TOWARD A MORE ACCURATE TIME-SCALE FOR THE HUMAN MITOCHONDRIAL-DNA TREE

Several estimates of the time of occurrence of the most recent common mitochondrial DNA (mtDNA) ancestor of modern humans have been made. Estimates derived from noncoding regions based on a model that classifies sites into two categories (variable and invariable) have been consistently older than those derived from the third positions of codons. This discrepancy can be attributed to a violation of the assumption of rate homogeneity among variable sites when analyzing the noncoding regions. Additional data from the partial control region sequences allow us to take into account some of this further heterogeneity. By assigning the sites to three classes (highly variable, moderately variable, and invariable) and by assuming that the last common mtDNA ancestor of humans and chimpanzees lived 4 million years ago, the most recent common mtDNA ancestor of humans is estimated to have occurred 211,000 +/- 111,000 years ago (+/- 1 SE), consistent with the estimate, 101,000 +/- 52,000 years, made from third positions of codons and also with those proposed previously. We used the same technique to estimate when a putative expansion of modem humans out of Africa took place and estimated a time of 89,000 +/- 69,000 years ago. Even though the standard errors of these estimates are large, they allow us to reject the multiregional hypothesis of modem human origin.