If the dystopian future dramatized in Waterworld ever comes to pass, the Bajau people of Indonesia have a head start, genetically speaking. The Bajau, renowned for their breath-holding and prolonged free-diving abilities, possess gene variants that result in larger spleens—and larger reservoirs of oxygenated red blood cells.

The discovery of the gene variants does more than eliminate the possibility that the Bajau’s larger spleens are a plastic response to diving; it also has implications for hypoxia research, an important medical issue. More generally, the new findings demonstrate how populations that maintain extreme lifestyles constitute living laboratories for the study of genetic adaptations, including adaptations that may prove to be medically significant.

To find the diving genes, an international team of scientists had to do some diving of their own—not into the South Asian seas, but into the genomes of the Bajau and the Saluan, the Bajau’s land-dwelling neighbors. The team, which included scientists from the Universities of Copenhagen, Cambridge, and California-Berkeley, also performed ultrasound scans of spleens.

The scans showed that the spleens of the Bajau, whether from divers or nondivers, are 50% larger than those of the Saluan. And genome sequencing showed that members of the Bajau have variants of the PDE10A gene that the Saluan lack. It is thought that the PDE10A gene controls the levels of thyroid hormone thyroxine, or T4.

“We believe that in the Bajau they have an adaptation that increases thyroid hormone levels and therefore increases their spleen size,” said Melissa A. Ilardo, a doctoral candidate at the University of Copenhagen. “It's been shown in mice that thyroid hormones and spleen size are connected. If you genetically alter mice to have an absence of the thyroid hormone T4, their spleen size is drastically reduced, but this effect is actually reversible with an injection of T4.”

This is the first time a genetic adaptation to diving has been tracked in humans. Ilardo added, “until now it has been entirely unknown whether sea nomad populations genetically adapt to their extreme lifestyle. The only trait previously studied is the superior underwater vision of Thai sea nomad children; however, this was shown to be a plastic response to training, and was replicable in a European cohort.”

Ilardo is the first author of a study (“Physiological and Genetic Adaptations to Diving in Sea Nomads”) that appeared April 19 in the journal Cell.

“In this study, we identify multiple candidate genes for adaptation to breath-hold diving in the Bajau,” the article’s authors wrote. “In the region of PDEA10, we find that the SNPs [single-nucleotide polymorphisms] at the selection scan peak are associated with thyroid function and spleen size. Because thyroid hormones regulate normal erythropoiesis during early postnatal development, the observed large spleen phenotype in the Bajau may be indicative of higher volume of erythrocytic cells….[The] resulting physiological change seems to have provided a functional adaptation to the conditions of acute hypoxia that is characteristic of breath-hold diving.”

The Bajau people, groups of whom are spread among the islands of Indonesia, and those of Malaysia and the Philippines, too, are often called sea nomads, and traditionally live on boats and harvest nearly everything they eat from the sea. Some have been shown to spend as much as 60% of their work day diving for food—spearing fish and octopus and gathering crustaceans and sea cucumbers—at depths greater than 70 meters, or 230 feet, using only a wooden mask. Linguistic analysis suggests that they have lived this way for more than a thousand years. A chronicler of one of Ferdinand Magellan's voyages recorded their unusual lifestyle in 1521.

For the Bajau, Ilardo believes that the decision to participate in this research is about better understanding themselves. “I basically just showed up at the house of the chief of the village, this bizarre, foreign girl with an ultrasound machine asking about spleens,” she recalled. “They're the most welcoming people I've ever met, but I wanted to make sure that they understood the science behind what I was doing, so that it wasn't just me taking measurements from them without giving back. And we do have a trip planned to return to the community to explain the results to them.”

“They're explorers, so I think they're inherently curious and want to know more about the world, including about their own biology,” she added.

Ilardo also believes her research has implications for how we think about natural selection in modern humans: “I think it's fascinating to see just how extraordinary this population is, to think that they're almost like superhumans living among us with these really extraordinary capabilities. But I also think natural selection is a lot more powerful than we sometimes give it credit for, and maybe we should be looking for it in more places than we thought.”

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