Tibetans Are Related to a Now-Extinct Human Species

Tibetans retain DNA from a species of human that they ironically helped push to extinction, according to a new study published in the journal Nature.

The gene allows Tibetans to adapt to high altitudes of 15,000 feet or more, researchers found.

“We have very clear evidence that this version of the gene came from Denisovans,” said principal author Rasmus Nielsen, a Berkeley professor of integrative biology, in a press release. Denisovans were a mysterious human relative that went extinct 40,000-50,000 years ago, around the same time as the more well-known Neanderthals, under pressure from modern humans.

“This shows very clearly and directly that humans evolved and adapted to new environments by getting their genes from another species,” Nielsen said.

That’s significant because it means we are all probably mutts, descended from more than one species of human. Homo sapiens didn’t just evolve and somehow lead to all modern humans today. Homo sapiens instead interbred with other species seemingly wherever they met, from Africa to Europe to, in this case, Asia.

What’s new and particularly interesting about this study is that it’s the first time a gene from another species of human has been shown unequivocally to help modern humans adapt to their environment.

The gene variant allows Tibetans to survive low-oxygen levels at high elevations that would cause others to develop thick blood, leading to cardiovascular problems. The gene passed down from Denisovans, called EPAS1, activates when oxygen levels in the blood drop, triggering production of more hemoglobin, which is the red-colored protein responsible for transporting oxygen in blood.

Too much hemoglobin thickens blood, which can result in heart attacks and death. The gene only leads to slight increases of hemoglobin — just enough to counter the oxygen reduction experienced on mountaintops.

“We found part of the EPAS1 gene in Tibetans is almost identical to the gene in Denisovans and very different from all other humans,” Nielsen said. “We can do a statistical analysis to show that this must have come from Denisovans. There is no other way of explaining the data.”

The researchers made this determination after conducting blood tests on Tibetans. Previously, researchers sequenced a Denisovan bone found in a Siberian cave, so Nielsen and his team were able to match the Denisovan gene to the one found in Tibetans’ blood.

Denisovans, in addition to passing the gene to Tibetans, also passed the gene to certain members of the Han Chinese population, according to the scientists. Native people from the islands of Melanesia, who are closely related to Denisovans — at least 5 percent of their DNA is Denisovan — don’t have the gene. Why? Probably because they don’t need it! Natural selection is pretty remarkable, giving us what we need over time.

“There might be many other species from which we also got DNA, but we don’t know because we don’t have the genomes,” Nielsen said. “The only reason we can say that this bit of DNA is Denisovan 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?”

Photo: A Chinese researcher collects a blood sample from an ethnic Tibetan man participating in the DNA study. Credit: Beijing Genomics Institute (BGI-Shenzhen)

Source: Discovery