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How did modern humans settle the world?

Gene ExpressionBy Razib KhanJuly 10, 2011 8:47 AM


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In lieu of lots of text, above is a stylized representation of the routes which Neo-Africans took ~50 thousand years ago from their point of departure to parts unknown. The two colors represent two models. The red lines show two major streams issuing out of Africa, a northern route which pushed into the heart of Central Asia, and a southern oceanic one, which pushed all the way into Australia. The second differs, with eastern and western branches of non-African humanity. The models really start to break down within the last ~10,000 years. For example, by either model India has seen an admixture even between the two branches in the Holocene. Additionally, there may have been "false dawns" and admixtures. In the early 2000s I accept the probable likelihood of the first model. But today I am more leaning toward the second. What's your stance, and why? I'll give my rationale below.... The primary reason I'm skeptical of the northern vs. southern route is that Melanesians are clearly southerners, while East Asians and Europeans are northerners. But the phylogenetic stuff I've run myself and that I've seen using autosomal data sets indicate that East Asians and Melanesians are far too close for Melanesians to be the distant outgroup to the European-East Asian clade. The archaeology is clear that Sahul, Papua and Australia, were settled ~40-50,000 years B.P., while the molecular evolutionary and archaeological evidence which points to a common origin of East and West Eurasians implies a date of ~20-30,000 years for the last common ancestor (probably the Gravettians or their relatives). Something doesn't smell right here. It may be that the contemporary Melanesians and Australians are more recent arrivals, but I'm skeptical of that. Additionally, in "Ancestral South Indians" are clearly closer to East Eurasians than they are to West Eurasians. It seems that the West Eurasians are best modeled as an outgroup to East Eurasians and ancient South Eurasians (granted, the latter clade has relatively distinct and distantly related branches). I could say more. But those are the big issues. I just have a hard time accepting the ~20-30,000 year figure to the last common ancestor for Europeans and East Asians if Melanesians settled Sahul ~40-50,000 years B.P. This is why I'm favorable to a shift of the mutation rate which recalibrates these estimates. Doubling the last common ancestor of East and West Eurasians to ~40-60,000 years would make much more sense of the model in my own head.

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