Forget climbing Mount Everest — for most humans, just eking out a living on the harsh Tibetan plateau is challenge enough. But Tibetan people have thrived there for thousands of years, and a new study says it is thanks to a genetic adaptation they inherited from an ancient human relative.
The study, published Wednesday in the journal Nature, identifies a long segment of DNA shared by the extinct people known as Denisovans and modern-day Tibetans. The segment contains the gene scientists think gives Tibetans a lung up over lowlanders at high altitudes.
No one knew the Denisovans ever roamed the Earth until four years ago, when scientists sequenced the DNA of a finger bone unearthed in a cave in the Altai Mountains of southern Siberia. The genome exhibited similarities to that of modern humans and our extinct Neanderthal relatives, but it was different enough to be considered a distinct species.
Like Neanderthals, Denisovans mated with their human contemporaries, scientists soon discovered. People of Melanesian descent who today inhabit Papua New Guinea share 5 percent of their genetic makeup with the Denisovans. Now, it appears that Tibetans can also trace part of their ancestry to this mysterious group.
In the new study, scientists collected blood samples from 40 Tibetans and sequenced more than 30,000 nucleotides on a segment of DNA containing EPAS1, the gene that makes Tibetans so well-suited for life at high altitude. Then the scientists compared that sequence with those of 1,000 individuals representing the 26 human populations in the Human Genome Diversity Panel. They found the high-altitude gene in only two of the 40 Han Chinese people in the panel, and no one else.
“Natural selection by itself could not explain that pattern,” said Rasmus Nielsen, a computational biologist at the University of California, Berkeley, and an author of the study. “The DNA sequence was too different from anything else we saw in other populations.”
So they investigated whether the gene might have been imported from extinct Neanderthals or Denisovans, and, bingo, they found a match.
But how did the gene end up in the genome of modern Tibetans? The scientists used computer models to test two different hypotheses. Were Denisovans and Tibetans descended from a common ancestor that gave the gene to both? Or did humans acquire the gene by mating with Denisovans?
Early humans and Denisovans probably diverged around half a million years ago, and it’s very unlikely that the gene could be maintained in both populations for so long, Mr. Nielsen said. “By the process of recombination, DNA segments become shorter and shorter and shorter,” he said. “But here, we have a very long segment that is shared. That’s very unlikely, statistically.”
Alternatively, the gene could have entered the Tibetan gene pool more recently via sex. Once transferred, the gene would have spread rapidly in the Tibetan population because of the merciless selective pressures of high-altitude living.