Scientists have long wondered why the sherpas of the Tibetan Highlands can negotiate with ease elevations that cause some humans to become life-threateningly ill. Tibetans live at altitudes of 13,000 feet, breathing air that has 40 percent less oxygen than is available at sea level, yet suffer very little mountain sickness. The reason, according to a team of biologists in China, is human evolution, in what may be the most recent and fastest instance detected so far.
Comparing the genomes of Tibetans and Han Chinese, the majority ethnic group in China, the biologists found that at least 30 genes had undergone evolutionary change in the Tibetans as they adapted to life on the high plateau. Tibetans and Han Chinese split apart as recently as 3,000 years ago, say the biologists, a group at the Beijing Genomics Institute led by Xin Yi and Jian Wang, according to recent reports in Science and the New York Times.
Even at elevations of 14,000 feet above sea level or higher, where the atmosphere contains much less oxygen than at sea level, most Tibetans do not overproduce red blood cells and do not develop lung or brain complications. The researchers found evidence that this might be related to at least 10 genes, two of which are specific genes strongly associated with hemoglobin, a molecule that transports oxygen in the blood.
High-altitude lung and brain complications threaten and even kill mountaineers who scale the world's tallest peaks. Others who find themselves at elevations significantly higher than where they normally live and work also can be stricken with the condition. Adaptations to living at higher altitudes have occurred in humans more than once, such as with people indigenous to the Andes Mountains in South America and people native to high altitude regions in the Ethiopian mountains in Africa.
But the Tibetans have evolved genes that others living at similar elevations have not developed, according to Lynn B. Jorde, Ph.D., professor and chair of human genetics at the U of U School of Medicine and a senior author on the study. "For the first time, we have genes that help explain that adaptation," Jorde said.
The study was undertaken after Josef T. Prchal, M.D., a hematologist and professor of internal medicine, approached Jorde about doing genetic analysis related to his research on polycythemia.
"What's unique about Tibetans is they don't develop high red blood cells counts," said Prchal, also a senior author on the study who has done research in Tibet. "If we can understand this, we can develop therapies for human disease."
Although much work remains, and there could be other physiological reasons for the Tibetans' ability to thrive at higher elevations, the researchers believe those 10 genes might have allowed the Tibetans to evolve more efficient metabolisms and not overproduce red blood cells in response to thinner air. The Tibetans also show higher levels of nitric oxide, a molecule that may help get more oxygen to tissues and prevent polycythemia.
"This might help make up for having fewer red blood cells," Jorde said. A detailed understanding of these changes may eventually lead to targeted therapies for common human maladies, including pulmonary hypertension and lung and brain edema, which affect people everywhere.
Casey Kazan via University of Utah