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Humans mostly out of Africa?

Genetic evidence and the modern human origins debate:

A continued debate in anthropology concerns the evolutionary origin of ‘anatomically modern humans’ (Homo sapiens sapiens). Different models have been proposed to examine the related questions of (1) where and when anatomically modern humans first appeared and (2) the genetic and evolutionary relationship between modern humans and earlier human populations. Genetic data have been increasingly used to address these questions. Genetic data on living human populations have been used to reconstruct the evolutionary history of the human species by considering how global patterns of human variation could be produced given different evolutionary scenarios. Of particular interest are gene trees that reconstruct the time and place of the most recent common ancestor of humanity for a given haplotype and the analysis of regional differences in genetic diversity. Ancient DNA has also allowed a direct assessment of genetic variation in European Neandertals. Together with the fossil record, genetic data provide insight into the origin of modern humans. The evidence points to an African origin of modern humans dating back to 200,000 years followed by later expansions of moderns out of Africa across the Old World. What is less clear is what happened when these early modern humans met preexisting ‘archaic human’ populations outside of Africa. At present, it is difficult to distinguish between a model of total genetic replacement and a model that includes some degree of genetic mixture.

The author, John Relethford, seems to have shifted from a Multiregionalist to Out-of-Africa bias over the past 10 years. But the review emphasizes that we don’t need to be shoe-horned into one parsimonious model. About 15 years ago it seems that there were two competing hypotheses, anagenetic Multiregionalism and a blitzkrieg Out-of-Africa scenario. The former emphasized the deep phylogenies localized to particular regions of the Old World, connected by the sweeping action of positively selected genes. It was an extension of a model of phyletic gradualism applied to to a world-wide mammalian species. The latter was often presented as an explosion of one deme of a few hundred individuals from East Africa who went on to conquer the whole world. Though it does seem that the strong likelihood is most of our ancestry as humans derives from Africa within the last 200,000 years, I think Relethford’s point that many of the genetic data adduced to support the blitzkrieg model can not establish that no alleles introgressed from archaic lineages must be taken seriously. Rather, the plausibility of this option must be evaluated in the framework of other data and assumptions (e.g., what is the expectation of sterility of hybrids between human species?). If in 5-10 years the results which imply introgression from archaic lineages do not hold up under scrutiny then I think it is time to modulate our priors.

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