Chimpanzee–human last common ancestor

Not to be confused with Hominoidea, Hominidae, Homininae, or Hominina.

Chimpanzee–human last common ancestor Temporal range: 5.4–0Ma PreꞒ Ꞓ O S D C P T J K Pg N
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	Primates
Infraorder:	Simiiformes
Superfamily:	Hominoidea
Family:	Hominidae
Subfamily:	Homininae
Type species
Homo sapiens Linnaeus, 1758
Genera
Tribe Panini Genus Pan Pan troglodytus (common chimpanzee) Pan paniscus (bonobo) Tribe Hominini Subtribe Hominina Homo Subtribe Australopithecina † Paranthropus † Australopithecus † Sahelanthropus † Orrorin † Ardipithecus † Kenyanthropus

The chimpanzee–human last common ancestor (CHLCA, CLCA, or C/H LCA) is the last species that the genera Homo and Pan (i.e. humans on one hand and bonobos and common chimpanzees on the other) share as a common ancestor. It is estimated to have lived during the late Miocene.

Speciation appears to have been unusually drawn-out, with an original divergence as much as 7 to 13 million years ago, but with ongoing hybridization during several millions of years, with the final separation dating to as late as 5 to 6 million years ago.^[1]

The species to which the CHLCA belonged is sometimes given the taxonomic name Pan prior, based on a suggestion by Richard Wrangham (2001). Wrangham argued that the CHLCA was so similar to chimpanzee (Pan troglodytes), that it should be classified as a member of the Pan genus.^[2] No fossil has directly been identified as representing Pan prior to date. The fossil finds of Sahelanthropus are closest in age and expected morphology to the CHLCA.

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, as the extant species most genetically similar to Homo sapiens.

Time estimates

Historical studies

The earliest studies of apes suggested the CHLCA may have been as old as 25 million years; however, protein studies in the 1970s suggested the CHLCA was less than 8 million years in age. Genetic methods based on orangutan–human and gibbon–human LCA times were then used to estimate a chimpanzee–human LCA of 5 to 7 million years.

“	One no longer has the option of considering a fossil older than about eight million years as a hominid no matter what it looks like.	”
—V. Sarich, Background for man^[3]

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. ^[4] Later, Vincent Sarich concluded that the T_CHLCA was no greater 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.^[5]

Current estimates

Since the 1990s, the estimation of the T_CHLCA has become less certain, and there is genetic as well as paleontological support for increasing T_CHLCA beyond the 5 to 7 million years range accepted during the 1970s and 1980s. An estimate of T_CHLCA at 10 to 13 million years was proposed in 1998,^[6] and a range of 7 to 10 million years ago is assumed by White et a. (2009):

In effect, there is now no a priori reason to presume that human-chimpanzee split times are especially recent, and the fossil evidence is now fully compatible with older chimpanzee–human divergence dates [7 to 10 Ma...
—White et al. (2009), ^[7]

A source of confusion in determining the exact age of the Pan–Homo split is evidence of a more complex speciation process rather 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 late as 6.3 to 5.4 million years ago according to Patterson et al. (2006).^[1] The assumption of late hybridization was in particular based on the similarity of the X chromosome in humans and chimpanzees, suggesting a divergence as late as some 4 million years ago. This conclusion was rejected as unwarranted by Wakeley (2008), who suggested alternative explanations, including selection pressure on the X chromosome in the populations ancestral to the CHLCA.^[8]

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.^[9]^[10] 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.

Taxonomy

The species to which the CHLCA belonged is sometimes given the taxonomic name Pan prior, based on a suggestion by Richard Wrangham (2001). Wrangham argued that the CHLCA was so similar to chimpanzees (Pan troglodytes), that it should be classified as a member of the Pan genus.^[2] No fossil has been directly identified as representing Pan prior to date. The fossil finds of Sahelanthropus are closest in age and expected morphology to the CHLCA, suggesting an age older than 7 million years, whereas the period of ongoing hybridization may include the temporal ranges of Orrorin and Ardipithecus.

The descendants of the CHLCA are the Hominini, defined as the descendants of the CHLCA to the inclusion of the genus Homo and exclusion of the genus Pan, i.e. representing "the human clade",^[11]^[12] and the Panini, the "chimpanzee clade". The taxon Hominini was proposed on the basis of the idea that the least similar species of a trichotomy should be separated from the other two. Some earlier classification schemes include the genus Pan (chimpanzees) within the Hominini, but this classification is now rarely followed. Sahelanthropus tchadensis is an extinct hominid species that lived 7 million years ago, very close to the time of the chimpanzee–human divergence; it is hence unclear whether it should be considered a member of the Hominini tribe, or as a member of CHLCA.

Few fossil specimens on the Pan side of the split have been found, the first fossil chimpanzee discovery being published in 2005,^[13] dating to between 545 ± 3 kyr (thousand years) and 284 ± 12 kyr via argon–argon dating, from Kenya's East African Rift Valley. All of the extinct genera listed in the table to the right are ancestral to Homo, or are offshoots of such. However, both Orrorin and Sahelanthropus existed around the time of the split, and so may be ancestral to all three extant species.

In the proposal of Mann and Weiss (1996),^[14] the tribe Hominini includes Pan as well as Homo, but within separate subtribes. Homo (and, by inference, all bipedal apes) is in the subtribe Hominina, while Pan is in the subtribe Panina. Wood (2010) discusses the different views of this taxonomy.^[15]

References