Medicine

Extra-large spleens help Indonesian 'fish people' tribe dive to 200 feet

April 20, 2018 08:15·
A population of Indonesian'fish people have evolved spleens are 50 per cent larger than normal people enabling them to free dive to depths of more than 200 feet

A population of Indonesian'fish people have evolved spleens are 50 per cent larger than normal people enabling them to free dive to depths of more than 200 feet

The Bajau people of Southeast Asia, known to many as "sea nomads", are renowned for their incredible diving abilities.

Melissa Ilardo, who was a graduate student at the University of Copenhagen when she did the work, sought to understand whether the Bajau had evolved their own strategies to cope with hypoxia while swimming.

It is the first known example of a human adaptation to deep diving. Their fascinating abilities have prompted the University of Copenhagen and the University of California-Berkeley to conduct scientific studies on the anatomy of the Bajau people and the results were shared on the Cell. They are known for an extraordinary ability to hold their breath. "There's nothing like seeing them in the water". She recalled how a Bajau man free diving alongside her suddenly dropped another 30 or 40 feet to scoop up a giant clam "like it was no problem at all".

In 2014, a different team published evidence for a genetic adaptation to living at high altitudes in Tibetan populations.

Among the top hits was a variant adjacent to the gene for PDE10A, a cyclic nucleotide phosphodiesterase involved in regulating smooth muscle contraction, including the muscles that surround the spleen. "I hadn't really heard much about the spleen". "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. Its roles include filtering out old cells and supporting the immune system.

Now experts say they have the answer: over time the Bajau people have undergone natural selection, resulting in certain versions of genes becoming widespread - many of which are linked to biological changes, including having a larger spleen, that could help the Bajau to hold their breath underwater for many minutes at a time.

Writing in the journal Cell, the 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.

In order to gain evidence for this study, Melissa Ilardo spent several months in Jaya Bakti, Indonesia taking genetic samples and performing ultrasound scans of the spleens from both the Bajau and their land-dwelling neighbours, the Saluan. And it didn't seem to matter whether a Bajau person was a diver or not, which suggested that this was not merely a result of underwater training over time.

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DNA analysis showed that the Bajau have a gene called PDE10A that is lacking in the Saluan.

There is no known link between thyroid hormones and spleen size in humans - but in mice, there is.

"We believe that in the Bajau they have an adaptation that increases Thyroid hormone levels and therefore increases their spleen size", said Melissa Ilardo. They live in boats and stilt houses around Indonesia and Malaysia. Feldman was not involved in the new study. "It was very exciting to find, and it just opens up so many possibilities", says Ilardo. "It's not like (body-mass index), height or levels of high cholesterol".

The team's top hit, a variant adjacent to the BDKRB2 gene, is the only other gene that has previously been found to be associated with a human diving response, but not with spleen size. The more these vessels constrict, the more they reroute oxygenated blood to crucial organs - such as the brain and heart - and potentially boost diving time, the researchers wrote.

"Free diving is extremely risky, and so even highly trained free divers often die because they lose consciousness on ascent and they drown", said Ilardo, who has been free diving recreationally since she was 4 years old.

While the Bajau divers today use equipment to help with their breathing when they search for the sea-critters, their ancestors have engaged in the breath-holding technique for thousands of years.

Astonishingly, these deep dives are performed only with a wooden mask or goggles and a weight belt.

"This isn't an adaptation to harsh conditions".

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