If multiregional evolution is the right portrayal of human history,...there is no difficulty understanding how genetic differences among populations arose. They are ancient and they reflect isolation by distance in a structured population with, of course, episodic population expansions such as the Bantu expansion into sub-Saharan Africa or the European expansion into the New World overlaid. But if the GOE [Garden of Eden, i.e., Out-of-Africa] hypothesis is correct, then it is hard to understand how human differentiation occurred (“Genetic Perspectives on Human Origins and Differentiation.” Annual Review of Genomics and Human Genetics, 2000, 379.In a nutshell, the problem is this: if we assume a common ancestry in Africa, with one small group branching off to West Asia and its descendants gradually making their way east, across the length of the continent to Southeast Asia and another group branching off at a later period to Europe, etc., in a fairly straightforward manner, then we would expect to see a relatively homogeneous mix of morphological and cultural traits among most people now living in the modern world.
Saturday, January 9, 2010
281. Babel 3
The striking differences, both physical and cultural, that we find among different peoples in different regions of the world can be explained quite easily according to the multiregional model, which was generally accepted by most anthropologists until fairly recently. If Asians evolved from homo erectus in Asia and Europeans evolved from Neanderthals in Europe, and Africans evolved from archaic humans in Africa, as was widely believed, then the differences can be explained on the basis of independent processes of evolution over millions of years. It is this simplistic interpretation of human history that gave rise to the notion of "race."
While some still cling to the multiregional model, recent developments in population genetics, involving the systematic study of literally millions of genetic markers, make it clear that all modern humans evolved relatively recently (150,000 to 200,000 years ago), from an ancestral group living in Africa. The Out of Africa model explains why we are all so similar, sharing the morphological traits that make us Anatomically Modern Humans (AMH), along with the capacity for language, music, religion and culture generally, which can now be explained on the basis of a common heritage. (For multiregionalists such similarities developed through "convergent evolution," a process which is extremely difficult to explain.)
The most challenging problem we now face is the existence of the regional differences outlined above, so easily explained by the multiregionalists, but apparently inconsistent with the Out of Africa model. For cosmologists, the homogeneity of the universe seemed incompatible with their (pre-Inflation) model of the Big Bang -- for modern anthropologists, it's the other way 'round: the striking differences among humans in different regions seem incompatible with the new model, based on a common ancestry.
The differences have puzzled geneticists Henry Harpending and Alan Rogers as well:
While it's certainly true that changes would have taken place over time, for various reasons, it's very difficult to understand how East Asians, Africans, Europeans, East Indians, etc., got to be so differentiated from one another in such large geographical areas, in a process that could be called "divergent evolution," but, as Harpending and Rogers argue, would be extremely difficult to explain.
The analogy with cosmology is especially useful in this respect, because it helps us understand the nature of the problem. For Inflation to account for the structure of the Universe as a whole, it was necessary for it to have occurred at a very early stage. If Inflation occurred too late, it would only have been able to produce local effects, not the universal pattern we now see.
By the same token, if we want to explain the large-scale differences we now see in the human "universe," we need to posit some one-time only event, roughly analogous to Inflation in this respect, that occurred early enough after the initial exodus to have such a powerful global impact even after tens of thousands of years. If it had occurred too late, after modern humans had spread too widely over the Earth, then its effects would be seen today only in the region where it occurred, and not globally.
Posted by DocG at 9:59 AM