Tibet has the highest average elevation of any other region in the world, at 4900m above sea level. There have been modern humans in Tibet for at least 21,000 years, with the ancestors of the current inhabitants moving in around 3000 BCE. A recent study led by Rasmus Nielsen of UC Berkeley suggests that Tibetans’ success at that high altitude may be partially credited to genes picked up when their ancestors interbred with an extinct cousin of humans, the Denisovans. The results of the study were published in Nature.
The decreased air pressure at high altitudes makes it harder to intake oxygen. Many people are able to temporarily compensate for the altitude by thickening the blood, though it can negatively affect the cardiovascular system after a while. However, residents of Tibet have been able to deal with the decreased oxygen permanently. This ability appears to be due to a variant of a gene that came from the Denisovans, an extinct group of humans that lived in Siberia and died out around 40,000 years ago.
“We have very clear evidence that this version of the gene came from Denisovans,” Nielson said in a press release. “This shows very clearly and directly that humans evolved and adapted to new environments by getting their genes from another species.”
The gene in question is EPAS1, which is regulated by oxygen. Prior genetic analysis of Denisovan DNA taken from a finger bone fragment revealed a variation of this gene which boosts hemoglobin and red blood cell production only slightly, avoiding the negative cardiovascular effects seen by those with other variants of the gene that cause thicker blood.
“We found that 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.”
This rare variation was found in 87 percent of Tibetans who were tested for the study, compared to only 9 percent of Han Chinese, who share a common ancestor with the Tibetans, though they live at lower elevations. The gene would have been more advantageous on the Tibetan plateau, where oxygen levels are about 40 percent lower than at sea level. It would not have been selected for at lower elevations, which is likely why it is so rare for non-Tibetans.
“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?”