Humans are not yet done cooking. We’re continuing to evolve and adjust to the world around us, the records of our adaptations written in our bodies.
We know that there are some environments that can make us unwell. Mountain climbers often succumb to altitude sickness – the body’s reaction to a significant drop in atmospheric pressure, which means less oxygen is taken in with each breath.
And yet, in high altitudes on the Tibetan Plateau, where oxygen levels in the air people breathe are notably lower than lower altitudes, human communities thrive.
In the more than 10,000 years the region has been settled, the bodies of those living there have changed in ways that allow the inhabitants to make the most of an atmosphere that for most humans would result in not enough oxygen being delivered via blood cells to the body’s tissues, a condition known as hypoxia.
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“Adaptation to high-altitude hypoxia is fascinating because the stress is severe, experienced equally by everyone at a given altitude, and quantifiable,” anthropologist Cynthia Beall of Case Western Reserve University in the US told ScienceAlert.
“It is a beautiful example of how and why our species has so much biological variation.”
Beall has been studying the human response to hypoxic living conditions for years. In research published in October 2024, she and her team unveiled some of the specific adaptations in Tibetan communities: traits that help the blood deliver oxygen.
To unlock this discovery, the researchers delved into one of the markers of what we call evolutionary fitness: reproductive success. Women who deliver live babies are those who pass on their traits to the next generation.
The traits that maximize an individual’s success in a given environment are most likely to be found in women who are able to survive the stresses of pregnancy and childbirth.
These women are more likely to give birth to more babies; and those babies, having inherited survivability traits from their mothers, are also more likely to survive to adulthood, and pass the traits on to the next generation.
That’s natural selection at work, and it can be a bit strange and counterintuitive; in places where malaria is common, for example, the incidence of sickle cell anemia is high, because it involves a gene that protects against malaria.
Beall and her team made a study of 417 women between the ages of 46 and 86 years who have lived all their lives in Nepal above altitudes of around 3,500 meters (11,480 feet). The researchers recorded the number of live births, ranging between 0 and 14 per woman for an average of 5.2, as well as health and physical information and measurements.
Among the things they measured were levels of hemoglobin, the protein in red blood cells responsible for delivering oxygen to tissues. They also measured how much oxygen was being carried by the hemoglobin. Interestingly, the women who demonstrated the highest rate of live births had hemoglobin levels that were neither high nor low, but average for the testing group.
But the oxygen saturation of the hemoglobin was high. Together, the results suggest that the adaptations are able to maximize oxygen delivery to cells and tissues without thickening the blood – a result that would place more stress on the heart as it struggles to pump a higher viscosity fluid more resistant to flow.
“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,” Beall said.
“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.”
The women with the highest reproductive success rate also had a high rate of blood flow into the lungs, and their hearts had wider than average left ventricles, the chamber of the heart responsible for pumping oxygenated blood into the body.
Taken all together, these traits increase the rate of oxygen transport and delivery, enabling the human body to make the most of the low oxygen in the air respired.
It’s important to note that cultural factors can play a role, too. Women who start reproducing young and have long marriages seem to have a longer exposure to the possibility of pregnancy, which also increases the number of live births, the researchers found.
Even taking that into account, however, the physical traits played a role. Nepalese women with physiologies most similar to women in unstressed, low altitude environments tended to have the highest rate of reproductive success.
“This is a case of ongoing natural selection,” Beall said. “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.
