Humans Are Still Evolving on the Tibetan Plateau, Study Finds Blood Traits Boost Reproductive Success

On the Tibetan Plateau, the air carries notably less oxygen with every breath, the same condition that gives mountain climbers altitude sickness. Yet communities there have lived and thrived for more than 10,000 years, and over that span the bodies of the people who settled there have changed to make the most of an atmosphere that, for most humans, would leave tissues short of oxygen, a state known as hypoxia.

Cynthia Beall, an anthropologist at Case Western Reserve University in the US, has studied how people respond to hypoxic living conditions for years. She told ScienceAlert that high-altitude adaptation is compelling because the stress is severe, felt equally by everyone at a given altitude, and measurable. In research published in October 2024, she and her team identified specific adaptations in Tibetan communities: traits that improve how well the blood delivers oxygen.

To find them, the researchers looked at reproductive success, one marker of evolutionary fitness. The logic is straightforward. Women who deliver live babies pass their traits to the next generation, and the traits that best suit an environment should be most common in women who survive the strains of pregnancy and childbirth. Those women tend to have more children, and those children inherit the survivability traits and are more likely to live, reproduce, and pass them on. That is natural selection. It can be counterintuitive: where malaria is common, sickle cell anemia is also common, because the gene involved protects against the disease.

The team studied 417 women aged 46 to 86 who had spent their entire lives in Nepal at altitudes above 3,500 meters, or 11,480 feet. They recorded each woman's number of live births, which ranged from 0 to 14 and averaged 5.2, along with physical and health measurements. These included hemoglobin, the protein in red blood cells that carries oxygen to tissues, and how much oxygen that hemoglobin was actually carrying.

The women with the highest rates of live births did not have high or low hemoglobin levels. Theirs were average for the group. But the oxygen saturation of their hemoglobin was high. The pattern suggests these adaptations maximize oxygen delivery without thickening the blood, which would force the heart to push a more viscous fluid that resists flow.

Beall said that researchers previously knew lower hemoglobin was beneficial, and now understand that an intermediate value gives the greatest benefit. They also knew higher saturation helped, and now see that the higher it is, the greater the benefit, with live births quantifying the gain. What surprised her was that women could have many live births despite low values on some oxygen-transport traits, provided other traits were favorable.

The most reproductively successful women also had high blood flow into the lungs, and their hearts had wider-than-average left ventricles, the chamber that pumps oxygenated blood to the body. Together these traits raise the rate of oxygen transport and delivery.

Culture played a part too. The researchers found that women who began reproducing young and had long marriages had longer exposure to the possibility of pregnancy, which raised their birth counts. Even accounting for that, the physical traits mattered. Nepalese women whose physiology most resembled that of women in unstressed, low-altitude environments tended to have the highest reproductive success.

"This is a case of ongoing natural selection," Beall said, adding that understanding how such populations adapt gives a better grasp of human evolution. The work appeared in the Proceedings of the National Academy of Sciences, and this article is an updated version of one first published in October 2024.