The genetic change discovered in the Bajau tribe is the first known example of a human adaptation to deep diving. Study author Melissa Llardo from the Center for Geogenetics at the University of Copenhagen wanted to see if the same characteristic was true for diving humans. She said: 'We know deep diving seals, like the Weddell seal, have disproportionately large spleens.
The Bajau, who are spread among the islands of Indonesia, Malaysia and the Philippines, have been living on houseboats for thousands of years and depend on the sea for most of their needs. According to Nielsen, this means they could mobilize perhaps 10 percent more red blood cells during a dive than those not adapted to the low-oxygen conditions of breath-hold diving.
Known as "sea nomads", the Bajau have spleens that are twice as big as those of their neighbours, the land-based Saluan people, which aids them as they dive for food in the ocean around Indonesia.
It helps their diving because the spleen reacts to submersion in water by injecting oxygenated red blood cells into the circulation.
The goal of Melissa Ilardo was to check if the size of their spleen was correlated with their breathing capacity, and to check if it was a genetic adaptation.
"We were so fascinated that they could stay underwater much longer than us local islanders". Both groups may have evolved the changes because hypoxia was common enough from living at higher altitudes or breath-holding under water that having the mutated genes gave them a significant advantage.
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Prior to her travels, Ilardo spent months learning the Indonesian language, Bahasa Indonesia, which the Bajau people speak in addition to their native language, so that she could communicate with them directly, and convey to them the research findings. The results of the study, published today in Cell Press, suggests their unusually large spleens are the result of a genetic mutation, and not a lifestyle-related phenomenon.
Then, scanning the Bajau DNA for clues, the researchers identified a possible culprit for their larger spleens: a variant of a gene called PDE10A, which has been linked to higher-than-average thyroid hormone levels in Europeans. As they never dive competitively it is uncertain exactly how long the Bajau can remain underwater, but one of them told researcher Melissa Ilardo that he had once dived for 13 minutes consecutively.
'It's fascinating to think that they're nearly like superhumans living among us with these extraordinary capabilities, ' said Dr Melissa Ilardo of the University of Copenhagen, who led a study on the divers.
The Bajau community the researchers studied was curious to learn more about their "genetic heritage" after Ilardo approached them, the study authors said. "I basically just showed up at the house of the chief of the village, this freakish, foreign girl with an ultrasound machine asking about spleens", she says.
There is no known link between thyroid hormones and spleen size in humans - but in mice, there is. This has been found to provide up to a 9 percent increase in oxygen, enabling longer dive times, the researchers explained. "This is the first time that we really have a system like that in humans to study", said Dr Rasmus Nielsen. Today, Bajau free divers use rudimentary equipment, like masks and a spears, while searching for fish, lobster, and octopus.
For the Bajau, Ilardo believes that the decision to participate in this research is about better understanding themselves.