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The chimpanzee–human last common ancestor (CHLCA) is the last common ancestor shared by the extant Homo (human) and Pan (chimpanzee and bonobo) genera of Hominini. Estimates of the divergence date vary widely from thirteen to five million years ago.

In human genetic studies, the CHLCA is useful as an anchor point for calculating single-nucleotide polymorphism (SNP) rates in human populations where chimpanzees are used as an outgroup, that is, as the extant species most genetically similar to Homo sapiens.

Despite extensive research, no direct fossil evidence of the CHLCA has been discovered. Fossil candidates like Sahelanthropus tchadensis, Orrorin tugenensis, and Ardipithecus ramidus have been debated as either being early hominins or close to the CHLCA. However, their classification remains uncertain due to incomplete evidence

Taxonomy

The taxon tribe Hominini was proposed to separate humans (genus Homo) from chimpanzees (Pan) and gorillas (genus Gorilla) on the notion that the least similar species should be separated from the other two. However, later evidence revealed that Pan and Homo are closer genetically than are Pan and Gorilla; thus, Pan was referred to the tribe Hominini with Homo. Gorilla now became the separated genus and was referred to the new taxon 'tribe Gorillini'.

Mann and Weiss (1996), proposed that the tribe Hominini should encompass Pan and Homo, grouped in separate subtribes. They classified Homo and all bipedal apes in the subtribe Hominina and Pan in the subtribe Panina. (Wood (2010) discussed the different views of this taxonomy.) A "chimpanzee clade" was posited by Wood and Richmond, who referred it to a tribe Panini, which was envisioned from the family Hominidae being composed of a trifurcation of subfamilies.

Richard Wrangham (2001) argued that the CHLCA species was very similar to the common chimpanzee (Pan troglodytes) – so much so that it should be classified as a member of the genus Pan and be given the taxonomic name Pan prior.

All the human-related genera of tribe Hominini that arose after divergence from Pan are members of the subtribe Hominina, including the genera Homo and Australopithecus. This group represents "the human clade" and its members are called "hominins".

Fossil evidence

No fossil has yet conclusively been identified as the CHLCA.

Sahelanthropus tchadensis is an extinct hominine with some morphology proposed (and disputed) to be as expected of the CHLCA, and it lived some 7 million years ago – close to the time of the chimpanzee–human divergence. But it is unclear whether it should be classified as a member of the tribe Hominini, that is, a hominin, as an ancestor of Homo and Pan and a potential candidate for the CHLCA species itself, or simply a Miocene ape with some convergent anatomical similarity to many later hominins.

Ardipithecus most likely appeared after the human-chimpanzee split, some 5.5 million years ago, at a time when gene flow may still have been ongoing. It has several shared characteristics with chimpanzees, but due to its fossil incompleteness and the proximity to the human-chimpanzee split, the exact position of Ardipithecus in the fossil record is unclear. It is most likely derived from the chimpanzee lineage and thus not ancestral to humans. However, Sarmiento (2010), noting that Ardipithecus does not share any characteristics exclusive to humans and some of its characteristics (those in the wrist and basicranium), suggested that it may have diverged from the common human/African ape stock prior to the human, chimpanzee and gorilla divergence.

Another candidate that has been suggested is Graecopithecus, though this claim is disputed as there is insufficient evidence to support the determination of Graecopithecus as hominin. This would put the CHLCA split in Southeast Europe instead of Africa.

The earliest fossils clearly in the human but not the chimpanzee lineage appear between about 4.5 to 4 million years ago, with Australopithecus anamensis.

Few fossil specimens on the "chimpanzee-side" of the split have been found; the first fossil chimpanzee, dating between 545 and 284 kyr (thousand years, radiometric), was discovered in Kenya's East African Rift Valley (McBrearty, 2005). All extinct genera listed in the taxobox are ancestral to Homo, or are offshoots of such. However, both Orrorin and Sahelanthropus existed around the time of the divergence, and so either one or both may be ancestral to both genera Homo and Pan.

