When you or I go up to high altitude, we gasp for a while, maybe faint, and then gradually adapt. The way we do it is by furiously generating more red blood cells, to increase the blood's ability to absorb oxygen, which gets thinner the higher we go. But we pay a price: all of those extra blood cells can make the blood sticky, leading to a risk of high blood pressure, heart attack and, in pregnant women, the delivery of low-birth-weight babies.
We pay that price, that is, unless we're natives of the Tibetan plateau, where people live more or less cheerfully at altitudes of 13,000 feet and more. The secret lies in their genes—mostly in a gene known as EPAS1, which allows them to absorb scarce oxygen without creating extra blood cells. But while genetic traits are often created by mutations within a given species, this one evidently came from outside. According to a paper just published in the current Nature, the Tibetans' ancestors evidently mated with a now extinct human species known as the Denisovans, which went extinct somewhere around 40,000 years ago.
It's no surprise that matings have happened between modern humans and other human species. We share a fair number of genes with the more familiar Neanderthals, for example, who were the Denisovan's distant cousins. But it's not clear (although it's certainly possible) that Neanderthal genes gave our ancestors any specific evolutionary advantages.
For Tibetans, though, the high-altitude gene allowed them to colonize a region nobody else could survive (some Han Chinese, which make up more than 90% of the population of China, also have the gene, but it's relatively rare). "We found part of the EPAS1 gene in Tibetans is almost identical to the gene in Denisovans," said lead author Rasmus Nielsen, of the University of California, Berkeley, in a statement, " and very different from all other humans."
What's perhaps even more surprising is that the scientists had Denisovan genes to work with in the first place. "The only reason we can say that this bit of DNA is Denisovan, said Nielsen, "is is because of this lucky accident of sequencing DNA from a little bone found in a cave in Siberia. We found the Denisovan species at the DNA level, but how many other species are out there that we haven't sequenced?"