CHLCA

The chimpanzee–human last common ancestor, or CHLCA, is the last common ancestor shared by the extant Homo (human) and Pan (chimpanzee) genera of Hominini. Due to complex hybrid speciation, it is not possible to give a precise estimate on the age of this ancestral individual. While "original divergence" between populations may have occurred as early as 13 million years ago (Miocene), hybridization may have been ongoing until as recent as 4 million years ago (Pliocene).

Speciation from Pan to Homo appears to have been a long, drawn-out process. After the "original" divergence(s), there were, according to Patterson (2006), periods of hybridization between population groups and a process of alternating divergence and hybridization that lasted over several millions of years. Sometime during the late Miocene or early Pliocene the earliest members of the human clade completed a final separation from the lineage of Pan — with dates estimated by several specialists ranging from 13 millionto as recent as 4 million years ago. The latter date and the argument for hybridization events are rejected by Wakeley (see current estimates regarding complex speciation).

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 Pan genus and be given the taxonomic name Pan prior. However, to date no fossil has been identified as a probable candidate for the CHLCA or the taxon Pan prior.

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

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