Where Tibetan highlanders live, the oxygen level is only about 60% of that at sea level. Since every cell in our body needs oxygen to survive, low oxygen levels pose a serious health concern for most people. At such altitudes, people whose ethnic roots trace to low-lying geographic regions are likely to experience nausea, vomiting, dizziness, high blood pressure, and worse — fluid in the lungs and swelling of the brain, which can be life-threatening. Lowlanders who live at high altitudes for long periods of time are likely to develop heart and lung disease. In addition, women from lowland backgrounds have decreased fertility at high altitudes, and the children they do have experience increased rates of infant mortality. For most people, there is a clear fitness cost associated with living at high altitudes.
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.1 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?
When the Beijing Genomics Institute approached the lab in which Emilia worked about a collaboration to investigate that question, she was excited to be part of the team to take a crack at it…
The adaptations that allow Tibetans to function at high altitudes are very different from the acclimatization process that most of us go through when we spend time at those heights. When lowlanders visit Denver, La Paz, or Lhasa, for example, their bodies acclimatize by producing more red blood cells. This change may help compensate for decreased oxygen levels in the short term, allowing breathing and heart rate to return to normal. What is involved is not a genetic change, but phenotypic plasticity, a change to a trait in response to environmental factors. True evolutionary adaptations, on the other hand, involve a change in the genetic makeup of individuals. For all the reasons described on this page, Emilia and her colleagues suspected that the heritable traits possessed by Tibetans represent true evolutionary adaptations.
1Moore, L.G., D. Young, R.E. McCullough, T. Droma, and S. Zamudio. 2001. Tibetan protection from intrauterine grown restriction (IUGR) and reproductive loss at high altitude. American Journal of Human Biology 13:635-644.
Beall, C.M. 2003. High-altitude adaptations. The Lancet 362:14-15.