Thechimpanzee–human last common ancestor (CHLCA) is thelast common ancestor shared by the extantHomo (human) andPan (chimpanzee andbonobo) genera ofHominini. 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 calculatingsingle-nucleotide polymorphism (SNP) rates in human populations where chimpanzees are used as anoutgroup, that is, as the extant species most genetically similar toHomo sapiens.
Despite extensive research, no direct fossil evidence of the CHLCA has been discovered. Fossil candidates likeSahelanthropus tchadensis,Orrorin tugenensis, andArdipithecus ramidus have been debated as either being early hominins or close to the CHLCA. However, their classification remains uncertain due to incomplete evidence.
| Hominoidea (hominoids, apes) | |
The taxontribeHominini was proposed to separate humans (genusHomo) from chimpanzees (Pan) and gorillas (genusGorilla) on the notion that the least similar species should be separated from the other two. However, later evidence revealed thatPan andHomo are closer genetically than arePan andGorilla; thus,Pan was referred to the tribeHominini withHomo.Gorilla now became the separated genus and was referred to the new taxon 'tribeGorillini'.
Mann and Weiss (1996), proposed that the tribeHominini should encompassPan andHomo, grouped in separate subtribes.[1] They classifiedHomo and all bipedal apes in the subtribeHominina andPan in the subtribePanina. (Wood (2010) discussed the different views of this taxonomy.)[2] A "chimpanzee clade" was posited by Wood and Richmond, who referred it to a tribePanini, which was envisioned from the familyHominidae being composed of a trifurcation of subfamilies.[3]
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 genusPan and be given the taxonomic namePan prior.[4]
All the human-related genera of tribe Hominini that aroseafter divergence fromPan are members of the subtribeHominina, including the generaHomo andAustralopithecus. This group represents "the human clade" and its members are called "hominins".[5]
No fossil has yet conclusively been identified as the CHLCA.
Sahelanthropus tchadensis is an extincthominine 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 ofHomo andPan 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 ofArdipithecus in the fossil record is unclear.[6] However, Sarmiento (2010), noting thatArdipithecus 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.[7]
Orrorin, which lived roughly 6 million years ago, seems, based on the fossils that were recovered, to share no derived features of hominoid great-ape relatives.[8] In contrast, "Orrorin seems to share several apomorphic features with modern humans, as well as some withaustralopithecines, including the presence of anobturator externus groove, elongated femoral neck, anteriorly twisted head (posterior twist inAustralopithecus),anteroposteriorly compressed femoral neck, asymmetric distribution of cortex in the femoral neck, shallow superior notch, and a well developed gluteal tuberosity which coalesces vertically with the crest that descends the femoral shaft posteriorly."[8] It does, however, also share many of such properties with several Miocene ape species, even showing some transitional elements between basal apes like theAegyptopithecus andAustralopithecus.[9]
Another candidate that has been suggested isGraecopithecus, though this claim is largely disputed as there is insufficient evidence to support the determination ofGraecopithecus as hominin or hominin-related.[10][11]
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 inKenya's East AfricanRift Valley (McBrearty, 2005).[12] All extinct genera listed in the taxobox[which?] are ancestral toHomo, or are offshoots of such. However, bothOrrorin andSahelanthropus existed around the time of the divergence, and so either one or both may be ancestral to both generaHomo andPan.
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 bothHomo andPan, having previously used this technique to visualize a skull of the last common ancestor ofNeanderthal andHomo sapiens.[13][14]
An estimate of 10 to 13 million years for the CHLCA was proposed in 1998,[15] and a range of 7 to 10 million years ago is assumed by White and colleagues in 2009.[16] A 2016 study analyzed transitions atCpG 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.[17]
Studies in the 2020s suggest a more recent CHLCA, such as between 9.3 and 6.5 million years ago in a 2021 article,[18] and between 6.6 and 4.7 million years ago in a 2022 article.[19] In a landmark 2025 paper incomparative genomics, the completetelomere-to-telomere sequences of sixhominoid genomes were used to estimate the CHLCA split as between 6.3 and 5.5 million years ago.[20]
A source of confusion in determining the exact age of thePan–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).[21]
Speciation betweenPan andHomo occurred over the last 9 million years.Ardipithecus probably branched off of thePan lineage in the middle MioceneMessinian.[22][23] 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.[21] Some time during the lateMiocene or earlyPliocene, the earliest members of the human clade completed a final separation from the lineage ofPan – with date estimates ranging from 13 million[15] to as recent as 4 million years ago.[21] The latter date was in particular based on the similarity of theX 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.[note 1]
Complex speciation andincomplete 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.[25][26] Such a scenario would explain why the divergence age between theHomo andPan has varied with the chosen method and why a single point has so far been hard to track down.
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