MBE has a couple of new articles on altitude adaptation in Tibetans. One talks about a particular mutation in EGLN1, a key gene in the response to hypoxia. The mutation, around 0.5% in most other populations, has soared to 70% frequency in Tibetans. They estimate that this selection began about 8000 years ago, while another Tibetan hypoxia defense (there are several) is a good deal older, something like 18,000 year old. The second paper, with many of the same authors, concludes that the Tibetan plateau was originally settled as far back as 30,000 years ago, while a fraction related to the Han was added to the Tibetan mix in the early Neolithic. They think that the people of the earlier, pre-agricultural phase originally lived in some of the lower parts of Tibet ( < 3000 meters), while colonization of the higher parts of Tibet happened after the domestication of the yak and barley. Colonization of higher altitudes led to increased selection for resistance to hypoxia.
This scenario makes a lot more sense than Rasmus Nielsen's notion that Tibetans adapted to high altitude in less than 3,000 years, which I never believed. Reminds me of the line in Dirty Harry where Callahan says that anybody could tell that he hadn’t beaten up Scorpio: “Because he looks too damn good, that’s how!”
The Tibetans deal with high altitude much more effectively than the Amerindians of the Altiplano. You have to think that they’ve lived there longer, been exposed to those selective pressures longer – and that’s quite feasible. Anatomically modern humans have been in Asia much longer than in the Americas, and it’s even possible that they picked up some adaptive altitude-adaptation genes from archaic humans that had been there for hundreds of thousands of years.
There’s another interesting point: the hunter-gatherers of Tibet appear to account for a lot of Tibetan ancestry, probably most of it, rather than than being almost entirely replaced by a wave of neolithic agriculturalists, which is the more common pattern. They had a trump card – altitude adaptation. A story like that which has left Bolivia mostly Amerindian.