Once non-genetic factors such as age, illness, or smoking were removed, a subset of the group seemed to have a blood-oxygen concentration that was 10% higher than normal. This trait was inherited in a way that suggested the difference was due to a single gene.
The researchers also found that the children of women with this putative gene are much more likely to survive to the age of 15, when they are old enough to have children of their own. In the low-oxygen group, each woman had on average 2.5 children that died during childhood. In the high-oxygen group, that average was just 0.4.
The fact that this genetic variation (which has not yet been identified) is under such active selection suggests that it is a variation of fairly recent vintage. A successful adaptation that has been around for a long time in some ecological niche will tend to be present in all organisms of the species which are in that niche unless they have only recently entered that niche. Of course, human conquest where one group wipes out most of another group in some niche could bring the invader group into the niche that the conquered group is already adapted to (even if the conquered group was not well enough adapted to warfare). So the adaptation could have been around for a long time and yet still happen to be getting rapidly selected for in the present time.
Human populations in less developed areas are not the only human groups still undergoing changes in frequencies of genetic variations due to natural selective pressures. All humans are still subject to selective pressures. Darwinian natural selection has not stopped in industrialized countries. What has changed is what is being selected for or against (e.g. intelligence is currently being selected against unfortunately).
The three major human population groups living at high altitudes in Ethiopia, the Himalayas, and the Andes have developed three different sets of adaptations to high altitude living. One reason that their sets of adaptations are different is likely just plain chance. But another reason is that they have not all been living at high altitudes for the same length of time. Initial genetic adaptations are usually not as ideal as adaptations that arise over longer periods of time.
The Andeans, whose lowland ancestors migrated from Asia perhaps 16,000 years ago, adjust to altitude essentially the same way as any lowlander would today -- and it is not a perfect solution.
"Creating more red cells is a pathological response," said Temple University anthropologist Charles Weitz. "If you have too many red cells, the blood's too thick, and it's like pumping oil. Eventually you have to move downhill."
By this same argument athletes who use erythropoietin (EPO) to boost their red blood cell counts are like evolutionary primitives. In the future better understanding of genetic variations for high altitude living will allow athletes to become more like Ethiopians or Himalyan Tibetans.
All of the Tibetans had significantly higher levels of a free radical-fighting enzyme, or antioxidant, called glutathione-S-transferase, and another enzyme, enoyl coenzyme A hydratase, that improves cellular energy production. Tibetans also possessed fewer mitochondria, which are the power manufacturers of cells.
Anthropologist Cynthia Beall of Case Western Reserve University said that Tibetans breathe more per minute than people who live at sea level. Andes residents are able to hold more oxygen in their blood.
Healthy sea-level dwellers have saturations of oxygen in their bloods that vary from 92-100 percent. In the Amabaras sample, the oxygen saturation averaged 95 percent, which surprised Beall, because the oxygen saturation in the Andean and Tibetan highlanders at similar altitudes was in the mid to high 80s.
Of course, once gene therapies and stem cell therapies become feasible then people will be able to have their choice of the best Himalayan and Ethiopian high altitude genetic enhancements. Some will use these enhancements for distance running. Others will enhance in order to do mountaineering. Expect to see extreme mountaineers use genetic engineering to so enhance their oxygen processing capacity that they will be able to easily climb Mount Everest without using supplemental oxygen and without suffering oxygen deprivation symptoms.
|Share |||Randall Parker, 2004 September 16 04:36 PM Trends, Human Evolution|