The secret of Tibetans’ success lies with ancestors who were not quite human:
RESEARCHERS have known for a while that many people alive today carry genes from human species other than Homo sapiens—the result of ancient interbreeding with Neanderthals and Denisovans. They have even worked out that this admixed DNA must often be doing something particularly useful, because its pattern suggests natural selection is actively retaining it. The specifics, though, have not been clear. But in one case they now are, for it is because of these occasional Denisovan ancestors that Tibetans thrive in Tibet.
Full article at THE ECONOMIST