Tibetans were able to adapt to high altitudes thanks to what is sometimes called the super-athlete gene, or more prosaically, EPAS1, they acquired when their ancestors bred with Denisovans – a mysterious group of prehistoric hominins that went extinct around 45,000 years ago.
Genetic researchers have found that part of the EPAS1 gene in Tibetans is almost identical to the gene in Denisovans. Image credit: Mauro Cutrona.
EPAS1 is activated when oxygen levels in the blood drop, triggering production of more hemoglobin. This gene has been referred to as the super-athlete gene because at low elevations, some variants of it help athletes quickly boost hemoglobin and thus the oxygen-carrying capacity of their blood, upping endurance.
An unusual variant of EPAS1 became widespread in Tibetans after they moved onto the high-altitude plateau several thousand years ago. This variant allowed them to survive despite low oxygen levels at elevations of 4,500 meters or more.
“We have very clear evidence that this version of the gene came from Denisovans. This shows very clearly and directly that humans evolved and adapted to new environments by getting their genes from another species,” said Prof Rasmus Nielsen of the University of California, Berkeley, who is the senior author of a paper published in the journal Nature.
For their study, Prof Nielsen’ team sequenced the EPAS1 gene in 40 Tibetans and 40 Han Chinese individuals.
The data revealed that the high-altitude variant of EPAS1 is so unusual that it could only have come from Denisovans. Aside from its low frequency in Han Chinese, it occurs in no other known humans, not even Melanesians whose genomes are nearly 5 percent Denisovan.
The variant (or allele) of EPAS1 found in Tibetans raises hemoglobin and red blood cell levels only slightly at high elevations, avoiding the side effects seen in most people who relocate to elevations above 4,000 meters.
“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. We can do a statistical analysis to show that this must have come from Denisovans. There is no other way of explaining the data,” Prof Nielsen said.
Prof Nielsen described a possible scenario leading to this result: “modern humans coming out of Africa interbred with Denisovan populations in Eurasia as they passed through that area into China, and their descendants still retain a small percentage – perhaps 0.1 percent – of Denisovan DNA.”
“The group that invaded China eventually split, with one population moving into Tibet and the other, now known as Han Chinese, dominating the lower elevations.”
Prof Nielsen added: “there might be many other species from which we also got DNA, but we don’t know because we don’t have the genomes.”
“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?”
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Emilia Huerta-Sánchez et al. Altitude adaptation in Tibetans caused by introgression of Denisovan-like DNA. Nature, published online July 02, 2014; doi: 10.1038/nature13408
