One of my met peeves is the confusion which some ethno-linguistic terms can cause. For example, the fact that there were Iranian language speakers on the plains of Ukraine ~2,000 years ago naturally indicates to people that Scythian nomads issued out of Iran northwards. Similarly, the existence of Indo-Aryan Mitanni in what is today Syria also suggests to people that there was a migration of Indians which traversed much of West Asia in a drive toward the Mediterranean from the Indus. Of course our perception of the center of gravity of these ethno-linguistic groups today is a function of historical contingency. If we didn't know much more about Antique and Medieval European history we might posit that the Celtic Galatians of ancient Anatolia were originally from Ireland, based on the contemporary distribution of Celtic languages!

This issue is now cropping up South Asian archaeogenetics. In my opinion the paper Reconstructing Indian population history is probably the most important contribution to the field in a generation. The authors explain technically why a "South Asian" ancestral component falls out of ancestry inference algorithms at the heart of ADMIXTURE, STRUCTURE, or frappe. In short, when you have a population which is a hybrid, but where the hybridization event is very distant in the past, recombination breaks up the signatures of that event (a decay of the linkage disequilibrium between two putative ancestral populations). Additionally, in the Indian case there doesn't seem to be a "pure" population of one of the two ancestral groups, what they termed "Ancestral South Indians" (ASI). The closest reference they found were Onge Andaman Islanders, whose last common ancestors with ASI was on the order of tens of thousands of years in the past. They do have excellent proxies for the other population, "Ancestral North Indians" (ANI). Compared to ASI all West Eurasians can be used as reasonable proxies for ANI.

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