Tibetan women on the high Tibetan Plateau have adapted to their oxygen-scarce environment over thousands of years, thriving despite thin air.
Recent studies reveal these adaptations include unique blood and heart traits that optimize oxygen delivery, supported by specific genetic variations such as the EPAS1 gene, linked to ancient Denisovans.
High Altitude Adaptation
Breathing thin air at high altitudes poses a serious challenge—there’s simply less oxygen with every breath. Yet, for over 10,000 years, Tibetan women living on the Tibetan Plateau have not only survived but thrived in this harsh environment.
A recent study led by Cynthia Beall, Distinguished University Professor Emerita at Case Western Reserve University, sheds light on this phenomenon. Published today (October 21) in the Proceedings of the National Academy of Sciences (PNAS), the research reveals how Tibetan women’s unique physiological traits improve their ability to reproduce in an oxygen-poor environment.
These findings highlight not only the remarkable resilience of Tibetan women but also offer valuable insights into how humans can adapt to extreme conditions. This research also provides clues about human evolution, how we might face future environmental challenges, and the biology of conditions linked to low oxygen levels at any altitude.
“Understanding how populations like these adapt,” Beall said, “gives us a better grasp of the processes of human evolution.”
Exploring Evolutionary Fitness at High Altitudes
Beall and her team research studied 417 Tibetan women aged 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.
Unveiling Natural Selection Through Reproductive Success
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.”
Tracing Genetic Contributions to Adaptation
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.
Reference: “Higher oxygen content and transport characterize high-altitude ethnic Tibetan women with the highest lifetime reproductive success” by Shenghao Ye, Jiayang Sun, Sienna R. Craig, Anna Di Rienzo, David Witonsky, James J. Yu, Esteban A. Moya, Tatum S. Simonson, Frank L. Powell, Buddha Basnyat, Kingman P. Strohl, Brian D. Hoit and Cynthia M. Beall, 21 October 2024, Proceedings of the National Academy of Sciences.
DOI: 10.1073/pnas.2403309121
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