At an altitude of nearly 3 miles, the Tibetan plateau is an extreme place to live. It's cold, it's hard to grow food, and there's about 40 percent less oxygen in the air than there is at sea level.
Somehow, though, native Tibetans are adapted to it. Their bodies — and their blood in particular — work differently than those of people used to lower altitudes. The Tibetans' advantage might be thanks to an ancient inheritance.
When someone used to living at low altitude travels to the oxygen-deprived Tibetan plateau, his or her body responds by producing more red blood cells to help circulate oxygen through the body.
Sounds like a good thing, right? Not quite.
"You don't want your blood to become too thick," says Rasmus Nielsen, a geneticist at University of California, Berkeley. Too many red blood cells can lead to thick blood that is harder for the heart to pump. People who aren't adapted to high altitudes have an increased risk of stroke. When pregnant women move up to high altitudes, they tend to have difficulties with high blood pressure, suffer a higher rate of infant mortality and are more likely to give birth to small babies.
Native Tibetans don't have those problems. Their blood doesn't contain extra red blood cells, yet it still manages to keep them alive and well. It's a mystery how they manage to function so well at high altitude without the extra help, but it's clear that they are able to avoid the health pitfalls that other people can encounter at high altitude.
According to Nielsen and a bunch of geneticists writing in the journal Nature, the Tibetans appear to have benefited from a genetic gift from the Denisovans, an extinct human ancestor known primarily from a little girl's tooth and pinkie bone.
Tibetans have a gene, EPAS1, that's known to help regulate how the body responds to low oxygen levels. "It's also been called the 'super athlete gene,' because we know that certain humans that have a special version of this gene have a better performance with certain types of athletics," says Nielsen.
At first, Nielsen and his colleagues weren't sure how Tibetans had gotten the gene. But now they have an idea. "We think we have very good evidence that it came from Denisovans," he says. The DNA patterns seen around that gene match those of the Denisovans, a sister group to the Neanderthals.
He says Tibetans were able to adapt because they got the genes from another human species that was already adapted to the environment. It's a lot more efficient than waiting around for evolution to do the job.
Here's how the (very speculative) story might have gone: Modern humans evolve in Africa about 100,000 years ago and then start spreading across the globe, encountering new environments, and also other archaic human species, like Neanderthals and Denisovans. They mingle and mate, trading genetic material. Some inherit the EPAS1 gene. Eventually, some move up to high altitudes. The ones with the EPAS1 gene thrive more at high altitude than those without it. Over generations and generations, the gene becomes more common in the population.
We probably have extinct human ancestors to thank (and curse) for a lot. Denisovans and Neanderthals might have contributed to modern human immune systems and skin pigmentation, but also to diseases like lupus and Crohn's.
"I think that it's very clear from the work of the last few years that ancient archaic humans interbred with modern humans as modern humans expanded out of Africa 50,000 years ago," says David Reich, a geneticist at Harvard Medical School.
As with many studies of ancient genetics, Reich cautions against jumping to conclusions. "What these authors show is that this genetic material is of archaic human origin, and that's important," says Reich. "But whether it's of Denisovan or of Neanderthal or of some other archaic source, that's not clear."
What is clear is that the genes of modern humans have elements of human species past.
"We exchanged genes with a lot of other lineages that existed 100,000 years ago or 50,000 years ago," says Nielsen. "We are in some sense mongrels made of DNA from many many different lineages of hominins."
In other words, we're mutts.
RENEE MONTAGNE, HOST:
Scientists have been learning some remarkable things about the sexual history of humans by digging DNA out of fossil teeth and bones. For example, they found DNA from Neanderthals that is also found in modern humans. Apparently, our own ancestors mated with them. Now there's evidence that early humans may have bred with yet another extinct species, and as NPR's Christopher Joyce reports, it may have done us some good.
CHRISTOPHER JOYCE, BYLINE: Early humans apparently were not too picky about who they had sex with. Don't take my word for it. Here's geneticist Rasmus Nielsen from the University of California at Berkeley.
RASMUS NIELSEN: We exchanged genes with a lot of other lineages that existed 100,000 years ago - 50,000 years ago. We are, in some sense, mongrels.
JOYCE: One of those lineages was the Neanderthals, usually thought of as dull, brutish and very hairy. As a result, a lot of us have a little Neanderthal for DNA.
DAVID REICH: Whether didn't anything useful or important, biologically, today - that was another question.
JOYCE: That's geneticist David Reich at Harvard University. He says yes, some of those Neanderthal genes actually made us resistant to some pathogens and more susceptible to some diseases as well. So now comes the latest news. It wasn't just Neanderthals that mated with our ancestors and gave us something useful - apparently, says Rasmus Nielsen, we mated with the Denisovans. Yes, Denisovans - not from outer space, but another extinct human relative. Scientists know their DNA from finger and toe bones and a tooth found in the Denisova Cave in Siberia. Writing in the journal "Nature," Nielsen says he's found what appears to be Denisovan DNA in one group of modern humans - tibetans. And it's actually a gene that does something quite useful.
NIELSEN: There's a characteristic of Tibetans that they have gained so that they're able to live in high altitudes.
JOYCE: Altitudes like two and a half miles up - the ancient Denisovan gene protects against stroke and other ailments that can afflict people who live where oxygen is very thin. Geneticist David Reich suggests the gene could possibly have come from some other archaic human cousin. There seem to have been quite a variety of early humans around the time, but he adds that the important point is there's value in a species being, shall we say, liberal in its mating habits.
REICH: These genetic variations were opportunities to adapt to different environments - different flavors that allow people to respond differently to environmental challenges like altitude.
JOYCE: Though, it's doubtful that's what the first human-Denisovan couple was thinking at the time. Christopher Joyce, NPR News. Transcript provided by NPR, Copyright NPR.