The physiological challenges of living so far above sea level are not trivial, and yet people with roots in these areas are largely able to avoid these problems. Biologists aren't sure how they do it. When lowlanders travel to high altitudes, their bodies produce more red blood cells, which house the oxygen-carrying protein hemoglobin. However, Tibetans living at high altitudes have red blood cell levels and hemoglobin levels similar to those of lowlanders at sea level — and as a consequence of the low oxygen levels at those altitudes, Tibetans live with 10% less oxygen in their blood than most other people. Lower oxygen levels might seem like a disadvantage, yet highland women have fewer fertility problems than lowlanders living at high altitudes, have better blood flow to the uterus during pregnancy, and deliver heavier, healthier babies. For these reasons, biologists are convinced that there is an evolutionary explanation for Tibetans' success — that over generations of living at high altitudes, natural selection has favored traits that allow Tibetans to survive and reproduce in this extreme environment. But which traits?
evolution.berkeley.edu/evolibrary/article/0_0_0/huertasan...