Tibetan and Ethiopian highlanders share a biological adaptation that enables them to thrive in the low oxygen levels of high altitudes, but the ability to pass on the trait appears to be linked to different genes in the two groups.

The adaptation is the share is the ability to maintain a low (for high altitudes) level of hemoglobin, the oxygen-carrying protein in red blood cells. Members of populations who historically live at low altitudes respond to the thin air by increasing hemoglobin levels. The response can help draw oxygen into the body to try and avoid hypoxia, but that increases blood viscosity and the risks for thrombosis, stroke and difficulties with pregnancies.

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