absolute answers, while orthodox science is qualified and reserved, a situation which has great appeal to students and the public
2. Unusual ideas challenge the authority of science and scientists, which appeals to anti-establishment sentiments
3. The AAH as developed by Morgan has a strong feminist component, which particularly appeals to a specific, feminist audience
4. The AAH can be explained simply and easily, lacking the myriad details and complicated theorizing involved in dealing with primary sources and materials
5. The AAH uses negative arguments, pointing to the flaws and gaps in conventional theories; though the criticisms of mainstream science and theories can be legitimate, the flaws in one theory do not automatically prove a proposed alternative is true
6. The consensus views of conventional anthropology are complicated, require specialized knowledge and qualified answers, and the investment of considerable time to understand.

See also

• Endurance running hypothesis

Footnotes

1. ^ Hardy, A. (1960). "Was man more aquatic in the past" (pdf). New Scientist. 7: 642–645.
2. Morgan, Elaine (1990). The Scars of Evolution. Souvenir Press. ISBN 0-285-62996-4.
3. ^ Morgan's books on AAH include:
   • Morgan, Elaine (1972). The Descent of Woman. Souvenir Press. ISBN 0 285 62700 7.
   • Morgan, Elaine (1982). The Aquatic Ape. Stein & Day Pub. ISBN 0-285-62509-8.
   • Morgan, Elaine (1990). The Scars of Evolution. Souvenir Press. ISBN 0-285-62996-4.
   • Morgan, Elaine (1994). The Descent of the child. Souvenir Press. ISBN 0-285-63377-5. {{cite book}}: Cite has empty unknown parameter: |month= (help)
   • Morgan, Elaine (1997). The Aquatic Ape Hypothesis. Penguin. ISBN 0-285-63518-2. {{cite book}}: Cite has empty unknown parameter: |month= (help)
   • Morgan, Elaine (2008). The Naked Darwinist. Eildon Press. ISBN 0-9525620-30. {{cite book}}: Cite has empty unknown parameter: |month= (help)
4. Vaneechoutte, Verhaegen & Kuliukas (eds) (2011). Was Man More Aquatic in the Past? ~ Fifty Years after Hardy: Waterside Hypothesis Of Human Evolution. Bentham Publishers. ISBN 978-1-60805-244-8. {{cite book}}: |author= has generic name (help)
5. Attention: This template ({{cite doi}}) is deprecated. To cite the publication identified by doi:10.1038/35011048, please use {{cite journal}} (if it was published in a bona fide academic journal, otherwise {{cite report}} with |doi=10.1038/35011048 instead.
6. Attention: This template ({{cite doi}}) is deprecated. To cite the publication identified by doi: 10.1073/pnas.0805474105, please use {{cite journal}} (if it was published in a bona fide academic journal, otherwise {{cite report}} with |doi= 10.1073/pnas.0805474105 instead.
7. ^ Langdon JH (1997). "Umbrella hypotheses and parsimony in human evolution: a critique of the Aquatic Ape Hypothesis". J. Hum. Evol. 33 (4): 479–94. doi:10.1006/jhev.1997.0146. PMID 9361254.
8. Westenhöfer, M. (1942). Der Eigenweg des Menschen. Mannstaedt & Co.
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10. Morris, Desmond (1967). The Naked Ape. McGraw-Hill. p. 29. ISBN 0 09 948201 0. {{cite book}}: Cite has empty unknown parameter: |month= (help)
11. ^ Laden, G (4 August 2009). "Musings on the Aquatic Ape Theory". ScienceBlogs. Retrieved 2 September 2009.
12. ^ Kurtén, Björn (1993). Our earliest ancestors. Columbia University Press. ISBN 0-231-08061-1.
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20. Morgan, E (1982). The Aquatic Ape. Stein & Day Pub. ISBN 0-285-62509-8.
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58. Reynolds, Vernon (1991). Cold and Watery? Hot and Dusty? Our Ancestral Environment and Our Ancestors Themselves: an Overview (in Roede et al. 1991). Souvenir Press. p. 340. ISBN 0 285 63033 4.
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60. Groves, Colin (with David W.Cameron) (2004). Bones, Stones and Molecules. Elsevier Academic Press. pp. 68. ISBN 0 121 56933 0.

External links

• Elaine Morgan's site that presents the Aquatic Ape Hypothesis and discusses criticism against it
• Aquatic ape theory: Sink or Swim? – website critical of the AAH by Jim Moore
• Presentation by Elaine Morgan at TED July, 2009; Comment on ScienceBlogs by paleoanthropologist Greg Laden
• Template:DMOZ - other web resources

Template:Link FA

• Use dmy dates from August 2010
• Human evolution
• Hypotheses
• Aquatic organisms