Many people, from medical scientists to athletes, have searched for ways to increase the human capacity to process oxygen. Increased blood oxygen levels have many benefits for those wishing to increase their aerobic capacity and endurance during exercise, and small increases in blood oxygen capacity can be achieved by training at high altitudes where oxygen levels are lower. While athletes have focussed on short time frames in single individuals, several scientists have begun asking how populations of people living at high altitudes, who breath less oxygen, have adapted to this environment on an evolutionary time scale. Peoples living at greater than 8,000 feet above sea level have clearly adapted to their relative lack of oxygen, as non-natives that attempt to live at such high altitudes often suffer from acute hypoxia, which is absent in natives.
One well-studied and understood example of this type of adaptation occurs in native South Americans living on the Andean Plateau. Compared to lowlanders, they breathe at the same rate, have a similar red blood cell count and do not have any novel hemoglobin variants. However, each red blood cell in the body holds a greater amount of total hemoglobin. This allows for the transport of more gasses throughout the body without increasing blood viscosity. This is in contrast to Tibetans, who have been found to have unchanged hemoglobin levels, but simply take more breaths during a given time, which increases the rate of gas exchange in the lungs. They have also been found to have abnormally high levels of nitrous oxide in their blood, which is a gas synthesized in the body that triggers vasodilatation. This increases blood flow to peripheral tissues and increase the rate of gas exchange. A third population located in the highlands of Ethiopia has been investigated as well, but no change in any of the variables already mentioned were found. This suggests that there are more subtle or complex factors regulating gas exchange, and may open up an exciting new line of investigation.
These distinct, yet similarly effective, mechanisms of human evolutionary adaptation to decreased atmospheric oxygen levels highlight the range of variables that evolution can act on to solve environmental challenges. It additionally illustrates the way in which natural selection is forced to utilize the first acceptable variation found in a population, rather than waiting for the best solution.
