Indonesian divers have evolved bigger spleens because of this.

Some of the most exciting discoveries in evolutionary biology in recent years have shown how humans have adapted to extreme conditions, such as living at high altitudes. Now, researchers have found that Indonesia’s Bajau people, who for generations have spent the majority of their days diving and hunting underwater, also have genetic adaptations for their unusual lifestyle. These “sea nomads” carry a gene variant that seems to lead to unusually large spleens that can supply an extra boost of oxygenated red blood cells on demand.

The new study suggests that people exposed to low-oxygen conditions have evolved “many pathways” for coping, says Gabriel Haddad, a physiologist at the University of California (UC), San Diego, who was not involved with the work.

The Bajau have lived for more than 1000 years on house boats in the waters around Malaysia, the Philippines, and Indonesia. Divers spend more than 60% of their 8-hour work days underwater, spearing fish, hunting sea cucumbers, and gathering black coral to fashion into jewelry. That lifestyle fascinated Melissa Ilardo, who first heard about the sea nomads as a graduate student studying corals in Southeast Asia. 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. “We were quite skeptical that it would be possible to find anything,” said one, Rasmus Nielsen, an evolutionary genomicist at UC Berkeley.

Physiologists know that most mammals—including humans—undergo a “diving response” when their faces hit cold water. Their heart rates slow; blood is shunted into the body’s core, and the spleen contracts, releasing some of its store of oxygenated red blood cells. In 1990, researchers learned that the spleens of Japanese pearl divers contract harder than normal, boosting blood oxygen by up to 9% during dives. Ilardo decided to see whether Bajau spleens were similarly special.

Using a portable ultrasound machine, she measured the organ in 59 Bajau and 34 people from a nearby farming community. The Bajau’s spleens were 50% larger than those of their land-based neighbors on average, she and her colleagues report today in Cell.

Ilardo then compared DNA—sequenced from saliva swabs—from the two groups. She found that the Bajau had 25 genes that differed from both the farmers and a large earlier study of Han Chinese. That makes the genes likely candidates for the evolutionary process known as natural selection. One such gene, PDE10A, has been shown to affect thyroid activity in mice, which in turn affects spleen size.

It’s often difficult to show that a gene has undergone recent evolutionary adaptations in living people, but in this case, there is strong evidence, even though the Bajau have been living the sea life for just a few thousand years, says study co-author Eske Willerslev, an evolutionary genomicist at the University of Copenhagen.

Mark Aldenderfer, an archaeologist at UC Merced who was not involved with the work, agrees: “Perhaps the biggest takeaway from this paper is that natural selection continues to work on human populations.”

Anthropologist Cynthia Beall from Case Western Reserve University in Cleveland, Ohio, isn’t yet convinced Ilardo and her colleagues fully understand what’s happened with the Bajau. She points out that although the Bajau dive more frequently than most people, they are not necessarily under the water longer for any given dive. Also, a larger spleen doesn’t necessarily mean a bigger expulsion of red blood cells, which is what actually helps the divers, she notes. 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.”

Still, Ilardo is hopeful the Bajau can offer some help to people experiencing temporary oxygen deficits, like those with sleep apnea or brain injuries. By finding out what makes the Bajau so adept at dealing with no air, she suggests, physicians may one day learn how to better treat those conditions in their patients.

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