Since Who We Are and How We Got Here is out I thought I would spoil the “India chapter” (though you should buy the book!).
– The “Ancestral North Indians” are best modeled as a 50/50 ratio of Yamna-type people from the steppes & “Iranian farmers.” The implication is that the Indo-Aryans mixed with agriculturalists in the BMC on the way into South Asia.
– The “Ancestral South Indians” have about ~25% “Iranian farmer”, along with the indigenous component more like the Andaman Islanders.
David Reich clearly believes in a model of the ethnogenesis of South Asian populations detailed in A genetic chronology for the Indian Subcontinent points to heavily sex-biased dispersals. Also, I think I can now say in public when I had lunch with him he indicated that he thinks this is the most likely model. Also, the West Eurasian admixture into South Asian populations is “male-mediated.” R1a1a-z93 for the win!
He also believes there were several admixtures. He notes that his group’s 2013 paper, Genetic Evidence for Recent Population Mixture in India, reported two admixture events in North India, but one in South India. And the North Indian populations had the most recent event. This makes more sense if you consider that much of the admixture probably happened in the Northwest, as a mixed population spread across the subcontinent.
Reich contends that long tracts of ANI ancestry in some North Indians indicate that later people arrived from the first ANI wave. Also, several populations have an atypical Yamna-Iranian ratio in their ANI ancestry, being enriched for Yamna, and not so enriched for Iranian. These are all Brahmin groups.
Finally, he unmasks some of the backstories of difficulties collaborating with researchers in India, who have to be sensitive to cultural and political pressures. 2009’s Reconstructing Indian Population History was hailed in India as refuting the “Aryan invasion theory,” but the evidence was on the contrary, and I said so at the time.
In Who We Are and How We Got Here David Reich makes an explicit analogy between the Indian subcontinent and Europe. Both protrusions from Eurasia are characterized by a synthesis of indigenous hunter-gatherers, intrusive pastoralists from the Eurasian steppe, and migrating West Asian farmers.