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## An example: Mitochondrial Eve

Cann et al. [1] sampled mitochondrial DNA from 147 humans of diverse racial and geographic origins. Based on the amount of sequence divergence they found among genomes in their sample and independent estimates of the rate of sequence evolution, they inferred that the mitochondria in their sample had their most recent common ancestor about 200,000 years ago. Because all of the most ancient lineages in their sample were from individuals of African ancestry, they also suggested that mitochondrial Eve lived in Africa. They used these arguments as evidence for the ``Out of Africa'' hypothesis for modern human origins, i.e., the hypothesis that anatomically modern humans arose in Africa about 200,000 years ago and displaced other members of the genus Homo in Europe and Asia as they spread. What does the coalescent tell us about their conclusion?

Well, we expect all mitochondrial genomes in the sample to have had a common ancestor about generations ago. Why rather than ? Because mitochondrial genomes are haploid. Furthermore, since we all got our mitochondria from our mothers, in this case refers to the effective number of females.

Given that a human generation is about 20 years, a coalescence time of 200,000 years implies that the mitochondrial genomes in the Cann et al. sample have their most recent common ancestor about 10,000 generations ago. If the effective number of females in the human populations is 5000, that's exactly what we'd expect. While 5000 may sound awfully small, given that there are about 3 billion women on the planet now, remember that until the recent historical past (no more than 500 generations ago) the human population was small and humans lived in small hunter-gatherer groups, so an effective number of females of 5000 and a total effective size of 10,000 may not be unreasonable. If that's true, then the geographical location of mitochondrial Eve need not tell us anything about the origin of modern human populations, because there had to be a coalescence somewhere. There's no guarantee, from this evidence alone, that the Y-chromosome Adam would have lived in Africa, too. Having said that, my limited reading of the literature suggests that other dara are consistent with the ``Out of Africa'' scenario. Y-chromosome polymorphisms, for example, are also consistent with the ``Out of Africa'' hypothesis [5]. Interestingly, dating of those polymorphisms suggetsts that Y-chromosome Adam left Africa 35,000 - 89,000 years ago.

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Kent Holsinger 2012-09-29