Divers’ spleens evolve to utilise oxygen better
A tribe of nomadic divers has evolved larger spleens, enabling them to catch fish more than 200ft underwater. Scientists discovered the Bajau people of Indonesia are superb divers because their genetically enlarged spleens allow them to use oxygen more efficiently.
A TRIBE of nomadic divers has evolved larger spleens to allow them to catch fish more than 200 feet underwater, scientists have discovered.
For more than 1,000 years the Bajau population of Indonesia has travelled the south-east Asian seas in houseboats, catching fish and seafood by free diving with spears.
Their itinerant lifestyle has earned them the nickname “sea nomads” or “sea gipsies”.
But now scientists have discovered the secret to their astonishing fishing prowess. They have evolved genetically enlarged spleens that allow them to use oxygen more efficiently so they can stay underwater for longer.
Melissa Ilardo of Cambridge University, the study leader, 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.
The results were sequenced at the University of Copenhagen and showed the Bajau have a median spleen size 50 per cent larger.
It has previously been hypothesised that the spleen plays an important role in enabling humans to free dive for prolonged periods, but the relationship between spleen size and dive capacity has never before been examined in humans.
“Until now it has been entirely unknown whether sea nomad populations genetically adapt to their extreme lifestyle,” said Miss Ilardo, a doctoral student. “There’s not a lot of information out there about human spleens in terms of physiology and genetics.
“But we know that deep-diving seals, like the Weddell, have disproportionately large spleens. I thought that if selection acted on the seals to give them larger spleens, it could potentially do the same in humans.”
Most of the Bajau community are now settled around the islands of Indonesia, where they are renowned for their extraordinary breath-holding abilities. Some members claim to be able to hold their breath for 13 minutes while diving.
Using weights and handmade wooden goggles, they can easily dive to impressive depths for lengthy periods.
The spleen plays a central role in prolonging free diving time as it forms part of what is known as the human dive response, which is triggered as a method of assisting the body to survive in an oxygen-deprived environment.
The heart rate slows down, blood vessels in the extremities shrink to preserve blood for vital organs, and the spleen contracts, which creates an oxygen boost of nine per cent by ejecting more oxygenated red blood cells into circulation. Researchers found enlarged spleens in non-diving Bajau individuals, as well as those who regularly free dive, proving the adaptation is genetic not acquired.
The team’s research, published in the journal Cell, also discovered that members of the Bajau have a gene called PDE10A which the Saluan do not. It is thought that the PDE10A gene controls the levels of thyroid hormone T4.
“We believe that in the Bajau they have an adaptation that increases thyroid hormone levels and therefore increases their spleen size,” added Miss Ilardo. The team now hopes to study similar populations such as the Moken hunter-gatherers of Thailand and the Haenyeo diving women of Jeju in South Korea.
“This study is a wonderful example of the value of studying these small populations living under extreme conditions,” said Professor Eske Willerslev of the University of Copenhagen.
“A lot of them are threatened, and this is not just a loss culturally and linguistically, but for genetics, medicine, and sciences in general.
“There’s still a lot of information to be gathered from these understudied populations.”
‘Until now it has been unknown whether sea nomad populations genetically adapt to their extreme lifestyle’