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May 22, 2005

Why no white aboriginals?

John Hawkes reviews a Jared Diamond review of some literature on the evolution of human skin color variation. It seems plausible that on a world-wide scale skin color variation is a reaction to a variety of environmental selection pressures, relaxation of functional constraints and social/sexual selection. Diamond focuses on the idea that balance of catabolism of folate and synthesis of vitamin D as a function of ultraviolent radiation (UVR) exposure is the primary determinant. But then he asks, what explains the dark skins of Australian Aboriginals and Tasmanians? In The Third Chimpanzee Diamond opined that sexual selection, not enviornmental selection, was the primary factor in diversifying human skin coloration. This was a qualitative assertion, and I do not not believe it has stood the test of time, seeing as how a strong positive quantitative correlation between UVR levels and melanin levels as well as evidence that purifying selection has been operative amongst these same dark skinned peoples in maintaining a low diversity on the MC1R locus both suggest long-term environmental selection (it being the central nexus in the regulation of melanin production).

The Australian Aboriginals are the great exception, and their extinct Tasmanian relatives were the specific example that Diamond noted when he offered that sexual selection was crucial, as these peoples remained dark skinned even after 10,000 years of isolation in their cool cloudy island home. John suggests that the low genetic variation common among island peoples might be the factor behind why the folks of Oz remained dark. Certainly in the New World there is some variation in skin color among the indigenous peoples, but the tribes of the north remained far darker than equivalent Old World groups (and similarly those of the equatorial regions do not rival Africans, South Indians or Melanesians in complexion). This suggests that the genetic background does matter.

Nevertheless, in the case of Australia proper, I suspect that the problem might have been less the isolation, than the fact that there was a fitness barrier over which alleles for lighter skin could not cross (note that Dingos only arrived in Australia over the last 5,000 years, so I suspect that some people did stray into Australia now and then as well). That is, the reservoir for alleles for lighter skin in eastern Eurasia would have had to have been passed from population to population across the archipelagos of Southeast Asia and likely via Papua New Guinea. Of course the relatively light complexions (in comparison to Melanesians) of the peoples of Polynesia suggests that such alleles could "hitchhike" with culturally adept folk. How likely would this have been over the ~40,000 years of human habitation of Australia? How about mutations for light skin?

In any case, what ever happened to those blonde Australian Aboriginals? Was that all the result of admixture? Internet searches come up blank, though C.S. Coon seemed to discount that they were byblows of Dutch sailors.

Related: Julian David has pointed out that much of Australia is tropical, and Tasmania is at the same latitude as Corsica. This is a fair point, but at the latitude of Corsica Eurasians generally have brunette white or pale olive skin. Still many shades fairer than the condition of the Tasmanians.

Posted by razib at 11:58 PM