Due to the scarcity of fossil evidence for CHLCA candidates, Mounier (2016) presented a project to create a "virtual fossil" by applying digital "morphometrics" and statistical algorithms to fossils from across the evolutionary history of both Homo and Pan, having previously used this technique to visualize a skull of the last common ancestor of Neanderthal and Homo sapiens.

Age estimates

An estimate of 10 to 13 million years for the CHLCA was proposed in 1998, and a range of 7 to 10 million years ago is assumed by White and colleagues in 2009. A 2016 study analyzed transitions at CpG sites in genome sequences, which exhibit a more clocklike behavior than other substitutions, arriving at an estimate for human and chimpanzee divergence time of 12.1 million years. Studies in the early 2020s suggest a more recent CHLCA, such as between 9.3 and 6.5 million years ago in 2021, and studies giving even more recent dates are cited in a 2022 article.

Gene flow

A source of confusion in determining the exact age of the Pan–Homo split is evidence of a more complex speciation process than a clean split between the two lineages. Different chromosomes appear to have split at different times, possibly over as much as a 4-million-year period, indicating a long and drawn out speciation process with large-scale gene flow events between the two emerging lineages as recently as 6.3 to 5.4 million years ago, according to Patterson et al. (2006).

Speciation between Pan and Homo occurred over the last 9 million years. Ardipithecus probably branched off of the Pan lineage in the middle Miocene Messinian. After the original divergences, there were, according to Patterson (2006), periods of gene flow between population groups and a process of alternating divergence and gene flow that lasted several million years. Some time during the late Miocene or early Pliocene, the earliest members of the human clade completed a final separation from the lineage of Pan – with date estimates ranging from 13 million to as recent as 4 million years ago. The latter date was in particular based on the similarity of the X chromosome in humans and chimpanzees, a conclusion rejected as unwarranted by Wakeley (2008), who suggested alternative explanations, including selection pressure on the X chromosome in the populations ancestral to the CHLCA.

Complex speciation and incomplete lineage sorting of genetic sequences seem to also have happened in the split between the human lineage and that of the gorilla, indicating "messy" speciation is the rule rather than the exception in large primates. Such a scenario would explain why the divergence age between the Homo and Pan has varied with the chosen method and why a single point has so far been hard to track down.

See also

• History of hominoid taxonomy
• List of human evolution fossils (with images)

Notes

1. "Patterson et al. suggest that the apparently short divergence time between humans and chimpanzees on the X chromosome is explained by a massive interspecific gene flow event in the ancestry of these two species. However, Patterson et al. do not statistically test their own null model of simple speciation before concluding that speciation was complex, and—even if the null model could be rejected—they do not consider other explanations of a short divergence time on the X chromosome. These include natural selection on the X chromosome in the common ancestor of humans and chimpanzees, changes in the ratio of male-to-female mutation rates over time, and less extreme divergence versions with gene flow. I, therefore, believe that their claim of gene flow is unwarranted."

