Saturday, January 19, 2008
I was talking with a friend about Native American skin color. From the Canadian north down to Chile it seems that though there is variation these populations exhibit some sort of brownish shade. There are no black-skinned Amazonians, nor are there pink-skinned peoples on the Canadian Arctic. So what gives?
First, it seems likely that Native Americans have been "native" to the New World for only around the past 10,000 years.1 A physical anthropologist once told me that the body proportions of the natives of the Amazon are still quite "Siberian," that is, they exhibit adaptations to cold weather after all these generations. And of course time is not the only parameter, Native American populations seem to have gone through a genetic bottleneck; they likely brought over very little standing genetic variation. So you have a relatively short period of time for selection to operate upon over a very limited range of trait value.2
But this isn't persuasive to me for skin color, at least in the totality. We seem to know the genes at work now. We know that they can be selected very fast, and we know that there have been convergent dynamics across the World Island. 10,000 years is plenty of time. So perhaps the second parameter, extant genetic variation, is at work? That is, the Siberian migrants didn't bring all the genes for selection to shift them toward new adaptive optimums. For dark skin the data suggest that there is a rough consensus sequence, a constrained set of alleles across skin color genes, which produces our species' dark "Wild Type." This suite of genes probably arose when we lost our fur and became strongly pigmented to counter the negative affects of radiation, and it seems like there hasn't been any reinvention of the wheel here. Melanesian populations which are quite distant from Africans on most genes exhibit the same consensus sequences for skin color loci, by and large. I think that it is likely that the brown-skinned Siberians did lose some alleles at particular loci (that is, they were fixed for loss of function variants), so that for true blackness to reemerge there needs to be new mutations which gain the function back. And as you likely know, gain of function is far less likely than loss of function.
But that only explains why Native Americans don't get very dark. As I imply above, loss of function isn't all that hard. That's why albinos can be found in most human societies, they're an extreme mutant, but the same principle seems to be operative on many of the skin lightening genes. So why didn't Native Americans get pink? I think the fact that Siberians and Inuit are relatively brown suggests that extreme depigmentation is not always entailed by life at high latitudes. As many workers have suggested groups like the Inuit consume marine animals who are heavily loaded with Vitamin D, the lack of which is one of the presumed selective pressures driving depigmentation. That being said, most Native American tribes did not live next to the sea. And yet the recent selection events for genes such as SLC24A5 and OCA2 strongly implies that European have become very depigmented very late in prehistory, perhaps almost into historical periods!
Why? I have proposed (following many others, such as L. L. Cavalli-Sforza) that the switch to agriculture resulted in a shift in diet and nutritional intake which entailed greater endogenous Vitamin D production by necessity. But there's a problem with this model: forms of agriculture existed in the New World as well, and spread up (eventually) into what became the eastern United States. Granted, the latitude of much of this region is about where the Middle East is, but even then it seems that the natives were relatively swarthy. I discount the notion that agriculture was too recent when SLC24A5 might have had selection coefficents on the order of 0.10. Perhaps the people of the New World, at least in North America, kept a more diverse diet, supplementing their agriculture with hunting and fishing to a far greater degree than in the Old World? Additionally, one might suppose that maize was nutritionally superior staple to wheat, barley, millet or rice (I have read that this is so). Ultimately these sorts of questions need to be addressed by a survey of the archaeological literature, as well as assessing the nutritional differences. I'll get to that at some point.
But there's one last thing I thought of: disease. I can't really explain with SLC24A5 goes so far south in India. You see frequencies as high as 25% in Tamil Nadu. Vitamin D deficiency? Certainly nutritional stress is a major issue, but, one thing is for sure, South Asia is subject to a lot of disease in comparison to any other densely populated part of the world. Of the Old World civilizational hearths India was certainly the one weighed down by the greatest endemic pathogen load, in large part because it was so far south and so wet. So perhaps it was disease.
Which brings us to Native Americans. Despite the recent uproar over syphilis, the New World was relatively pathogen free for humans. Granted, with greater population densities disease would have been a major issue among the agriculture populations of the New World, but there were structural reasons why they would have been less prone to epidemic outbreaks than Old World civilizations. The relative lack of domestic animals, the non-existence of closely related species (think of ape strains of viruses), the smaller and more fragmented population networks, and of course the fact that the original migrants probably only brought a small subset of the diseases of the Old World originally. Empirically we know that the Native Americans died like flies when the Eurasians showed up. Their civilization simply didn't prepare them for Old World plagues. What I'm proposing here is that disease was a major driver of skin color evolution over the last 10,000 years. Or, at least, the same loci which control and modulate melanin production are critical in immune defenses.
I need to do a lot more digging for this to be anything more than a guess. But the disease angle seemed to be the last best hope in explaining why the New World was different. If they were subject to the same nutritional stress, why didn't they go down the same path as Eurasians? The reason may be that the path was being forged by the threat of disease (Vitamin D deficiency increases susceptibility to infectious agents), which was a less important parameter in the New World. Implausible as it may sound, it seems the most plausible of the various explanations to me.
Note: If you are really curious about the topic, check out the many posts on skin color on this GNXP and the other.
1 - Even if Clovis First is debunked, it seems more and more likely that there are problems with genetic studies which claim that the earliest migrations date to 20-40 thousand years BP.
2 - All things being equal the rate of adaptive evolution is proportional to the extant genetic variation. If there is no genetic variation evolution has no raw material to work with.