How do Tibetans thrive at such a high altitude? New research shows they owe this extraordinary ability to a rogue gene from long-lost cousins that enables them to live and work and climb ...
Tibetans live in a mountainous home where the air contains 40 percent less oxygen than low elevations. They can thank an extinct human relative for providing a rogue gene that helps them adapt to the high altitude, a study said.
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 for thousands of years, and a study says it’s thanks to a genetic adaptation they inherited from an ancient relative.
The study, published 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 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.
Natural selection could not explain the pattern
In the 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. 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 2 of the 40 Han Chinese in the panel and no one else.
“Natural selection by itself could not explain that pattern,” said author Rasmus Nielsen, a computational biologist at the University of California, Berkeley. “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 hypotheses.
‘Extremely differentiated from each other’
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, Nielsen said. 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. “Genetically, Han Chinese and Tibetans are very similar throughout the genome,” Nielsen said. “But for this particular gene, they are extremely differentiated from each other, which is something you only see with very strong or very recent selection.”
The reason Tibetans need EPAS1 is that their mountainous home — a crease of buckled crust thrust upward by the tectonic collision of India and Asia — lies about 15,000 feet above sea level, on average. Up there, the air contains 40 percent less oxygen than low elevations.
Although previous studies had identified the importance of EPAS1, scientists still don’t know exactly what the gene does. They know only that it leads to lower levels of hemoglobin — the oxygen-toting protein in blood — in Tibetans who live at high altitude compared with people from low elevations who have acclimatized.
Abigail Bigham, an anthropologist at the University of Michigan who was not involved in the study, said it adds to a growing body of work that has reshaped the way scientists think about human evolution and our relationship to our extinct relatives. For a long time, most scientists believed Neanderthals and Denisovans had nothing to do with modern humans. Now they realize that these species are responsible for introducing some of the genetic diversity that let people adapt to unique environments.
“We’ve come full circle,” Bigham said. “Not only has there been interbreeding, but in fact that interbreeding has led to important functional changes in the human genome.”