Humans are still evolving and adapting to their surroundings, with evidence of these changes visible in our own bodies. Some environments can adversely affect our health. For instance, mountain climbers often encounter altitude sickness, a result of decreased atmospheric pressure that reduces oxygen intake with each breath.
However, on the Tibetan Plateau, despite the low oxygen levels in the air, human communities have thrived. Over the past 10,000 years, the bodies of those living in this high-altitude region have adapted significantly.
These adaptations enable the inhabitants to efficiently utilize an atmosphere that would typically cause hypoxia, a condition where insufficient oxygen is delivered to body tissues through blood cells, in most humans.
Watch the video below for a summary of the research:
frameborder=”0″ allow=”accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share” referrerpolicy=”strict-origin-when-cross-origin” allowfullscreen>“Adaptation to high-altitude hypoxia is fascinating because the stress is severe, experienced equally by everyone at a given altitude, and quantifiable,” said anthropologist Cynthia Beall from Case Western Reserve University in the US to ScienceAlert. “It is a beautiful example of how and why our species has so much biological variation.”
Cynthia Beall has been investigating the human response to hypoxic environments for years. In research published in October 2024, she and her team uncovered specific adaptations in Tibetan communities, enhancing the blood’s ability to transport oxygen.
To understand these adaptations, the researchers examined a key indicator of evolutionary fitness: reproductive success. Women who give birth to live babies pass their traits to future generations.
Traits that enhance an individual’s success in a specific environment are often found in women who endure the challenges of pregnancy and childbirth. These women tend to have more children, passing on advantageous traits to their offspring, who are then more likely to survive and reproduce. This cycle exemplifies natural selection.
Natural selection can yield unexpected results; for instance, in regions plagued by malaria, sickle cell anemia is prevalent because it involves a gene that offers malaria resistance.
Beall and her team conducted a study involving 417 women, aged 46 to 86, who spent their entire lives in Nepal at elevations exceeding 3,500 meters (11,480 feet).
The study recorded the number of live births per woman, ranging from 0 to 14, with an average of 5.2, along with various physical and health measurements.
Key measurements included levels of hemoglobin, a protein in red blood cells vital for oxygen transport to tissues, and the amount of oxygen carried by the hemoglobin.
Interestingly, women with the highest live birth rates had hemoglobin levels that were neither high nor low but rather average for the group. However, their hemoglobin’s oxygen saturation was notably high.
The findings imply that these adaptations optimize oxygen delivery to cells without thickening the blood, which would otherwise increase cardiac stress by requiring the heart to pump a more viscous fluid.
Beall noted, “Previously, we knew that lower hemoglobin was beneficial; now we understand that an intermediate value has the highest benefit. We knew that higher oxygen saturation of hemoglobin was beneficial; now we understand that the higher the saturation, the more beneficial. The number of live births quantifies the benefits.”
“It was unexpected to find that women can have many live births with low values of some oxygen transport traits if they have favorable values of other oxygen transport traits.”
Women with the highest reproductive success also exhibited increased blood flow to the lungs and had wider-than-average left ventricles, the heart chamber that pumps oxygenated blood to the body. Together, these traits enhance oxygen transport and utilization, enabling effective adaptation to low oxygen levels in the air.
Cultural factors also influence outcomes. Women who begin childbearing at a young age and maintain long marriages have more opportunities for pregnancy, resulting in more live births, the researchers observed.
Despite cultural influences, physical traits played a significant role. Nepalese women whose physiologies closely resembled those in low-altitude environments had the highest reproductive success rates.
Related: Humans in The Andes Appear to Have Evolved a Strange Genetic Ability
“This is a case of ongoing natural selection,” Beall remarked. “Understanding how populations like these adapt gives us a better grasp of the processes of human evolution.”
The research was published in the Proceedings of the National Academy of Sciences.
An earlier version of this article was published in October 2024.
