Fans of mountaineering likely know all about altitude sickness. They may even have experienced it — difficulty breathing, dizziness, nausea, dangerous fluid buildups in the lungs and brain, thickened blood, and a stressed heart. With time and patience, people can adapt somewhat to altitude. But it’s long-term adaptations that concerned a team of scientists led by Cynthia Beall, an anthropologist at Case Western Reserve University in Cleveland, Ohio.
And by long-term, we mean really long-term.
In a paper published in The Proceedings of the National Academy of Sciences, Beall and her colleagues delved into the mechanisms that have allowed the Tibetan people to adapt to the ferocious altitudes of their homeland in the more than 10,000 years they’ve existed there.
To do so, they studied a population of over 400 Tibetan women who’ve lived their entire lives above 3,500m. Specifically, they measured their reproductive success (the number of live births, ranging from zero to 14, the women successfully carried out during their childbearing years).
Then they collected information on hemoglobin levels in the women’s blood. Hemoglobin is the protein that allows red blood cells to transfer oxygen to tissues. Finally, they measured the oxygen levels that the women’s hemoglobin could deliver.
If you don’t want to be gasping for breath up here, you’d better be the product of 10,000 years of human evolution. Photo: Shutterstock
More oxygen without thicker blood
The result? The women with the most successful pregnancies were those with average hemoglobin levels but better-than-average oxygen delivery by hemoglobin. This particular combination of traits maximizes tissue oxygenation without thickening the blood and adding stress to the heart — a godsend for high-altitude living.
“Previously, we knew that lower hemoglobin was beneficial,” Beall told ScienceAlert. “Now we understand that an intermediate value has the highest benefit. We [also] knew that higher oxygen saturation of hemoglobin was beneficial. Now we understand that the higher the saturation, the more beneficial.”
The study also revealed that the women with the most live births had larger-than-average left ventricles in their hearts. Their lungs were also more effective at oxygenating red blood cells.
The work is a fascinating window into natural selection and human evolution, unfolding practically in real time before our eyes. You can adapt a little to elevation in the short term, and being born and living your life at altitude also helps. But to truly thrive in the clouds, you need to be the end result of millennia of human adaptation.
“It is a beautiful example of how and why our species has so much biological variation,” Beall said.
