Chimpanzee–human last common ancestor
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. The CHLCA is frequently cited as an anchor for the molecular most recent common ancestor (MRCA) determination because the two species of the genus Pan, the bonobos and the chimpanzee, are the species most genetically similar to Homo sapiens.
The age of the CHLCA is an estimate. The fossil find of Ardipithecus kadabba, Sahelanthropus tchadensis, and Orrorin tugenensis are closest in age and expected morphology to the CHLCA and suggest the LCA (last common ancestor) is older than 7 million years. 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 6 million years, and LCA times between 5 and 7 million years ago are currently used in the literature.[note 1]
|“||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
Because chimps and humans share a matrilineal ancestor, establishing the geological age of that last ancestor allows the estimation of the mutation rate. However, fossils of the exact last common ancestor would be an extremely rare find. The CHLCA is frequently cited as an anchor for mt-TMRCA determination because chimpanzees are the species most genetically similar to humans. However, there are no known fossils that represent that CHLCA. It is believed that there are no proto-chimpanzee fossils or proto-gorilla fossils that have been clearly identified. However, Richard Dawkins, in his book The Ancestor's Tale, proposes that robust australopithecines such as Paranthropus are the ancestors of gorillas, whereas some of the gracile australopithecines are the ancestors of chimpanzees (see Homininae).
|“||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), 
Some researchers tried to estimate the age of the CHLCA (TCHLCA) using biopolymer structures which 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 determined that apes were closer to humans than some paleontologists perceived based on the fossil record.[note 2] Later Vincent Sarich concluded that the TCHLCA 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, when others began questioning the certainty of the assumption. Currently, the estimation of the TCHLCA is less certain, and there is genetic as well as paleontological support for increasing TCHLCA. A 13 million year TCHLCA is one proposed age.
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. Particularly the X chromosome shows very little difference between Humans and chimpanzees, though this effect may also partly be the result of rapid evolution of the X chromosome in the last common ancestors. Complex speciation and incomplete lineage sorting of genetic sequences seem to also have happened in the split between our lineage and that of the gorilla, indicating "messy" speciation is the rule rather than exception in large-bodied primates. Such a scenario would explain why divergence age between the Homo and Pan has varied with the chosen method and why a single point has been so far hard to track down.
- Studies have pointed to the slowing molecular clock as monkeys evolved into apes and apes evolved into humans. In particular, Macaque monkey mtDNA has evolved 30% more rapidly than African ape mtDNA.
- "If man and old world monkeys last shared a common ancestor 30 million years ago, then man and African apes shared a common ancestor 5 million years ago..." Sarich & Wilson (1971)
- Background for man: readings in physical anthropology, 1971
- White TD, Asfaw B, Beyene Y, et al. (October 2009). "Ardipithecus ramidus and the paleobiology of early hominids". Science 326 (5949): 75–86. doi:10.1126/science.1175802. PMID 19810190.
- Arnason U, Gullberg A, Janke A (December 1998). "Molecular timing of primate divergences as estimated by two nonprimate calibration points". J. Mol. Evol. 47 (6): 718–27. doi:10.1007/PL00006431. PMID 9847414.
- Patterson N, Richter DJ, Gnerre S, Lander ES, Reich D (June 2006). "Genetic evidence for complex speciation of humans and chimpanzees". Nature 441 (7097): 1103–8. doi:10.1038/nature04789. PMID 16710306.
- Wakeley J (March 2008). "Complex speciation of humans and chimpanzees". Nature 452 (7184): E3–4; discussion E4. doi:10.1038/nature06805. PMID 18337768.
- Scally A, Dutheil JY, Hillier LW, et al. (March 2012). "Insights into hominid evolution from the gorilla genome sequence". Nature 483 (7388): 169–75. doi:10.1038/nature10842. PMC 3303130. PMID 22398555.
- Van Arsdale, A.P. "Go, go, Gorilla genome". The Pleistocene Scene – A.P. Van Arsdale Blog. Retrieved 16 November 2012.
- De Waal, Frans B. M (2002-10-15). Tree of Origin: What Primate Behavior Can Tell Us About Human Social Evolution. pp. 124–126. ISBN 9780674010048.