Some can hold their breath for minutes at a time, plunging dozens of meters below the surface of the sea with nothing more than goggles and weights.
"That doesn't really compare to any other humans. The closest thing to that is sea otters," said evolutionary geneticist Melissa Ilardo, whose latest research focuses on a Bajau community in Sulawesi, Indonesia.
"The way they dive is so natural," Ilardo said. "There's nothing like seeing them in the water."
Ilardo was struck by how the Bajau seemed "so incredibly comfortable in the water." She recalled how a Bajau man free diving alongside her suddenly dropped another 30 or 40 feet to scoop up a giant clam "like it was no problem at all."
To understand what the team then discovered, you need to understand what happens to the human body when it dives into the water, Ilardo explained.
First, the heart slows, which lowers the amount of oxygen consumed. Blood vessels in the extremities also constrict, channeling oxygen-rich blood to vital organs. Lastly, the spleen contracts.
"The spleen is a weird one," Ilardo said. "I hadn't really heard much about the spleen. I know that you can live without a spleen, so it was kind of like, 'What is the spleen even doing?' "
The spleen is a fist-size organ
that sits alongside the stomach. Its roles include filtering out old cells and supporting the immune system. But the spleen also serves as a reservoir of oxygen-carrying red blood cells; when it contracts, it gives the body what Ilardo called an "oxygen boost," allowing you to hold your breath for longer.
on diving seals has suggested that larger spleens could be linked to longer dive times.
"If natural selection had acted in seals to give them larger spleens, then it might've done the same thing in humans," Ilardo said.
Spleens that are 50% larger
Using an ultrasound machine, Ilardo found that the Bajau had spleens that were roughly 50% larger than those of their land-dwelling neighbors on the Indonesian island of Sulawesi. And it didn't seem to matter whether a Bajau person was a diver or not, which suggested that this was not merely a result of underwater training over time.
Then, scanning the Bajau DNA for clues, the researchers identified a possible culprit for their larger spleens: a variant of a gene called PDE10A, which has been linked to higher-than-average thyroid hormone levels in Europeans.
There is no known link between thyroid hormones and spleen size in humans -- but in mice
, there is. However, the researchers did not measure the thyroid hormone levels of the Bajau participants.
The genetic variant they identified wasn't specific to the Bajau, Ilardo said, and there could be many other genes that affect spleen size.
"That's where you need much bigger studies asking people to see how long they can hold their breath," said Marc Feldman, professor of biology at Stanford University and co-director of its Center for Computational, Evolutionary and Human Genomics.
Feldman was not involved in the new study.
"People vary in that particular phenotype (or trait), but it's not one that we go around testing very much. It's not like (body-mass index), height or levels of high cholesterol."
Feldman said that the relationship between the PDE10A gene, spleen size and diving ability couldn't be proved solely on this study of 43 Bajau; it would require thousands more participants on a much larger scale.
"There aren't enough people who spend so much time underwater to make a really definitive statistical statement," Feldman said.
More genes at play
Ilardo's team found three additional genetic variants that they say might be connected to the underwater prowess of the Bajau.
The strongest hit overall came from a gene called BDKRB2
, which may increase how much peripheral blood vessels constrict. The more these vessels constrict, the more they reroute oxygenated blood to crucial organs -- such as the brain and heart -- and potentially boost diving time, the researchers wrote.
It's genes like these that may have given some Bajau divers the best shot at surviving deep free dives and collecting food for their families over many generations, Ilardo said.
"Free diving is extremely dangerous, and so even highly trained free divers often die because they lose consciousness on ascent and they drown," said Ilardo, who has been free diving recreationally since she was 4 years old.
"If that's happening over thousands of years, then the people who are surviving are those that are carrying the genes that give them an advantage."
Though evolution is typically described on the order of millions of years
, the Bajau have been diving this way -- by holding their breath -- on the order of only thousands.
"What we're taking about is evolution over a relatively short time span," Feldman said.
This isn't the only evidence of "recent" human evolution. For example, researchers identified a "genetic adaptation to high altitude"
What makes the Bajau different, Ilardo said, is that their historically seafaring lifestyle and fishing practices may have driven the evolutionary process.
"This isn't an adaptation to harsh conditions. It's the co-evolution of human culture and humans as a species," she said.