In response to my post below a friend emailed me the above sentence. As I suggest below it sounds crazy, and I don't know if I believe it. But here's an abstract from the Reich lab from June:
Estimating a date of mixture of ancestral South Asian populations Linguistic and genetic studies have demonstrated that almost all groups in South Asia today descend from a mixture of two highly divergent populations: Ancestral North Indians (ANI) related to Central Asians, Middle Easterners and Europeans, and Ancestral South Indians (ASI) not related to any populations outside the Indian subcontinent. ANI and ASI have been estimated to have diverged from a common ancestor as much as 60,000 years ago, but the date of the ANI-ASI mixture is unknown. Here we analyze data from about 60 South Asian groups to estimate that major ANI-ASI mixture occurred 1,200-4,000 years ago. Some mixture may also be older—beyond the time we can query using admixture linkage disequilibrium—since it is universal throughout the subcontinent: present in every group speaking Indo-European or Dravidian languages, in all caste levels, and in primitive tribes. After the ANI-ASI mixture that occurred within the last four thousand years, a cultural shift led to widespread endogamy, decreasing the rate of additional mixture.
The most likely candidate population for an admixture event in the Indian subcontinent within such a time frame are Indo-Aryans. But, it does make some sense in light of the fact that the Northwest Eurasian variant of the lactase persistence allele is found in India, as that is presumably a relatively new variant. Let's assume that in fact Indo-Aryans did arrive in India within this time frame, and were demographically numerous enough that they left a population genetic stamp.
What is the probability that they did not do the same in Europe?
I'd say that's low. In other words, if the above results are correct, that Indo-Aryans had a significant impact on already heavily populated South Asia, then it stands to reason that the same would be true for Europe. Why hasn't this signal been as easily detected? I Assume it's because the Indo-Europeans were genetically closer to non-Indo-European Europeans in the first place. This sort of phenomenon may explain the recent divergence between Tibetans and Chinese. Linguistically the two populations are very different, and it is hard to credit that Tibetan and Chinese dialects diverged within the last ~3,000 years. But it makes much more sense if the Han demographic radiation was from a set of genetically similar, but culturally diverse, populations. Though gene flow maintained a degree of coherency, it may be that there were deep linguistic fissures across the North China plain before the rise of the Han. Similarly, the peoples of the Caucasus exhibit a lot of linguistic diversity, far more than you'd predict from simple genetics. The Indo-Europeans in Europe may have assimilated linguistically very different people, who were genetically quite similar. In India, they may have assimilated linguistically very different people, who were also genetically distinct. Addendum: I do believe that the highest probability is that Europe and India saw multiple intrusive populations after the rise of agriculture. So the larger proportion of the ANI signal probably derives back to the early West Asian farmers.