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The chimpanzee–human last common ancestor, or CHLCA, is the last species shared as a common ancestor by humans and chimpanzees; it represents the node point at which the line to genus ''Homo'' split from genus ''Pan''. The last common ancestor of humans and chimps is estimated to have lived during the late Miocene, but possibly as late as Pliocene times — that is more recent than 5.3 million years ago. 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.〔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.〕 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 million 〔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.〕 to as recent as 4 million years ago.〔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.〕 The latter date and the argument for hybridization events are rejected by Wakeley〔Wakeley J (March 2008). "Complex speciation of humans and chimpanzees". Nature 452 (7184): E3–4; discussion E4. doi:10.1038/nature06805. PMID 18337768. "Patterson et al. suggest that the apparently short divergence time between humans and chimpanzees on the X chromosome is explained by a massive interspecific hybridization event in the ancestry of these two species. However, Patterson et al. do not statistically test their own null model of simple speciation before concluding that speciation was complex, and—even if the null model could be rejected—they do not consider other explanations of a short divergence time on the X chromosome. These include natural selection on the X chromosome in the common ancestor of humans and chimpanzees, changes in the ratio of male-to-female mutation rates over time, and less extreme versions of divergence with gene flow. I therefore believe that their claim of hybridization is unwarranted."〕 (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''. == Time estimates == 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Chimpanzee–human last common ancestor」の詳細全文を読む スポンサード リンク
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