Chimpanzee–human last common ancestor

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''."Out of the ''Pan'', Into the Fire" in:

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. 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

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