Scientists reveal how they unpicked the mystery following a clue from previous research: species of seals which can dive for longer have larger than expected spleens - an organ which, among its functions, can store oxygen-carrying red blood cells. The research was published Thursday in the journal Cell.
The group is known as "sea nomads", given their sea-dependent traditions: The Bajau spend almost two-thirds of their work day in the water diving to collect crustaceans and sea cucumbers or spearfish octopuses and other aquatic food at depths as far as 230 feet, researchers said.
The Bajau have lived for more than 1000 years on house boats in the waters around Malaysia, the Philippines, and Indonesia.
Spleens are important in diving - and are also enlarged in some seals - because they release more oxygen into the blood when the body is under stress, or a person is holding their breath underwater.
This is not the first time that examples of continuing human evolution among specific groups of people have been discovered.
The researchers from the Universities of Copenhagen, Cambridge and Berkeley, write that the indigenous nomadic tribes of Bajau have been called "sea nomads" and they have travelled all over the Southeast Asian seas. Mostly they are colonized presently around the islands of Indonesia.
"They're explorers, so I think they're inherently curious and want to know more about the world, including about their own biology", she says.
The team spent some time in Jaya Bakti, Indonesia to examine the people of this tribe. "I wanted to make sure that this was a cooperative effort", says Ilardo.
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Specifically, the genetic mutation researchers found that had spread through the Bajau population "upregulates" thyroid hormone - a hormone that, in mice at least, has been linked to bigger spleens.
Ilardo then compared DNA-sequenced from saliva swabs-from the two groups. Scanning for variants, the group identified the top 25 polymorphisms that were unique to the Bajau genomes, suggesting natural selection pressures were at work. This has been found to provide up to a 9 percent increase in oxygen, enabling longer dive times, the researchers explained. Crucially, a contraction of the spleen is one of the features of the so called "diving reflex" - a set of responses in mammals that occur when the head is submerged. When breathing stops, our bodies trigger a series of physiological changes: our heart rate slows down, the blood vessels in our extremities constrict, and our spleen shrinks down in size.
"I think the most surprising part was the genetic component that underlies that physiological adaptation and that seems to be connected to thyroid hormone levels".
Author of the latest study, Melissa Ilardo of the Center for Geogenetics at the University of Copenhagen noted that, "if there's something going on at the genetic level, you should have a certain sized spleen", according to National Geographic. But it had never previously been seen in humans.
"There we saw this hugely significant difference", she said.
In mice, "PDE10A is known for regulating a thyroid hormone that controls spleen size, lending support for the idea that the Bajau might have evolved the spleen size necessary to sustain their long and frequent dives, ' said the study. There's still a lot of information to be gathered from these understudied populations".
Findings of a new study now reveal that their incredible ability to dive deeper underwater and go longer without oxygen is a genetic adaptation.
Studies suggest that the Bajau experience the most extreme low-oxygen situations humans encounter. Ilardo added, "until now it has been entirely unknown whether Sea Nomad populations genetically adapt to their extreme lifestyle".