Thursday, December 31, 2009

269. The Baseline Scenarios -- 45: The Migration

As I've already suggested, it's possible that there was some sort of bifurcation among the migrants when they reached the Indus Delta, with some heading northward along the west bank of the river, and the rest continuing east along the Indian Ocean coast. Since there are two separate mtDNA lines out of Africa, M and N, it's tempting to speculate on the possibility that M went one way at this juncture and N the other. However, while we do find evidence of N and its descendents in Europe, the picture is not quite so simple, since we find N elsewhere as well, notably in both South and East Asia.

We can assume most or all of the migrants would have continued eastward along the coast of what are now Pakistan, India and Bangla Desh. Sri Lanka would, at that time, have been part of the mainland, so they would have passed through that region as well. The question is whether or not they left colonies in place as they traveled, or simply bypassed South Asia altogether, gradually progressing eastward as a mobile unit. Everything depends on how we interpret the genetic, archaeological and ethnographic evidence.

One possibility is a so-called wave of advance, with each generation taking root in a particular spot and subsequent generations moving on only when feeling increased pressure as the region becomes saturated. This would produce not so much isolated colonies as an ever growing society continually expanding in all possible directions. If this had been the case, then we would expect to find evidence that the Out of Africa migrants expanded upward into all inhabitable regions of South Asia before continuing east along the coast -- the migration would not in fact have been a coastal migration at all, but a progressive saturation of much or most of the South Asiatic peninsula.

There is in fact some evidence consistent with such a wave of advance, particularly since so many tribal groups can be found well to the north of the coastal region. On the other hand, if the migrants moved on simply due to population pressure, there would have been no coastal migration at all, there would have been an advance in all directions, leaving numerous archaeological sites, and the alleles found throughout central and northern Asia today would be far older than any found further along the coast -- which is decidedly not the case. In fact some of the oldest evidence of the migration, both archaeological and genetic, can be found in Southeast Asia and the Sahul.

Which tells us that the migrants must have had a coastal culture, constraining them to the ocean front as they progressed, another example of the power of tradition over the sort of Darwinian adaptation favored by functionalism (since hugging the coast meant always having an uncertain and unreliable water supply). So if there was a "wave of advance" it would have been confined to the coast. Another possibility is that they could have traveled largely in boats, leaving an occasional colony behind from time to time. Or, possibly, they could have bypassed South Asia altogether, progressing very rapidly along the coast by boat. (While this seems unlikely it's one of the possibilities proposed in Soares et al.: "an origin of haplogroup M in East Asia and a later migration back into South Asia.")

What I presently think most likely was a gradual migration, with the migrants moving largely along the seacoast by boat, leaving various colonies behind as they went, eventually progressing beyond South Asia to Southeast Asia, and what is now Indonesia, which would have been a large body of land at that time when the ocean level was much lower than it now is, as well as many other places in that same general area, including the Sahul (New Guinea and Australia linked at that time by a substantial land bridge).

Something must then have happened, however, centered in South Asia, some sort of major event, a disaster . . .

(to be continued . . . )

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