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Evolution

Extreme Diver Community Evolved Spleens Similar to Seals

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A Badjao kid diving for coins

On an archipelago in Southeast Asia live the Bajau, an ancient people world-renowned for a unique cultural tradition. Every work day, the Bajau dive hundreds of feet below water to harvest fish and octopuses from the sea floor. To do so, they must hold their breath on the order of minutes.

Ancient epics describe the Bajau originating from a land-based community dispatched onto the sea, stranded after a fruitless search for an abducted princess. These oral epics placed the Bajau’s origin at an uncertain point in time, and it was not until Magellan’s voyage around the world in 1521 that the Bajau were officially documented in historical records.

Melissa Ilardo, of the Center for GeoGenetics, predicted that the Bajau had been seafaring long enough for diving advantages to emerge as heritable traits in the community. After all, the culture’s survival relies heavily upon harvesting food from the ocean’s depths; if a genetic diving advantage arose by chance, it would probably enhance survival and persist in the community for future generations. Ilardo told Berkeley News that “the closest thing to the Bajau in terms of underwater working time is sea otters; they are also spending about 60% of their time in the water. That is really remarkable, even compared to other professional or traditional divers.”

Armed with a portable ultrasound machine and DNA isolation kit, Ilardo set out to measure traits unique to extreme diver physiology.

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Although the spleen is mainly known for its blood-filtering properties, the spleen also acts like a reservoir, housing a large percentage of the body’s blood. Spleen enlargement is a major component of highly specialized mammalian divers, like Weddell seals and whales, and competition divers tend to have larger-than-average spleens. Large spleens assist in diving by contracting & thereby increasing the number of red blood cells circulating oxygen throughout the body. Some whales have even evolved a multi-spleen system to magnify the dive benefits, with 14 spleens reported in some species.

Since the Bajau are superb divers, Ilardo postulated that they might have acquired an adaptation affecting spleen size. She measured spleen sizes of 59 Bajau, and then compared Bajau spleens to 34 people from Saluans, a neighboring village that does not historically dive.

Ilardo discovered Bajau spleens were 50% larger than their land-loving counterparts. Even Bajau individuals who avoided diving altogether still presented increased spleen size—suggesting this trait is heritable, rather than an artifact of diving itself.

Enticed by the penetrance of this physical feature, Ilardo wanted to know for sure whether the Bajau’s enlarged spleens had genetic underpinnings. Scouring Bajau genomes, Ilardo identified two unique protein variants very likely to contribute to the Bajau’s otherworldly diving skills. One unique gene variant, PDE10A, is related to spleen growth, while another, FAM178B, influences carbon dioxide levels in the blood.

Indeed, the Bajau have developed heritable adaptations that imitate diving features of seals. This convening of traits, called convergent evolution, is an instance where two totally unrelated species (seals and humans) evolved separately towards the same beneficial feature (large spleen size).

Overall, Ilardo’s research illuminates genetic features contributing to the Bajau’s unusual diving skills, and demonstrate how much there is to learn about how human evolution might intertwine with cultural trends. Ilardo plans to return to the archipelago, and continue working with the Bajau people, who are curious about their own genetic history. Ultimately, Ilardo hopes her work spreads awareness and appreciation for understudied, indigenous populations such as the Bajau.

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Editors Notes