Chimpanzee-human last common ancestor

The chimpanzee-human last common ancestor (CHLCA, CLCA, or C/H LCA) is the last species, a species of African apes, that humans, bonobos and chimpanzees share as a common ancestor.

The CHLCA is generally used as an anchor point for calculating single-nucleotide polymorphism (SNP) rates in human genetic studies where chimpanzees are used as an outgroup. The CHLCA is frequently cited as an anchor for molecular TMRCA (Time to most recent common ancestor) determination because the two species of the genus pan, the Bonobos and the Chimpanzee, are the species most genetically similar to Homo sapiens.

Time estimates

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 of the CHLCA and suggest the LCA is older than 7 million years. Earliest studies of apes suggest 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 have then been used to estimate Chimpanzee/Human LCA of 6 million years and LCA times between 5 and 7 million years 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[1]

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-T_MRCA 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: Paranthropus, are the ancestors of gorillas, whereas some of the gracile australopithecus are the ancestors of chimpanzees (see Homininae).

“	In effect, there is now not 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), [2]

Some researchers tried to estimate the age of the CHLCA (T_CHLCA) 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 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, when others began questioning the certainty of the assumption. Currently, the estimation of the T_CHLCA is less certain, and there is genetic as well as paleontological support for increasing T_CHLCA. A 13 million year T_CHLCA is one proposed age.^[2][3]

Pan Prior

Pan prior is the name suggested by British biological anthropologist Richard Wrangham for the last common ancestor of humans (Homo sapiens) and chimpanzees (Pan troglodytes). This species is supposed, on the basis of DNA reconstruction (no fossil remains have been found), to have existed prior to six million years ago, when the human and chimpanzee lines are thought to have diverged. Pan prior lived an arboreal existence in the forests of Africa. It was initially thought that an Ice Age, around seven million years ago, caused forests to shrink thus prompting some members of the species to venture into the savannah, becoming the ancestors of humanity.^[4] However, the discovery of Ardipithecus ramidus shows that bipedal locomotion was used both on the ground and in the trees around 4.4 million years ago. Researchers infer from the form of her pelvis and limbs and the presence of her abductable hallux, that she was a Facultative biped: bipedal when moving on the ground, but quadrupedal when moving about in tree branches.

See also

• Ardipithecus ramidus
• Australopithecus
• Homininae
• Human evolution
• Human evolutionary genetics
• Humanzee
• Timeline of human evolution

Notes

1. ^ Studies have pointed to the slowing molecular clock as monkeys evolved into apes and apes evolved into humans. In particular Macaque mtDNA is evolved 30% more rapidly than African Ape mtDNA.
2. ^ "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)

References

1. ^ Background for man: readings in physical anthropology, 1971
2. ^ ^{a b} 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. 
3. ^ 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. 
4. ^ The World Without Us by Alan Weisman, 2007.