There's been a lot of information coming out of the Human Genome Project and other studies lately. We now know that in Europe homo sapiens sapiens interbred with Neanderthals.
In Asia, we appear to have mated with a different extinct human species, Denisovan man, identified only by a few bones. We know almost nothing about the Denisovans other than that they existed, interbred with modern Asians and then went extinct, leaving a few stray genes behind.
Except, not so stray. Interspecies crossing when the resulting offspring are fertile or semi-fertile (the evidence indicates that in modern/Neanderthal crosses only the females were fertile, an interesting pattern also seen in small cats) is one of the mechanisms of evolution. Crossing brings new genes into the population, some of which are going to be useful - and spread.
Enter...Tibet. Most humans can't survive at elevations above 13,000 feet without either a lot of training or supplemental oxygen. We aren't designed for it, and our red blood cell production shoots up (causing altitude sickness and other dangerous symptoms).
Not so for the Tibetans. They carry a gene that reduces this effect, allowing them to be perfectly comfortable at altitude. And this gene has been seen only in Tibetans and a few Han Chinese.
It's a Denisovan gene. (With as much certainty as we can give at this point). Does this mean that the Denisovans were high altitude humans?
It proves that inter-species crossing can be a way to get stable (over 90% of Tibetans carry the gene) genetic variance that can allow a species, especially one migrating, to adapt to new environments more quickly than by evolution. In fact, the fact that this variant gene doesn't exist in other high altitude populations such as some of the South American and African groups...although nowhere is as high as the Himalayas...indicates that mutation and selection alone hasn't come up with it yet.
But it also tells us that inter-species crossing can result in the extinction of one of the species involved. Apparently, what we call modern man was overall better adapted - and thus the only genes that survived from our cousins were the ones we found useful.
We're all gene thiefs, apparently. Unless all of your ancestors are African.