How did groups of modern humans inhabiting high-altitude environments adapt to live in such low oxygen levels? Some, such as the Andeans, cope with these conditions by increasing their capacity for oxygen. Tibetans have adapted by doing the opposite but have evolved ways to use the scarce oxygen more efficiently. They have a unique version of a gene called EPAS1 that is thought to reduce levels of hemoglobin and protect against altitude sickness, and a new report suggests that this gene was inherited from an extinct group of humans.
Tibetans separated from the Han Chinese population only ~3,000 years ago, and the version of EPAS1 found in most Tibetans is only found at a very low frequency in sea-level Han Chinese. This would be a remarkably short period of time for this variant of the gene to be selected for in the Tibetan population. Moreover, archaeological evidence suggests modern humans occupied the Tibetan plateau for at least 20,000 years. The study by Rasmus Nielsen and colleagues resolves this problem by showing that the Tibetan version of EPAS1 is almost identical to the version found in an extinct group of humans related to Neanderthals called Denisovans. The fact that the Tibetan EPAS1 shows more similarity to the Denisovan gene than to the version found in any modern human population suggests that the Tibetan EPAS1 was acquired via interbreeding between the ancestors of Tibetans and the Denisovans some 40,000 years ago. Tibetans and their ancestors who then went on to colonize the plateau retained it due to its beneficial effects at high altitudes.
These findings raise the exciting possibility that many of the population-specific adaptations in modern humans may have originated in other, now extinct groups of humans. Since the genomes for many of these archaic groups is now available, it will be interesting to see if similar findings are reported in the near future for other adaptive traits.
Edited by SITN Waves Editor Tyler Huycke. Thanks to Elizabeth Brown, a graduate student in the Department of Human Evolutionary Biology at Harvard University and M. Holly Elmore, a graduate student in the Department of Organismic and Evolutionary Biology at Harvard University, for providing expert commentary on the article.
Further Reading on the topic: