Humanity hasn't yet reached the end of its culinary career. We continue to evolve and adapt to the world around us, and the traces of our adaptations are imprinted on our bodies.
We know that certain environmental conditions can cause discomfort. Mountain climbers often suffer from altitude sickness—the body's reaction to a significant drop in atmospheric pressure, which reduces the oxygen intake with each breath.
Yet at high altitudes on the Tibetan Plateau, where oxygen levels in the air people breathe are noticeably low, human settlements thrive.
Over the more than 10,000 years of inhabitation of this region, the bodies of its inhabitants have undergone changes.
They have been modified in such a way that they allow the inhabitants to make the most efficient use of an atmosphere that, for most people, would result in insufficient oxygen supply to the body's tissues through blood cells, a condition known as hypoxia.
Watch the video below for a brief overview of the study:
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«"Adaptation to high-altitude hypoxia is a remarkable process because the stress is so intense, so uniformly felt by everyone at a given altitude, and so measurable," anthropologist Cynthia Beall of Case Western Reserve University in the US told ScienceAlert.
«"This is a wonderful example of how and why our species has such enormous biological diversity.".
Bill has been studying the human response to hypoxic living conditions for many years. In a study published in October 2024, she and her team identified some specific adaptations in Tibetan communities: traits that improve the blood's ability to deliver oxygen.
To unlock this secret, researchers studied one measure of what we call evolutionary fitness: reproductive success.
Women who give birth to live children are the ones who pass on their traits to the next generation.
Lo Manthang in Nepal, where some of the data was collected. (James J. Yu)
The qualities that best contribute to a person'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 have more children.
Offspring that inherit survival traits from their mothers are also more likely to survive, reproduce, and pass on those same traits to their offspring.
This is natural selection in action.
Natural selection can be somewhat strange and counterintuitive; for example, in places where malaria is common, sickle cell disease is high because it is linked to a gene that protects against malaria.
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Bill and her team studied data from 417 women, aged 46 to 86, who had lived their entire lives in Nepal at altitudes above 3,500 metres (11,480 feet).
The researchers recorded the number of live births per woman—from 0 to 14, with an average of 5.2—as well as physical and medical indicators.
Among other things, they measured the level of hemoglobin, a protein in red blood cells responsible for delivering oxygen to tissues.
They also measured how much oxygen was carried by hemoglobin.
Notably, women with the highest live birth rate had hemoglobin levels that were neither high nor low, but rather average for the entire study group.
But the hemoglobin oxygen saturation was high.
Hemoglobin in red blood cells is responsible for delivering oxygen to tissues. (Photo archive Science/Canva)
The results show that these adaptations allow for maximal oxygen delivery to cells and tissues. without blood thickening, a condition that would increase the workload of the heart as it would have difficulty pumping fluid with higher viscosity and greater resistance to flow.
«"We used to know that lower hemoglobin levels were beneficial; now we understand that the most benefit comes from somewhere in between," Bill said.
«"We knew that higher hemoglobin oxygen saturation was beneficial; now we understand that the higher the saturation, the more beneficial. Live birth rates confirm this benefit.".
«"It was unexpectedly discovered that women can have many live-born babies with low values of some oxygen transport parameters, if other oxygen transport parameters have favorable values.".
Noninvasive measurement of hemoglobin concentration and blood oxygen saturation. (Sienna R. Craig)
Women with the highest conception success rates also had high blood flow to the lungs, and their hearts had larger-than-average left ventricles—the chambers of the heart responsible for pumping oxygen-rich blood to the body.
Together, these properties increase the rate of oxygen transport and delivery, allowing the human body to make maximum use of the low oxygen content in the air during breathing.
It's important to note that cultural factors may also play a role. Researchers have found that women who begin having children at a young age and are in long-term marriages have longer pregnancy rates, which also increases the number of live births.
However, even taking this into account, physical characteristics also played a role. Nepalese women, whose physiology most closely resembled that of women living in low-altitude environments and experiencing less stress, tended to have the highest reproductive success rates.
See also: People in the Andes appear to have developed a strange genetic ability.
«"It's an example of natural selection going on," Bill said.
«"Understanding how such populations adapt allows us to better understand the processes of human evolution.".
The results of the study were published in the journal Proceedings of the National Academy of Sciences.
An earlier version of this article was published in October 2024.
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