A genetic mutation among the Bajau tribe allows them to become better freedivers

November 21, 2018

The Bajau people of Southeast Asia some of the best freedivers in the world. In 2015, Melissa Ilardo went on a diving trip with Pai Bayubu, who had already gone fairly deep when he saw a giant clam, 30 to 50 feet below him.

“He just dropped down,” Ilardo recalls.

“He pointed at it, and then he was there. Underwater, the Bajau are as comfortable as most people are on land. They walk on the seafloor. They have complete control of their breath and body. They spear fish, no problem, first try.”

Sometimes known as sea nomads, the Bajau have lived at sea for more than 1,000 years, on small houseboats that float in the waters off Indonesia, Malaysia, and the Philippines. Traditionally, they came ashore only to trade for supplies or to shelter from storms. They collect their food by free diving to depths of more than 230 feet. They have no wet suits or flippers, and use only wooden goggles and spearguns of their own making. Sometimes, they rupture their own eardrums at an early age to make diving easier.

Each day, they’ll spend more than five hours underwater, capturing between two and 18 pounds of fish and octopuses. The average dive lasts for just half a minute, but the Bajau can hold their breath for far longer.

While much of their abilities are shaped by experience and training, Ilardo has found evidence that they are also genetically adapted to life in the sea.

Over three trips in the summer of 2015, she got to know people from the Bajau village of Jaya Bakti in Indonesia. She explained her work as a geneticist, went diving with them, and learned about their lifestyles. On one trip, she brought along an ultrasound machine, and scanned the bodies of 59 villagers. That’s when she realized that the Bajau have unusually large spleens – 50% bigger than those of the Saluan, a neighboring group who barely interact with the sea.

The spleen stores and filters oxygen-carrying red blood cells. When mammals hold their breath, the spleen contracts, expelling those cells and boosting oxygen levels by up to 10%. For that reason, the best competition free divers tend to have the largest spleens, as do the deepest-diving seals. It’s even possible to train your spleen. Erika Schagatay, from Mid Sweden University, found that after climbing Mount Everest, mountaineers empty more of their spleens while holding their breath than they could before.

But Ilardo and her team, led by Eske Willerslev and Rasmus Nielsen at the University of Copenhagen, found that even Bajau villagers who never dive still have disproportionately large spleens, leading them to believe it was genetic.

Using blood samples collected from the same 59 Bajau villagers, she and her team compared their DNA to that of 34 Saluan individuals and 60 Han Chinese. They looked for genes with variants that are more common in the Bajau than in the other populations – a sign of natural selection at work. And they found several contenders.

In Bajau people, the gene PDE10A, active in the thyroid gland and controls the release of hormones, was found with higher levels of thyroid hormones, which in turn, makes spleens grow bigger. This might explain why the Bajau have such large spleens, and thus, such extraordinary breath-holding skills.

Ilardo’s team also found signs of adaptation in other genes, which they now plan to study further. One of these, BDKRB2, is the only gene that’s been previously linked to diving in humans. It affects the constriction of blood vessels in the extremities, and so controls how much oxygen reaches the core organs like the brain, heart, and lungs.

Another gene, FAM178B, influences the levels of carbon dioxide in the blood—which is also an important factor to control when holding one’s breath. The version of FAM178B that’s common in the Bajau seems to have come from the Denisovans, a group of ancient hominids who lived in Asia. It’s clear that when modern humans entered Asia, they had mated with Denisovans and inherited some of their DNA. One Denisovan gene provides modern Tibetans with a crucial adaptation that allows them to survive at high altitudes. It’s possible that another gives the Bajau an advantage underwater, although more analysis is yet to be conducted.

These discoveries might have important medical implications. Several disorders, including strokes and heart attacks, starve the body of oxygen, so the genetic tricks that help the Bajau thrive underwater might inspire new ways of protecting patients on dry land. For the same reasons, Nielsen and his colleagues are studying the genes of people in Tibet and Ethiopia who live at high altitudes, and other people who live in extreme environments.

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