Case Study: Tibetans

If you live in the lowlands, you may have experienced the huffing  and puffing that typically accompanies a trip to higher altitudes. That's because oxygen levels go down as one goes up. Traveling to  

Denver from sea level means a 17% decrease in available oxygen. Our  bodies compensate​ for even this small change with faster breathing and  a higher heart rate — at least until we acclimate​ to the thinner  atmosphere.  

What happens when you travel to the mountains?  

As you increase elevation the PO₂ in the air drops which affects the pressure in arterial blood (PAO₂). The brain detects these changes and sends a message to increase respiration rate, a  condition called hyperventilation​. You are likely to take deeper breaths in addition to breathing faster and your heart  rate will increase. You may experience dizziness, nausea, fatigue, and headaches. Low arterial PO₂ will cause the release of erythropoietin​ from the kidneys. EPO will stimulate the bone marrow to  produce more red blood cells to increase the concentration of hemoglobin in the blood. This hemoglobin will have  the effect of providing tissues with more oxygen.  

Extra hemoglobin may compensate for decreased oxygen levels, allowing breathing and heart rate to return to  normal. This is an example of phenotypic plasticity​, shifts in an organism's body, physiology, or behavior that are  dependent upon the environment it occupies, it is not a genetic change. People can usually acclimate to higher  altitudes within a couple of weeks.  

Tibetan highlanders have no trouble living at 13,000, and many of them can climb parts of Mount Everest without supplemental oxygen. How do they do it? New research makes it clear that Tibetan highlanders haven't just acclimated to their mountain home; they've evolved​ unique physiological mechanisms for dealing with low oxygen levels. 

The evolutionary 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 in those places.  

One of these adaptations is almost exactly the opposite of a lowlander  response to high altitude: Tibetans have gene​ versions that cause them to produce fewer red blood cells. How is that helpful? It turns out that extra red blood cells make blood thicker — more like honey than water — and after a certain point, this cell-laden blood can  actually get so thick that it doesn't pass through capillaries efficiently to oxygenate cells. Having blood with too many red blood cells  can be particularly problematic during pregnancy since it is linked to slow fetal growth and high rates of fetal mortality​.  

The basis for the Tibetans' adaptation is not a change in a gene that produces hemoglobin or any one of the  other proteins that make up red blood cells. Instead, the key change seems to be in a stretch of DNA, called EPAS1​, which  helps control the process of producing red blood cells. The change in EPAS1 seems to make Tibetans less likely to overproduce  red blood cells at extreme altitudes.  

Biologists compared the genomes of ethnic Tibetans to the genomes of Han Chinese individuals. The basic reasoning was that  if a particular gene version was found in Tibetans, but not in their close relatives who lived in lowlands (Han), then that gene  likely arose from natural selection​. It was found that the Tibetans were much more likely to have this gene than Han Chinese.  

Genetic studies estimate that the Tibetans split from the Han Chinese population and began migrating to the highlands less than  3000 years ago, which means adaptation for living at high altitudes occurred in the population in about a hundred generations.  That would represent the fastest example of human evolution ever documented!