Humans Still Evolving: How Tibetan Plateau Residents Adapt to Life at High Altitude

Human evolution is not finished. Populations living for millennia on the high Tibetan Plateau offer a clear example of ongoing adaptation: their bodies have developed traits that improve oxygen delivery in an environment with chronically low atmospheric oxygen.

Mountain climbers who ascend rapidly often experience altitude sickness because lower air pressure reduces the oxygen taken in with each breath. Yet communities that have lived above 3,500 meters (11,480 feet) for more than 10,000 years thrive under those conditions. Recent research led by anthropologist Cynthia Beall examined physiological and reproductive data from longtime residents to identify which traits improve survival and reproductive success in hypoxic environments.

Study design and key findings

Beall and colleagues studied 417 women aged 46–86 who had lived their entire lives at elevations above 3,500 meters in Nepal. The researchers recorded the number of live births per woman (ranging from 0 to 14, mean 5.2) and measured several oxygen-transport traits, including hemoglobin concentration, hemoglobin oxygen saturation, pulmonary blood flow, and cardiac structure.

Women with the highest numbers of live births tended to have intermediate hemoglobin concentrations—not exceptionally high—and higher oxygen saturation of hemoglobin. They also showed greater pulmonary blood flow and wider-than-average left ventricles, the heart chamber that pumps oxygenated blood. Together these traits maximize oxygen delivery without excessively thickening the blood, which would increase cardiac workload and raise health risks.

Why these traits matter

High hemoglobin concentration can compensate for low oxygen but also increases blood viscosity, making the heart work harder. The combination seen in the most reproductively successful women—moderate hemoglobin levels with high oxygen saturation and improved blood flow—appears to provide a better balance: efficient oxygen transport with lower circulatory strain.

Role of culture and natural selection

The researchers also considered cultural factors. Women who started childbearing at younger ages and who had longer marriages had more reproductive opportunities, which increases lifetime live births. Even after accounting for these social variables, physiological traits tied to efficient oxygen transport remained significant predictors of reproductive success. According to Beall, these results represent ongoing natural selection in a human population adapting to a severe, quantifiable environmental stress.

The study was published in the Proceedings of the National Academy of Sciences in October 2024. Understanding these adaptations improves our grasp of human biological variation and the processes of evolution in extreme environments.