Longer dives because of larger spleen Seanomads in Southeast Asia have larger spleens because of natural selection

20 April 2018

Foto: Melissa Ilardo The Bajau people of Southeast Asia, known as the last Sea Nomads, spend their whole lives at sea, working eight-hour diving shifts with traditional equipment and short breaks to catch fish and shellfish for their families till depths of 70 meters. They don’t use compressed air but hold their breaths. In a study published April 19 in the journal Cell, researchers of Radboudumc and international colleagues, report that the extraordinary diving abilities of the Bajau may be thanks in part to their unusually large spleens. The adaptation, the researchers say, is a rare example of natural selection in modern humans­and one that could provide medically relevant insight into how humans manage acute hypoxia.

Humans adapt to a number of different extreme environments just through our lifestyle changes or our behavioral changes. So it wasn't necessarily likely that the team of researchers would find an actual genetic adaptation to diving. “But when we saw that both the Bajau divers and non-divers had larger spleens than the Saluan, a nearby, non-diving population, we knew we were onto something", says first author Melissa Ilardo from the University of Copenhagen.

Seals

Spleen size is significant because of the organ's role in the human dive response, which occurs when our faces are submerged in water and we hold our breath. As our heart rate slows and blood vessels in our extremities constrict, the spleen contracts, releasing oxygenated red blood cells and making more oxygen available in the bloodstream. A larger spleen means that more oxygen gets released. Perhaps for this reason, large spleens have also been documented in diving seals.

When the researchers scanned the genomes of the Bajau, they identified 25 sites that differed significantly from two comparison populations, the Saluan and the Han Chinese. Of these, one site on a gene known as PDE10A was found to be correlated with the Bajau's larger spleen size, even after accounting for confounding factors like age, sex, and height. In mice, PDE10A is known for regulating a thyroid hormone that controls spleen size, lending support for the idea that the Bajau might have evolved the spleen size necessary to sustain their long and frequent dives.


Een Bajau duiker vist met een traditionele speer (foto: Melissa Ilardo)

COPD and sleep apnea

Professor Experimental Internal Medicine Mihai Netea from Radboud university medical center, was also involved in the study. “It is unique that we found evidence of population-specific natural slection.” The research provides interesting information for the medical context. “A lack of oxygen, for instance plays an important role in  chronic obstructive pulmonary disease and surgery.” Hypoxia has been well studied in populations living at high altitudes, where the lack of oxygen is much more chronic. But not as much research has been done on diving populations. Here it's more of an acute hypoxia, almost similar to what's experienced with sleep apnea.


Een man demonstreert het gebruik van een traditionele duikbril van de Bajau (foto: Melissa Ilardo)

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