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. Due to complex hybrid speciation, it is not currently possible to give a precise estimate on the age of this ancestral population. While "original divergence" between populations may have occurred as early as 13 million years ago (Miocene), hybridization may have been ongoing until as recently as 4 million years ago (Pliocene (SNP) rates in human populations where chimpanzees are used as an outgroup, that is, as the extant species most genetically similar to ''Homo sapiens''.

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. A possible candidate is '' Graecopithecus''. This would put the CHLCA split in Southeast Europe instead of Africa.

'' 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 hybridization 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.

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 T_CHLCA of 10 to 13 million years was proposed in 1998, and a range of 7 to 10 million years ago is assumed by White et al. (2009):

Some researchers tried to estimate the age of the CHLCA (T_CHLCA) using biopolymer structures that differ slightly between closely related animals. Among these researchers, Allan C. Wilson and Vincent Sarich were pioneers in the development of the molecular clock for humans. Working on protein sequences, they eventually (1971) determined that apes were closer to humans than some paleontologists perceived based on the fossil record. Later, Vincent Sarich concluded that the T_CHLCA was no older than 8 million years in age, with a favored range between 4 and 6 million years before present.

This paradigmatic age has stuck with molecular anthropology until the late 1990s. Since the 1990s, the estimate has again been pushed towards more-remote times, because studies have found evidence for a slowing of the molecular clock as apes evolved from a common monkey-like ancestor with monkeys, and humans evolved from a common ape-like ancestor with non-human apes.

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.

Hybrid speciation

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 hybridization 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 hybridization between population groups and a process of alternating divergence and hybridization that lasted several million years. Some time during the late Miocene or early Pliocene

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