Evolution in action: How ethnic Tibetan women thrive in thin oxygen at high altitudes

A new study led by Cynthia Beall, Distinguished University Professor Emerita at Case Western Reserve University, answers some of those questions. The new research, recently published in the journal Proceedings of the National Academy of Sciences of the United States of America(PNAS), reveals how the Tibetan women's physiological traits enhance their ability to reproduce in such an oxygen-scarce environment.

The findings, Beall said, not only underscore the remarkable resilience of Tibetan women but also provide valuable insights into the ways humans can adapt in extreme environments. Such research also offers clues about human development, how we might respond to future environmental challenges, and the pathobiology of people with illnesses associated with hypoxia at all altitudes.

"Understanding how populations like these adapt," Beall said, "gives us a better grasp of the processes of human evolution."

The study

Beall and her team research studied 417 Tibetan women age 46 to 86 who live between 12,000 and 14,000 feet above sea level in location in Upper Mustang, Nepal on the southern edge of the Tibetan Plateau.

They collected data on the women's reproductive histories, physiological measurements, DNA samples and social factors. They wanted to understand how oxygen delivery traits in the face of high-altitude hypoxia (low levels of oxygen in the air and the blood) influence the number of live births -- a key measure of evolutionary fitness.

Adaptation into thin air

They discovered that the women who had the most children had a unique set of blood and heart traits that helped their bodies deliver oxygen. Women reporting the most live births, had levels of hemoglobin, the molecule that carries oxygen, near the sample's average, but their oxygen saturation was higher, allowing more efficient oxygen delivery to cells without increasing blood viscosity; the thicker the blood, the more strain on the heart.

"This is a case of ongoing natural selection," said Beall, also the university's Sarah Idell Pyle Professor of Anthropology. "Tibetan women have evolved in a way that balances the body's oxygen needs without overworking the heart."

A window into human evolution

Beall's interdisciplinary research team, which included longtime collaborators Brian Hoit and Kingman Strohl, from the Case Western Reserve School of Medicine, and other U.S. and international researchers, conducted fieldwork in 2019. The team worked closely with local communities in the Nepal Himalayas, hiring local women as research assistants and collaborating with community leaders.

One genetic trait they studied likely originated from the Denisovans who lived In Siberia about 50,000 years ago; their descendants later migrated onto the Tibetan Plateau. The trait is a variant of the EPAS1 gene that is unique to populations indigenous to the Tibetan Plateau and regulates hemoglobin concentration. Other traits, such as increased blood-flow to the lungs and wider heart ventricles, further enhanced oxygen delivery. These traits contributed to greater reproductive success, offering insight into how humans adapt to lifelong levels of low oxygen in the air and their bodies.