Over three trips in the summer of 2015, she got to know people from the Bajau village of Jaya Bakti in Indonesia. She explained her work as a geneticist, went diving with them, and learned about their lifestyles. On one trip, she brought along an ultrasound machine, and scanned the bodies of 59 villagers. That’s when she realized that the Bajau have unusually large spleens—50 percent bigger than those of the Saluan, a neighboring group who barely interact with the sea.

The spleen acts as a warehouse for oxygen-carrying red blood cells. When mammals hold their breath, the spleen contracts, expelling those cells and boosting oxygen levels by up to 10 percent. For that reason, the best competition free divers tend to have the largest spleens, as do the deepest-diving seals. It’s even possible to train your spleen: Erika Schagatay, from Mid Sweden University, found that after climbing Mount Everest, mountaineers empty more of their spleens while holding their breath than they could before.

But Bajau spleens aren’t big just because of training. Ilardo and her team, led by Eske Willerslev and Rasmus Nielsen at the University of Copenhagen, found that even Bajau villagers who never dive still have disproportionately large spleens. “When we saw that, we thought, okay, something’s going on and it’s likely genetic,” Ilardo says.

Using blood samples collected from the same 59 Bajau villagers, she and her team compared their DNA to that of 34 Saluan individuals and 60 Han Chinese. They looked for genes with variants that are more common in the Bajau than in the other populations—a sign of natural selection at work. And they found several contenders.

One gene, known as PDE10A, stood out. It does many things, but it’s especially active in the thyroid gland, and controls the release of hormones. The version of PDE10A that’s common in the Bajau is associated with higher levels of thyroid hormones, and those hormones, in turn, make spleens grow bigger—at least, in rodents. This might explain why the Bajau have such large spleens, and thus, such extraordinary breath-holding skills. “This shows, for the first time, that there may be a genetic background to the spleen response in humans,” says Schagatay, who was not involved in the study.

PDE10A is only part of the story. Ilardo’s team also found signs of adaptation in other genes, which they now plan to study further. One of these, BDKRB2, is the only gene that’s been previously linked to diving in humans. It affects the constriction of blood vessels in the extremities, and so controls how much oxygen reaches the core organs like the brain, heart, and lungs.

Another candidate, FAM178B, influences the levels of carbon dioxide in the blood—which is also an important factor to control when holding one’s breath. The version of FAM178B that’s common in the Bajau seems to have come from the Denisovans, a group of ancient hominids who lived in Asia. It’s clear that when modern humans entered Asia, they had sex with Denisovans and inherited some of their DNA. One Denisovan gene provides modern Tibetans with a crucial adaptation that allows them to survive at high altitudes. It’s possible that another gives the Bajau an advantage underwater. “But we haven’t confirmed that yet,” says Ilardo. “We need to do more analyses.”