Thursday, July 14, 2005

Model rising....(?)   posted by Razib @ 7/14/2005 06:03:00 AM

Eric Trinkaus says: conjunction with the emerging chronology of the earliest modern humans, the paleontological data indicate an assimilation model for modern human origins, in which the earliest modern humans emerged in eastern Africa, dispersed briefly into southwestern Asia, and then subsequently spread into the remainder of Africa and southern Asia, eventually into higher latitude Eurasia. The earliest modern humans outside of the core area of eastern Africa can be understood only if a variable degree of admixture with regional groups of late archaic humans occurred....

This should not surprise readers. John foreshadowed these comments, and I'm sure he'll have more to say later. Henry has been pointing out the differences between a few neutral markers (Y and mtDNA) and various functional loci for years in terms of the picture they present of the human past. Greg has been pushing the 2s hypothesis recently. These models imply that we have to rework the "Human Species Concept" a bit. Additionally, if there are ancient alleles from "archaics" which have introgressed into a predominantly neo-African worldwide human population, that might explain some of the incredible human adaptational flexibility. Where as before a pure African replacement model envisages the development of localized races out of the variational genetic background of a small founder population of East Africans within the last 100,000 years, this model can leverage the genetic background of the entire extent homonid population of the Old World. For instance, I have already pointed out some groups in the southern hemisphere seem to be rather dark for their latitude. The hypothesis I offered was that perhaps MC1R, the locus at the heart of skin color regulation, jumped from northern Eurasian archaics but could not cross the fitness valley of Southeast Asia over to Australia.1 While the southern moderns swept past ancient homonid populations who were tropically adapted just as they were, the northerners encountered peoples with hundreds of thousands and possibly millions of years of different adaptationally relevant alleles. Heather Norton at Penn State has already done research which suggests that East Eurasians and West Eurasians have different variants for the MC1R locus which controls melanin production and so results in lighter skin color outside the tropics (unpublished, though presented at a conference in Europe). So if the replacement model is correct the African populations brought different variants to the north and directional selection fixed alternative alleles (or the mutation emerged in situ within a particular geographic zone, which would fit in with the thesis of relaxation of functional constraint outside of the tropics). The assimilation model implies that the local Eurasian archaics had long independent existences and different alleles might have been dominant at the antipodes of the World Island. As a point of comparison the phenotypic variation of the Native Americans is on subjective grounds often evaluated to have been rather narrow for the wide latitudes and habitats that they inhabited, so it does not seem that 10,000 years (lowbound) is enough time for the "full range" of human coloration to manifest itself out of the genetic background. One obvious expalantion is that founder effect severely limited the genetic variation of the New World populations from which selection could work, but note that the original East African expansion event is often hypothesized to have been an almost explosive demographic event, likely going through several bottlenecks.

Via Dienekes.

Related: What's your s?

1 - The main issue I might have with this is that there seem to have been migrations into Australia within the last 10,000 years. But nevertheless these populations would still have to squeeze through the high UV regions of the tropics. And though Julian O'Dea has pointed out that Tasmania is at the latitude of Corsica, my impression is that Napoleon wasn't that swarthy (he certainly thought of himself as white, as illustrated by his declaration that he was "for the whites" in Haiti because "he was white").