Humans are not done cooking yet. We constantly evolve and adapt to the world around us, and the record of our adaptations is written in our bodies.
We know that certain environments can make us uncomfortable. Climbers often suffer from altitude sickness, which is the body’s response to a significant drop in air pressure, meaning less oxygen is taken in with each breath.
However, in the high altitudes of the Tibetan Plateau, where the air people breathe contains significantly less oxygen, human communities thrive.
Over the more than 10,000 years since the area was settled, the bodies of the people who lived there underwent changes that allowed the inhabitants to take advantage of an atmosphere that, for most people, causes blood cells to be unable to deliver enough oxygen to body tissues, a condition known as hypoxia.
Watch the video below for a summary of the research:
“Adaptation to high-altitude hypoxia is fascinating because the stress is so severe, it’s experienced equally by everyone at a given altitude, and it’s quantifiable,” Cynthia Beall, an anthropologist at Case Western Reserve University, told ScienceAlert.
“This is a great example of how and why our species has so much biological variation.”
For many years, Bill has been studying human responses to oxygen-depleted living conditions. In research published in October 2024, she and her team revealed some specific adaptations in Tibetan communities: traits that increase the blood’s ability to carry oxygen.
To unravel this finding, the researchers looked at one of the hallmarks of what we call evolutionary fitness: reproductive success. Women who give birth to live babies are the ones who pass on their traits to the next generation.
The traits that make an individual most successful in a given environment are most likely to be found in women who are able to withstand the stress of pregnancy and childbirth.
These women are more likely to have more babies. These offspring inherit survival traits from their mother and are more likely to survive, reproduce, and inherit those same traits.
This is what natural selection does, but it may be a little weird and counterintuitive. For example, where malaria is endemic, sickle cell anemia occurs at high rates because it involves a gene that protects against malaria.
Beall and her team studied 417 women aged 46 to 86 who had lived their entire lives in Nepal at an altitude of more than 3,500 meters (11,480 feet). The researchers recorded the number of live births per woman (ranging from 0 to 14, with an average of 5.2) as well as physical and health measurements.
Sign up for ScienceAlert’s free fact-checking newsletter
Among the things they measured were levels of hemoglobin, the protein in red blood cells responsible for carrying oxygen to tissues. They also measured the amount of oxygen carried by the hemoglobin.
Interestingly, the women with the highest live birth rates had hemoglobin levels that were neither high nor low, but average for the test group. But their hemoglobin oxygen saturation was high.
Results suggest these adaptations maximize oxygen delivery to cells and tissues No Thickening of the blood – This result increases stress on the heart as it struggles to pump more viscous fluids that are harder to flow.
“Before we knew that lower hemoglobin was beneficial, now we know that the middle value has the highest benefit. We know that higher oxygen saturation of hemoglobin is beneficial, and now we know that higher saturation is more beneficial. The number of live births can quantify the benefit,” Beall said.
“Unexpectedly, women can have many live births with low values for some oxygen transport traits if they have good values for other oxygen transport traits.”
Women with the highest reproductive success rates also had high blood flow to their lungs and their hearts were wider than average in their left ventricles, which pump oxygenated blood into the body.
Collectively, these properties increase the rate of oxygen transport and delivery, allowing the body to take advantage of low oxygen levels in the air we breathe.
It’s worth noting that cultural factors also come into play. Researchers found that women who started having children at a younger age and were in longer-term marriages appeared to have a longer likelihood of becoming pregnant, which also increased the number of live births.
However, even taking this into account, physical characteristics play a role. Nepalese women whose anatomy was most similar to women in stress-free, low-altitude environments tended to have the highest reproductive success.
Related: New evidence suggests evolution itself may actually be evolving
“This is an example of ongoing natural selection,” Beer said. “Understanding how these populations adapted could give us a better grasp of the process of human evolution.”
The research was published in Proceedings of the National Academy of Sciences.
An earlier version of this article was published in October 2024.
