When a human is submerged in water, within seconds the body begins to reflexively adjust. The heart rate slows; blood vessels in the extremities tighten, diverting blood flow to vital organs. And, crucially, the spleen constricts, expelling a precious reserve of oxygenated red blood cells into the bloodstream. All of this extends the time we can go without gasping.

Now a new study suggests some seafaring people may have evolved over thousands of years to push the limits of typical dive responses even further. Genetic changes have allowed one population in Southeast Asia to grow plus-size spleens that may enhance their breath-holding capabilities, according to an international research team’s analysis. Some scientists have likened these evolutionary adaptations to the ones that have allowed Tibetans to thrive at high elevations.

The new study dealt with people who are often locally called “Sea Nomads” and live among the islands and coastlines of Southeast Asia. “Traditionally, they live on houseboats and come to land only occasionally,” says Melissa Ilardo, a postdoctoral researcher at The University of Utah and first author on the study. “They have a reputation for being incredible divers, and for their connection to the sea. I went diving with them, and their abilities are just unreal.”

Among the Bajau—one group of people who live on houseboats in the waterways around and between the Philippines, Malaysia and Indonesia—divers have been recorded holding their breath for over five minutes while hunting for fish or shellfish. In comparison, average people might be able to stay underwater for one to two minutes, and world-class free divers can hold their breath in competitive settings for up to three or close to four and a half minutes.

Previously, a crew filming for the BBC documentary series Human Planet recorded a Bajau hunter during dives, and noted his heart rate plummeted to a mere 30 beats per minute. (The diving reflex in most humans only drops the heart rate to perhaps 50 beats per minute in a healthy adult.) “They’ve been observed diving over 70 meters with only a weight belt and a set of goggles,” Ilardo says. “If they’re just collecting shellfish at 10 meters, they could spend all day doing these shallow dives. We were diving at one point and [a Bajau friend] looked down and saw a large clam. He dropped another 15 meters in an instant and grabbed it. It’s pretty remarkable.”

Ilardo, an evolutionary geneticist, wanted to know if the Bajau’s abilities were a result of being trained from birth or if they evolved to be elite divers over generations of marine living. So she asked the Bajau, along with the Saluan—a genetically similar group of farmers—to let her sequence their genomes and measure the size of their spleens. A larger spleen may store greater amounts of oxygenated red blood cells, allowing divers to remain submerged longer. “I had a portable ultrasound machine. I brought it to the villages, and people would come and let me measure their spleens,” she says. 43 Bajau and 33 Saluan participated in the study, which was published in Cell on Thursday.

The Bajau had significantly larger spleens than the nondiving Saluan, says Rasmus Nielsen, a computational biologist at the University of California, Berkeley, and a senior author on the study. “We found the Bajau had 50 percent larger spleen size,” he says. But among the Bajau themselves, divers had only slightly larger spleens (roughly 10 percent larger) than those who eschewed the traditional diving lifestyle, he notes. That surprising finding raised the possibility that the reason for the Bajau’s larger spleens was genetic—not from a lifetime of underwater training.

Next, the researchers scoured the Bajau genome for signs of natural selection and found 25 gene variants that seemed unique to this population. When Ilardo and colleagues cross-referenced what the involved genes do, they discovered a few of them seem to be related to breath-holding and oxygen deprivation. “[That] was absolutely thrilling when we saw all of these genes that were under selection that had potential relevance for diving,” Ilardo says.

One of the genes the team identified is called PDE10A. “We know that this gene controls thyroid hormone levels, and we know that controls spleen size,” Nielsen says. Just under half of the Bajau carry the version of this gene that is associated with larger spleen, compared with 6 percent of the Saluan and 3 percent of Han Chinese (a population chosen for comparison because they are not closely related to either group), he says. Two other genes that the analysis suggested had evolved in the Bajau were BDKRB2, which controls blood vessel constriction in the extremities, and FAM178B, which helps regulate carbon dioxide balance in the blood. Both could be important for oxygen conservation and breath-holding ability underwater, according to the researchers.

The new study offers a stunning example of human genetic diversity and adaptation to the underwater environment, says Anna Di Rienzo, a human population geneticist at the University of Chicago who was not involved with the research. “This is obviously a very big piece of work, a very substantial piece of work,” she says. “I thought it was a really fascinating story of populations that live in very different and extreme environments.”

But Di Rienzo says researchers will need to do extensive follow-up work to understand how the 25 genetic variations the team identified are functioning differently in the Bajau. They still “need to figure out more precisely how the genes they suggest actually affect the physiology, and actually affect the ability to do these dives,” she says. Currently, Ilardo and her colleagues only know these genes are different in the Bajau. It remains unclear what most of these genes actually do that would confer a diving advantage.

Part of that research will require returning to the Bajau and Saluan communities, and collecting more physiological measurements beyond spleen size—such as levels of carbon dioxide and oxygen in the blood, says Cynthia Beall, an evolutionary anthropologist at Case Western Reserve University who did not work on the study. “Their hypothesis is reasonable just as a beginning, but before making all of these biological stories, there’s some data that are obtainable,” Beall says. “Like how does their blood oxygen saturation fall over the course of an average dive? Their heart rate?” These data would help show how much stress Bajau divers’ bodies are under when hunting, and might elucidate how their specialized genetics translates to underwater prowess.

Ilardo and Nielsen agree, and plan to go back to Indonesia to continue studying the Bajau. Eventually, they say, this work may help unearth fresh clues about how different people respond to oxygen deprivation—critical knowledge in medical settings like surgery.