WHEN DID EVE LIVE? AN EVOLUTIONARY DETECTIVE STORY

Abstract

Various estimates of the time at which the human mitochondrial Eve lived have ranged from as little as 60,000 yr to more than 500,000 yr ago. Because of this immense range, it is impossible to distinguish between single-origin and multiple-origins hypotheses for the evolution of our species. In an attempt to reduce the uncertainty, I have examined the largest available body of sequence information, comprising the mitochondrial control region, for clues to how the observed diversity arose. In this region it is possible to show, by examining the distribution of polymorphic sites, that transitions have occurred at some sites at a much higher rate than at others. Computer simulations can, when two rates for transitions are postulated, provide close approximations to the distribution of substitutions seen in the actual data. The "best fit" was obtained when the rate at ¾ of the sites was 4 times the transversion rate, and the rate at the remainder 160 times the transversion rate. The likelihood of such a high rate at some sites helps to explain why tree-building methods employing these data have provided so little phylogenetic information. Furthermore, it is possible to show that transversions do not appear to occur preferentially at these transition "hot-spot" sites and that such huge differences in substitution rates are not seen for transversions, suggesting that the rules governing the mutation and acceptance or rejection of transversions are different from those governing transitions. The great majority of transversions appear to occur at a low rate throughout the region. Thus, methods for determining the age of Eve that are based on recent divergence in human populations, or on applying a mutation probability matrix based on an assumption of uniform mutation rates, are likely to result in underestimates. The rate of accumulation of transversions is shown to be a more accurate estimator of the age of Eve. The conclusion is reached that Eve probably lived (depending on when the ancestors of humans and chimpanzees diverged) between 436,000 and 806,000 yr ago.

Keywords: D-loop; transitions; transversions; unequal mutation rates.