Indonesian 'Sea Nomads' Have Spleens That Are Twice As Big As Normal

A Bajau diver swims among coral looking for fish to spear. Melissa Ilardo

The Bajau, who live in Indonesia, Malaysia, and the Philippines, sustain themselves by diving for fish, and have a noticeable ability to dive long and deep, even compared to other local divers.

Her new study may have uncovered genetic adaptations that allow the Bajau to go for longer periods without oxygen - and which might give them unusually large spleens. New research suggests these impressive feats aren't the result of training, but rather, an example of natural selection at work - which, in this case, has endowed Bajau individuals with abnormally large spleens. Intrigued by work pinpointing the genes that help Tibetans, Ethiopians, and South Americans adapt to chronic oxygen deprivation at high altitude, Ilardo-now a postdoc at the University of Utah in Salt Lake City-approached her colleagues about doing similar studies in the Bajau.

Men and women of the Bajau tribe row a boat at a fishing village on Kaledupa island, Southeast Sulawesi, Indonesia. Instead they rely on weights, handmade wooden goggles - and a single breath of air. Their spleens are 50 per cent larger than those of their land-dwelling neighbours, the Saluan.

Contraction of the spleen to release oxygenated red blood cells into the circulation is a key part of humans' and all mammals' innate diving reflex, which is initiated when you hold your breath and at the same time immerse your face in cold water.

"Nothing is really known about the genetic basis of spleen size in humans, so it is hard to validate without further research", Rasmus Nielsen, a professor of integrative biology at the University of California, Berkeley, said in a statement.

The findings, which are being published in the research journal Cell, could also have medical implications in relation to the condition known as Acute Hypoxia, which can cause complications in emergency medical care. There was no difference in spleen size between Bajau who dived and Bajau who did not dive.

Dr Ilardo explained,"There's a human dive response that's triggered by holding your breath and submerging yourself in water".

A Bajau diver with homemade mask
Nomadic divers evolve larger spleens to stay underwater for 13 minutes, scientists find

The team spent some time in Jaya Bakti, Indonesia to examine the people of this tribe.

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.

The Bajau people of South-East Asia are sea nomads who spend most of their workday under the sea hunting for fish and sea cucumbers. After they agreed to the study, she returned with an ultrasound machine.

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. The study authors created a phylogenetic tree, calculating that the Bajau and Saluans diverged about 15,000 years ago, suggesting that the Bajau-unique genetic variants evolved some time after this divergence.

"We told Melissa that this was a very risky Ph.D. and she needed to be aware that it was likely she would find nothing", said Willerslev. The diving as well as the non-diving Bajau had enlarged spleens which meant that there was genetics at play. And Edward Gilbert-Kawai, a physician-physiologist at the University of London, notes also "it is highly unlikely that spleen size is controlled by only one gene".

Bajau "sea nomads" are the first known humans to be genetically adapted to diving, which could explain why they are able to stay underwater for up to 13 minutes at depths of around 200 feet, National Geographic reported. By making their data freely available to other researchers, she and her co-authors hope that some of what they've learned from the Bajau can be applied in medical contexts.

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