"Remarkably, they acquired this capacity in as little as 3,000 years — and it would now appear that interbreeding with another human species [the Denisovans] may have had something to do with it."
Article on http://ow.ly/yKX5C :
Tibetans Got Their High-Altitude Gene From An Extinct Human Species
In what’s considered one of the finest examples of natural selection in action, Tibetans have acquired the ability to thrive at extremely high altitudes. Incredibly, researchers say the gene required for this adaptation was inherited from the now-extinct Denisovans.