Researchers have long wondered why Tibetans can live at elevations that cause some humans to become life-threateningly ill.

A new study answers that mystery, in part, by showing that through thousands of years of natural selection, those hardy inhabitants of south-central Asia evolved 10 unique oxygen-processing genes that help them live in higher climes.

A study by researchers from the University of Utah School of Medicine and Qinghai University Medical School in China reports that thousands of years ago, Tibetan highlanders began to genetically adapt to prevent polycythemia (a process in which the body produces excess red blood cells in response to oxygen deprivation). It also prevented other health abnormalities such as swelling of the lungs and brain (edema) and hypertension of the lung vessels leading to eventual respiratory failure.

Even at elevations of 4,267 meters above sea level or higher, where the atmosphere contains much less oxygen than at sea level, most Tibetans do not overproduce red cells and do not develop lung or brain complications.

The researchers found evidence this might be related to at least 10 genes, two of which are 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 they're used to can also be stricken with the condition.

The Tibetans have evolved genes that others living at similar elevations have not developed, according to Utah's Lynn Jorde, a senior author on the study.

"For the first time, we have genes that help explain that adaptation," Jorde said.

A detailed understanding of how these genes have evolved may eventually lead to new therapies for common maladies, including pulmonary hypertension and lung and brain edema.