It's hard to breathe when oxygen is scarce. But some Tibetan women who live at high altitudes can carry more oxygen in their blood than others. New research shows that children of these women have a survival advantage--a dramatic example of natural selection at work in humans.
People who live at high altitudes adjust to the thin air by increasing the percentage of red blood cells, the oxygen-carrying cells, in their blood. Previous studies have suggested that some Tibetans also inherit the ability to boost the amount of oxygen carried by their hemoglobin--the molecule in red blood cells that snags oxygen.
To see if higher oxygen capacity of hemoglobin confers a survival advantage on children, physical anthropologist Cynthia Beall of Case Western Reserve University in Cleveland, Ohio, and others investigated all the women--more than 1700--who lived in three areas in Tibet above 3800 meters. By examining the family tree for each village and testing the women's blood to see how much oxygen their hemoglobin could carry, the researchers were able to determine how hemoglobin's oxygen capacity is inherited.
This analysis suggested that there are two genetic variations, or alleles: one for high oxygen capacity, and one for low oxygen capacity. Because the trait is passed down from both parents, a woman can have two copies of the high-capacity allele, two copies of the low-capacity allele, or one of each. Mom's draw in this genetic lottery had a big impact on her baby's chances, the researchers found. If a woman had two copies of the low-capacity allele, her baby had only a 60% chance of surviving the first year of life. On the other hand, if mom had just one copy of the high-capacity allele, her baby's chances improved to 90%. Two copies improved the odds to 94%, the team reported online 7 September in Proceedings of the National Academy of Sciences.
The work shows that low oxygen at high altitudes encourages survival of people whose blood can carry more oxygen, says anthropologist Henry Harpending of the University of Utah, Salt Lake City. The result is "a dramatic example of natural selection," he says. But Harpending is puzzled that the low-capacity allele is still around, given the lower survival rates of children who carry it. It could be that the high-capacity allele came along only recently or that it somehow becomes a disadvantage for adults, he says.