Competitive breath-hold divers have only two options to increase their time underwater - through training, they can try to boost their lung capacity or increase their red blood cell count. Over hundreds, if not thousands of years, however, a group of Southeast Asian "sea nomads" known for their deep-diving prowess has evolved a better solution: larger spleens. The spleen holds oxygenated red blood cells, so presumably an enlarged spleen (those of the sea nomads, or Bajau people, are about 50 percent larger than the spleens of unrelated, non-diving neighboring groups) injects more blood cells into the circulation and makes more oxygen available for basic body functions during prolonged dives. The physical and genetic changes that have enabled the Bajau to dive longer and deeper is yet another example of the immense variety of human adaption to extreme environments, in this case, environments with low levels of oxygen, said Dr. Rasmus Nielsen, a Professor of Integrative Biology at the University of California, Berkeley. These examples can be key to understanding human physiology and human genetics. "We can't really make experiments in humans, where we expose people to new conditions and have controlled genetic experiments in the same way we can do in fruit flies and mice," Dr. Nielsen said. "But nature has made experiments for us that tell us how humans react and adapt genetically to a whole new set of physiological conditions, so that we can explore and learn much more about the interaction between genetics and physiology." The surprise finding led researchers from the University of Copenhagen and UC Berkeley to a genetic mutation that appears to have spread throughout the population to increase spleen size. This genetic variant upregulates thyroid hormone, which in mice has been linked to larger spleen size.
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