The Denisovans’ Legacy

Tibetans are more capable than most other people of living at extreme altitudes thanks to a mutation in a gene called EPAS1, which allows them to cope with the oxygen-poor air of the 4,000-meter-high Tibetan Plateau. Now, geneticists have identified the same mutation in the Denisovans, an extinct group of humans with whom our ancestors may have interbred some 30,000-40,000 years ago:

To date, this is still “strongest instance of natural selection documented in a human A Phuwa villager stands for a portrait a few miles from thepopulation”—the EPAS1 mutation is found in 87 percent of Tibetans and just 9 percent of Han Chinese, even though the two groups have been separated for less than 3,000 years. But when the team sequenced EPAS1 in 40 more Tibetans and 40 Han Chinese, they noticed that the Tibetan version is incredibly different to those in other people. It was so different that it couldn’t have gradually arisen in the Tibetan lineage. Instead, it looked like it was inherited from a different group of people. By searching other complete genomes, the team finally found the source: the Denisovans! …

This discovery is all the more astonishing because we still have absolutely no idea what the Denisovans looked like. The only fossils that we have are a finger bone, a toe, and two teeth. Just by sequencing DNA from these fragments, scientists divined the existence of this previously unknown group of humans, deciphered their entire genome, and showed how their genes live on in modern people. Denisovan DNA makes up 5 to 7 percent of the genomes of people from the Pacific islands of Melanesia. Much tinier proportions live on in East Asians. And now, we know that some very useful Denisovan DNA lives on in Tibetans.

But Catherine Brahic deflates some of the excitement, noting that the findings are not conclusive:

The Tibetan EPAS1 probably got there by interbreeding, but more evidence is needed to confirm which archaic humans were the source, says David Reich of Harvard Medical School in Boston. “There is no proof in the paper that the origin of the [DNA] is Denisovan.” He says it could just as easily have come from Neanderthals, whose EPAS1 looks similar to the Tibetans’. That might make more sense as they were common on mainland Asia, whereas the Denisovan heartland seems to have been in South-East Asia.

It is still unclear how the modified EPAS1 gene helps Tibetans survive 4000 metres above sea level. It seems to cut the number of red blood cells, which carry oxygen, being made. That is odd: most people make more of these cells when they travel to high altitudes, to carry more oxygen. But they thicken the blood, possibly making strokes more likely. Nielsen thinks that, by thinning the blood, the Tibetan gene may help lower this risk.

Regardless, Boer Deng concludes, genetic discoveries such as these should require us to rethink our definition of “humans”:

Denisovans and Neanderthals are called extinct human “species”—a term that used to demark a clear line between two organisms incapable of interbreeding to produce fertile offspring. But the definition is no longer so clear. We know that these hominin cousins did couple with our Homo sapiens ancestors—and some of us have inherited from them valuable modern traits. How we define “humans” past and present is a subject to contemplate—as fitting for scientists as for pilgrims to think about on their journeys across Tibetan plains.

Update from a reader, who bolsters the fact that “the genetic results of the last few years demonstrate pretty clearly that Denisovans and Neanderthals are us”:

See, for example, my take (“Denisovans are us“,  “Neanderthals are us– More evidence“,  “Neanderthals are us?”), Jerry Coyne’s (“How many species of humans were there?“, and John Hawks’ (“Naming archaic human populations“, “The ‘braided stream’ at year-end“, “Is the Biological Species Concept a ‘minority view’?“) . I’m a zoologist who studies lizards, Coyne is a geneticist whose most important work has been on the nature and origin of species, and Hawks is a paleoanthropologist who studies both bones and genes, and we all came to the same conclusion.

Hawks, who studies the issue most closely, states simply, “Ancient human populations like the Neandertals and Denisovans were not separate biological species.” Here’s how I put it: “The current work tends to confirm the conclusion that archaic humans (Neanderthals and Denisovans) were part of a group of interbreeding populations in nature that included the immediate ancestors of modern humans, and thus were members of the species Homo sapiens.”

That Neanderthals and Densiovans were human can, I think, not be doubted, because they are, under any point of view, fellow members with us of the genus Homo, and thus men in the generic sense (in both the vernacular and technical senses of generic).

But are they members of the same species as us, Homo sapiens? There are, regrettably, quite a few historical instances where two peoples (both of course undoubted modern H. sapiens) met, and one people quickly disappeared, with relatively little gene flow having occurred. That Denisovans and Neanderthals contributed “only” several percent to our genomes before disappearing is no bar to their having been the same species as us moderns; it is, in fact, strong evidence of all of us (modern, Denisovan, Neanderthal) being the same species.

(Photo: A Phuwa villager stands for a portrait a few miles from the Chinese border. Hidden in the rain shadow of the Himalaya in one of the most remote corners of Nepal lies Mustang, or the former Kingdom of Lo. Hemmed in by the world’s highest mountain range to the south and an occupied and shuttered Tibet to the north, this tiny Tibetan kingdom has remained virtually unchanged since the 15th century. Today, Mustang is arguably the best-preserved example of traditional Tibetan life left in the world. By Taylor Weidman/LightRocket via Getty Images)