References

1. Mann, Alan; Mark Weiss (1996). "Hominoid Phylogeny and Taxonomy: a consideration of the molecular and Fossil Evidence in an Historical Perspective". Molecular Phylogenetics and Evolution. 5 (1): 169–181. doi:10.1006/mpev.1996.0011. PMID 8673284.
2. B. Wood (2010). "Reconstructing human evolution: Achievements, challenges, and opportunities". Proceedings of the National Academy of Sciences. 107 (Suppl 2): 8902–8909. Bibcode:2010PNAS..107.8902W. doi:10.1073/pnas.1001649107. PMC 3024019. PMID 20445105.
3. Wood and Richmond.; Richmond, BG (2000). "Human evolution: taxonomy and paleobiology". Journal of Anatomy. 197 (Pt 1): 19–60. doi:10.1046/j.1469-7580.2000.19710019.x. PMC 1468107. PMID 10999270.
4. "Out of the Pan, Into the Fire" in: Frans B. M. De Waal, ed. (2001). Tree of Origin: What Primate Behavior Can Tell Us About Human Social Evolution. Harvard University Press. pp. 124–126. ISBN 9780674010048.
5. Bradley, B. J. (2006). "Reconstructing Phylogenies and Phenotypes: A Molecular View of Human Evolution". Journal of Anatomy. 212 (4): 337–353. doi:10.1111/j.1469-7580.2007.00840.x. PMC 2409108. PMID 18380860.
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7. ^ Wood, Bernard; Harrison, Terry (2011). "The evolutionary context of the first hominins". Nature. 470 (7334): 347–52. Bibcode:2011Natur.470..347W. doi:10.1038/nature09709. PMID 21331035. S2CID 4428052.
8. ^ Wolpoff, Milford H. (1996). Human Evolution. McGraw-Hill, Incorporated. ISBN 978-0070718333.
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10. Fuss, Jochen; Spassov, Nikolai; Begun, David R.; Böhme, Madelaine (2017). "Potential hominin affinities of Graecopithecus from the Late Miocene of Europe". PLOS ONE. 12 (5): e0177127. Bibcode:2017PLoSO..1277127F. doi:10.1371/journal.pone.0177127. PMC 5439669. PMID 28531170.
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12. Dickinson, Kevin (20 April 2019). "New fossils suggest human ancestors evolved in Europe, not Africa". Big Think. Retrieved 23 October 2022.
13. McBrearty, Sally; Nina G. Jablonski (2005). "First fossil chimpanzee". Nature. 437 (7055): 105–108. Bibcode:2005Natur.437..105M. doi:10.1038/nature04008. PMID 16136135. S2CID 4423286.
14. "'Virtual fossil' reveals last common ancestor of humans and Neanderthals". 18 December 2015. Retrieved 30 June 2019.
15. Mounier, Aurélien; Mirazón Lahr, Marta (2016). "Virtual ancestor reconstruction: Revealing the ancestor of modern humans and Neandertals". Journal of Human Evolution. 91: 57–72. doi:10.1016/j.jhevol.2015.11.002. PMID 26852813.
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18. Moorjani, Priya; Amorim, Carlos Eduardo G.; Arndt, Peter F.; Przeworski, Molly (2016). "Variation in the molecular clock of primates". Proceedings of the National Academy of Sciences. 113 (38): 10607–10612. Bibcode:2016PNAS..11310607M. doi:10.1073/pnas.1600374113. ISSN 0027-8424. PMC 5035889. PMID 27601674.
19. Almécija, Sergio; et al. (7 May 2021). "Fossil apes and human evolution". Science. 372 (6542). doi:10.1126/science.abb4.
20. Poszewiecka, Barbara; et al. (25 August 2022). "Revised time estimation of the ancestral human chromosome 2 fusion". BMC Genomics. 23 (616).
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External links

• Human Timeline (Interactive) – Smithsonian, National Museum of Natural History (August 2016).
• Locomotion and posture from the common hominoid ancestor to fully modern hominins, with special reference to the last common panin/hominin ancestor - R H Crompton E E Vereecke and S K S Thorpe, Journal of Anatomy, April 2008

v t e Apes
Extant ape species	Homo Human (H. sapiens) Pan Bonobo (P. paniscus) Chimpanzee (P. troglodytes) Gorilla Eastern gorilla (G. beringei) Western gorilla (G. gorilla) Orangutan Sumatran orangutan (P. abelii) Bornean orangutan (P. pygmaeus) Tapanuli orangutan (P. tapanuliensis) Gibbon (family: Hylobatidae)
Study of apes	Great ape language Dian Fossey Birutė Galdikas Jane Goodall Chimpanzee genome project Human Genome Project Gladys Kalema-Zikusoka Neanderthal genome project Willie Smits Lone Drøscher Nielsen Ian Redmond Elgin Center Iowa Primate Learning Sanctuary Borneo Orangutan Survival Primate archaeology
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