The internal branch lengths estimated by distance methods such as neighbor-joining are shown to be biased to be short when the evolutionary rate differs among sites. The variable-invariable model for site heterogeneity fits the amino acid sequence data encoded by the mitochondrial DNA from Hominoidea remarkably well. By assuming the orangutan separation to be 13 or 16 Myr old, a maximum-likelihood analysis estimates a young date of 3.6 +/- 0.6 or 4.4 +/- 0.7 Myr (+/- 1 SE) for the human/chimpanzee separation, and these estimates turn out to be robust against differences in the assumed model for amino acid substitutions. Although some uncertainties still exist in our estimates, this analysis suggests that humans separated from chimpanzees some 4-5 Myr ago.
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