Human adaptation to extreme environments is an essential subject of study to understand the mechanisms of survival. High altitudes, where oxygen becomes scarce, pose a major biological challenge. However, populations living on the Tibetan plateau, at more than 3,500 meters, have developed remarkable physiological adaptations, particularly women.
A recent study, led by Cynthia Beall of Case Western Reserve University and published in the journal Proceedings of the National Academy of Sciencesfocuses specifically on Tibetan women. The goal is to explore how these women evolved to survive and reproduce in an environment marked by constant hypoxia. By measuring parameters such as hemoglobin concentration and blood oxygenation, this research uncovers the processes of natural selection that continue to shape these populations.
The challenges of living at high altitude
Living at altitudes exceeding 3,500 meters imposes intense physiological constraints on human beings. Hypoxia, characterized by a low concentration of oxygen in the air, forces the body to operate with a reduced amount of oxygen. Which obviously disrupts vital functions such as breathing and blood circulation. For the majority of individuals, this situation leads to symptoms of shortness of breath and sleep problems.
We can go as far as severe pathologies such as pulmonary or cerebral edema at altitude. The heart must pump harder and more often, and the blood must carry oxygen more efficiently. In certain populations, this can lead to complications. On the other hand, Tibetans, and particularly Tibetan women, have developed unique mechanisms allowing them to live normally, to work, and even to give birth in this environment where atmospheric pressure and oxygen availability are drastically reduced.
Anthropologist Cynthia Beall of Case Western Reserve University has conducted several studies on Tibetan women living in the Upper Mustang Mountains of Nepal. This region is located between 3,500 and 4,100 meters above sea level. Beall’s team observed that these women exhibited specific physiological adaptations that allowed them not only to survive, but also to have large families. A key indicator of evolutionary success. They then studied 417 women aged 46 to 86, and highlighted unique traits.
Hemoglobin and oxygen saturation: the keys to success for women?
The reproductive success of Tibetan women is deeply linked to their ability to maintain a delicate balance in the transport of oxygen in their body. Cynthia Beall’s team found that women who gave birth to more children had moderate levels of hemoglobin, associated with particularly high oxygen saturation.
The importance of women’s hearts in survival and reproduction
Besides hemoglobin levels and oxygen saturation, certain cardiac traits play a fundamental role in Tibetan women. Cynthia Beall’s team’s research found that those who had the most children had high blood flow to the lungs. In fact, blood oxygenation is considerably improved. This is because increased blood flow to the lungs allows for maximum oxygen absorption in depleted air, thereby optimizing the amount of oxygen the body can use.
Additionally, these women have larger heart ventricles. This allows their hearts to efficiently pump larger volumes of oxygenated blood to the rest of the body. This adaptation reduces the load on the heart. It improves oxygen distribution, particularly essential in low oxygen conditions such as those of the Tibetan Plateau.
These physiological adaptations illustrate how the human body can adjust to survive in extreme conditions. Researchers suggest that some of these traits may have an ancient genetic origin, inherited from the Denisovans. They inhabited the region around 50,000 years ago. The EPAS1 gene, in particular, plays a central role in this adaptation. It regulates hemoglobin production and the oxygen balance in the blood.
It may have been positively selected in Tibetan populations. It thus promotes optimal oxygen supply without the risks associated with too thick blood. This genetic heritage, combined with specific cardiovascular traits, allows Tibetan women to maintain high rates of reproductive success despite the harsh high-altitude environment.
Natural selection at work
The example of Tibetan women clearly illustrates that natural selection continues to shape human evolution. The physiological adaptations they exhibit are not simple vestiges of the past, but evolutionary mechanisms still active. These traits allow Tibetan women to not only survive in environments where oxygen is scarce, but also to reproduce successfully.
Cynthia Beall underlines the importance of these discoveries for understanding human evolution. These studies are not limited to shedding light on the evolution of Tibetan populations. They also have broader repercussions, particularly for medicine. By understanding how certain human groups have adapted to low oxygen levels, these results open perspectives on the treatment of diseases associated with hypoxia, such as certain forms of heart failure. They provide valuable biological models for studying how the human organism can adjust to extreme stress conditions.
Source : Beall, Cynthia M., et al., “Higher oxygen content and transport characterize high-altitude ethnic Tibetan women with the highest lifetime reproductive success”, Proceedings of the National Academy of Sciences (2024)
