Razib, at Gene Expression has an interesting post on a gene that shows strong signs of selective pressure in East Asia prior to the migration of the founding population of the Americas called EDAR based on a paper published in Cell, Modeling Recent Human Evolution in Mice by Expression of a Selected EDAR Variant.
Razib notes that the authors of the Cell paper fail to make a convincing argument for what selective pressure was involved. Neither their climate adaptation theory, nor their sexual selection for small breast and lusterous black hair theory withstand serious scrutiny. I advance another hypothesis which I believe is a more convincing explanation for this selective swep and another striking East Asian racial type defining mutation (low levels of body hair), although it is a hypothesis to be tested rather than a proven theory.
The Selective Sweep At The EDAR Locus Was A Protective Response To Bubonic Plague.
My hypothesis is that the EDAR gene and low levels of body hair were protective against flea carried bubonic plague, an extremely lethal disease with its origins in China. This disease became a powerful selective pressure in East Asia when the domestication of dogs provided a vector for this flea borne disease in human communities.
These communities also became more vulnerable to the plague because their transition to behavioral modernity in the Upper Paleolithic revolution led to the formation of semi-sedentary communities who fished and hunted other coastal animals, and this in connection with the advantage conferred on them by domesticating dogs allowed them to live in villages with population densities sufficient to sustain plague outbreaks.
The communities of semi-sedentary fishing and foraging villages with domesticated dogs where the bubonic plague outbreaks began were probably similar in diet and social organization to those of the early Jomon people of Japan, the Comb Ceramic and Pitted Ware people of Northeast Europe, the Paleo-Eskimos of Greenland, the Inuit and the Native Americans of the Pacific Northwest.
The mass deaths in Inuit communities from exposure to diseases carried by Basque whalers who were their first contact with Europeans supports the hypothesis that these communities had sufficient population density and intercommunity trade links to sustain a major outbreak of bubonic plague without causing it to fizzle out by killing people before the disease could be spread.
Bubonic plague acts on the lymph system. The derived EDAR allelle might have been protective because lymph nodes are warehouses for the immune system, so a higher frequency of lymph nodes would strengthen immune response, and because small breasts would have reduced the volume of vulnerable plague targeted lymph tissue there making women with smaller breasts more likely to survive a plague infection.
The hair type phenotype associated with the derived EDAR allelle, and also one or more other allelles coding phenotypes for low levels of body hair which are a racial type defining East Asian allelle, may have reduced the vulnerability of people with this phenotype to infection with the plague's flea vector.
Other hard selective sweeps on allelles that confer resistence to diseases, are documented in recent human evolution (e.g. malaria), and hard sweep on the genes for lactose persistence can to some extent also be seen as genes enhancing immune capacity via better nutrition. So the hypothesis that EDAR conferred selective fitness by conferring disease resistence is a plausible one.
But, since the timing of the EDAR selective sweep was clearly too early for it to be driven by the Neolithic revolution, and very low population density terrestrial hunter-gatherer populations who moved very slowly before horses were domesticated are ill suited to harboring infectious disease outbreaks which tend to quickly run their couse and fizzle out, they must have arisen in more tightly tied, higher population density communities.
Upper Paleolithic fishing villages whose prosperity is also enhanced by the use of domesticated dogs could account would have had nearly Neolithic population densities. Their use of domesticated dogs would simultaneously provide a bubonic plague flea vector in a community where the rat vector of the bubonic plague flea Middle Ages was not available (since there were no large human grain stores for the rats to plunder) and explain why this happened 30,000 years ago around the time that the dog was domesticated, rather than at some earlier or later point in prehistory.
The hypothesis that EDAR conferred resistence in particular to bubonic plague rather than some other disease is driven by the particularized importance of the lymph system that the derived EDAR allelle appears to enhance to bubonic plague in particular relative to other lethal infectious diseases.
Why Don't Finns and Russians Have The Derived EDAR Allelle?
How do I explain the absence of the derived EDAR allelle in Northeast European populations who show clear genetic signs of East Eurasian genetic contributions, probably through linguistically Uralic populations distingished by the Y-DNA haplogroup N which is a sister clade to the dominant Asian Y-DNA haplogroup O?
The Last Glacial Maximum sterilized Northern Asia, killing off the plague and killing or exiling all of its modern humans. North Asia was repopulated from one or more genetically East Eurasian refugia, probably including Tibet and the Altai Mountains, that had become isolated from coastal China before the selective sweep took place ca. 30,000 years ago in response to a series of bubonic plague outbreaks.
These refugia included the place of origin in North Asia of Y-DNA haplogroup N whose population did not undergo the selective sweep for the derived EDAR allelle that coastal villages in China from which this mutation expanded did, perhaps because these nomadic terrestrial hunter-gatherers lacked the relatively high population densities in semi-sedentary villages of the fishing people in which this mutation arose, or perhaps because the colder climate there was protective.
Even if the derived EDAR allelle was present in low frequencies in the North Asians who repopulated North Asia, genetic drift and founder effects could have purged it from the populations who made East Eurasian genetic contributions to Northeastern Europeans, because the derived EDAR allelle was not actively conferring selective advantage in either the tundras of Siberia or in Europe, where bubonic plague did not arrive until the 14th century from a Chinese source via Siberia's Southern fringe.
The story of Y-DNA haplogroup N securing its current distribution in a counterclockwise expansion from Southeast Asia is wrong according to this hypothesis. Instead, haplogroup N subsequently introgressed into the Han Chinese population before or in the early phases of the East Asian Neolithic revolution and expanded with its rice and millet farmers, all of the way to to South Asia where Munda rice farmers carried its most far flung extension.
Later North Asian populations acquired the ubiquitous EDAR allelle from their expanding Han Chinese neighbors to the South who in the Neolithic era that followed the East Eurasian genetic introgression into Northeast Europe, brought higher density settlements that were subject to EDAR alllelle selective pressures associated with the periodic plague outbreaks that followed Han Chinese trade routes into North Asia.
The biggest gap in this theory is that I have no idea whether the derived EDAR alllele (or the derived allelle for low levels of body hair found in East Asians) would have indeed been protective against death from bubonic plague. But, this is something that should be relatively easy to examine once the question is well posed. If it isn't protective, we need to find out what the gene did that did confer selective advantage, because neither the climate conditions theory, nor the small breast fashion fad theory make sense to explain such a powerful selective sweep.
DNA phylogeny evidence already in print should be able to establish whether the hypothesis regarding the Northeast European infusion of East Eurasian DNA is consistent with the evidence.