If you read this weblog you are aware that I have a fascination with the intersection of human history and human evolutionary genetics. There are many questions I have about the finding from evolutionary genomic studies that light skin evolved at least twice independently in Eurasia within the last 20,000 years or so at the extremities. The selection coefficients are large, so I am confused as to why even minimal gene flow did not result in equilibration and homogenization of the allelic profiles of the populations. I have posited that the answer has to do with very low population densities verging upon nil in Central Asia. From The Horse, the Wheel, and Language: How Bronze-Age Riders from the Eurasian Steppes Shaped the Modern World (a book I can not recommend enough!):
…On the Ukok plateau, where the early Afanasievo cemetery at Bertek 33 was found, the Afanasieva immigrants occupied a virgin landscape-there were no earlier Mesolithic or Neolithic sites. Afanasievo sites also contained the earliest bones of domesticated cattle, sheep, and horses in the Altai.
Cemeteries of the local Kuznetsk-Altai foragers like Lebedii II were located in the forest and forest-meadow zone higher up on the slopes of the Altai….
The Ukok Plateau is about 2000 kilometers from the Ural-Volga regions from which the Afanasievo likel originated. It seems that around 3500 BCE they “leap-frogged” across what would become the northern Kazakh steppe for the pine forests of south Siberian fringe. The point though is this: this sort of volkswanderung only became possible with the emergence of the nomadic herder lifestyle. The selective sweeps which one infers from evolutionary studies likely occurred before the Afanasievo trek….