The evolutionary quirk, developed over hundreds or thousands of years, enables the Bajau tribe of "fish people" in Indonesia to work eight-hour diving shifts to spear fish and octopus for their families. This hormone causes increases in metabolic rate (the amount of energy the body can use in a given time period), which can help to combat low oxygen levels, but is also associated with larger spleen size in mice.
But while the Bajau people's talents have always been known, it was unclear whether the skill was the result of practice, as in the case of the excellent underwater vision of Thai "sea nomad" children, or the result of adaptations which have their roots in the Bajau people's DNA. She suspected this involved the spleen, since contraction of the spleen is a key part of the mammalian diving reflex, and some deep-diving seals, such as the Weddell, have enlarged spleens. "But I also think natural selection is a lot more powerful than we sometimes give it credit for, and maybe we should be looking for it in more places than we thought".
The tribe described as sea nomads are now settled in Indonesia and are known for their ability to hold their breath for a long time. They are able to dive 70 ft with only wooden glasses and a set of weights.
More research is needed to understand how thyroid hormone affects human spleen size.
"We were so fascinated that they could stay underwater much longer than us local islanders. [The] resulting physiological change seems to have provided a functional adaptation to the conditions of acute hypoxia that is characteristic of breath-hold diving".
It's genes like these that may have given some Bajau divers the best shot at surviving deep free dives and collecting food for their families over many generations, Ilardo said.
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.
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The researcher spent several months in Jaya Bakti, Indonesia, taking genetic samples and conducting ultrasound scans of people from the Bajau and land-dwelling Saluan tribes.
Researcher Melissa Ilardo, the lead author of the new study, chose to study the Bajau people by examining their spleens with an ultrasound device and collecting genetic material both from Bajau people who dived and those who did not. For generations, the Bajau people have built their homes above the water and spend most of their time at sea. "I basically just showed up at the house of the chief of the village, this freaky, foreign girl with an ultrasound machine asking about spleens", she says. A specific gene called PDE10A was found among the Bajau. The width of the Bajau's spleen thus indicate a higher volume of red blood cells. Thus, according to anthropologist Cynthia Beall of Case Western Reserve University in Cleveland, although the Bajau dive more frequently than most people, each of these dives is not necessarily very long.
Ilardo, first author on the paper, suspected that the Bajau could have genetically adapted spleens as a result of their marine hunter-gatherer lifestyle, based on findings in other mammals.
Studying free-divers such as the Bajau could help improve the understanding of acute hypoxia.
This gene is also though to control the thyroid gland and its principal hormone Thyroxine or T4.
According to study co-author Rasmus Nielsen, "This is the first time that we really have a system like that in humans to study".
Willerslev admitted he initially urged Ilardo not to pursue the research for her PhD thesis, believing it was too risky and that she may find nothing.