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Study finds that Tibetans with Oxygen Rich Blood have more Surviving Children |
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Written by Staff Writer
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May 16, 2007 at 09:53 PM |
Researchers seldom see Darwinian natural selection happening in living people. So physical anthropologist Cynthia Beall was delighted in 2004 when she discovered a trait that boosts the survival of some Tibetan children, apparently by raising the level of oxygen in their mothers' tissues — a crucial advantage during pregnancy 4 kilometers above sea level.
Now Beall has updated her study by exploring a possible mechanism for the adaptation and by documenting that the adaptation represents some of the strongest natural selection yet measured in humans. Her team is showing how a genetic trait can dramatically improve survival in real life, in living mothers and babies. "I love this work,' says Mark Gladwin, chief of the vascular medicine branch of the National Heart, Lung, and Blood Institute in Bethesda, Maryland. "It shows you need a strong adaptive response to have children survive at high altitude.
Beall, of Case Western Reserve University in Cleveland, Ohio, and her colleagues have for a decade been gathering all sorts of family, biological, and fertility data from men, women, and children living in more than 900 households in 14 villages in Tibet. As part of the study, they also tracked the survival of children born to mothers who had high and low oxygen saturation in their blood. Beall reported at the meeting that women with high Levels of oxygen in their blood had more than twice as many surviving babies as had those with low oxygen levels—a ratio of 1:0.44. This is a startlingly strong selection pressure, she says — even stronger than that on the sickle cell gene, which protects against malaria and has a fitness ratio of 1:0,66.
But exactly how do these women manage to carry extra oxygen in their blood? They do not produce more hemoglobin the way Andeans living at high altitude do. One possibility is that the women with high oxygen have an adaptation that Beall is exploring independently in these same Tibetan villagers. She found that some villagers exhale extra nitric oxide in their breath, a sign of additional amounts of the gas in their blood. In those tibetans, nitric oxide dilates the blood vessels so they can pump more blood and oxygen to organs and tissues, as measured by images of the heart and lung blood vessels. The tibetans can boost their blood volume - and so pump more oxygen to their tissues - without producing more haemoglobin or raiging the blood pressure in their lungs. That's the reverse of what happens when mountaineers suffer from oxygen deficiency: The blood pressure in their lungs rises, the blood vessels tanstrIct. and fluid builds up, suffocating the lungs.
The next step, says Beall, is to try to see whether these two lines of research meet. She wants to find the underlying gene behind the women's high oxygen blood — and see whether it is relayed to genes that regulate levels of nitric oxide in the blood. She notes, however, that it's quite possible that the Tibetans have evolved more than one way to boost blood oxygen, and that these are independent adaptations. Gladwin suggests that Beall's team also measure nitric oxide and blood pressure in the lungs in pregnant women, who are under the most physiological stress at altitude and presumably would benefit most from this adaptation. "Study the pregnant wornen,' he says. "because that's where you'll see evolution in action.'
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Last Updated ( Aug 29, 2007 at 10:41 AM )
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