The Bajau Laut, a sea-faring hunter-gatherer tribe with a 1,000-plus year history in the Southeast Asian islands, are known for their remarkable free-diving ability. Depending almost entirely on food and resources harvested from the ocean, the Bajau, or sea nomads, spend about five cumulative hours underwater every day.

If humans are a terrestrial species, how are the Bajau able to function more like sea otters? According to a study by researchers from the University of Copenhagen and UC Berkeley, published in Cell, the sea nomads can to repeatedly hold their breath for long periods without harming their bodies thanks to the recent evolution of enlarged spleens.

Past physiological studies have discovered that when diving, all humans are capable of the same tricks that are employed by true marine mammals, such as seals: Our heart rate lowers, peripheral blood vessels constrict to conserve flow of warm, oxygenated blood to the vital organs; and the spleen contracts to send an extra reserve of oxygen-carrying red blood cells (RBCs) into the circulatory system.

Yet Bajau men and women put the most seasoned western divers to shame – they regularly plunge more than 70 meters deep (230 feet) with nothing more than a pair of wooden goggles and some weights. Thus, for decades scientists have wondered whether this extreme breath-holding ability stems from genetic adaptations, or if it is simply the result of repeated training beginning early in life.

To answer this question, the authors set out to compare the spleens and genetic backgrounds of Bajau individuals living near the Indonesian island of Sulawesi to their land-preferring neighbors, the Saluan. Splenic ultrasounds and DNA samples were collected from 43 non-closely related Bajau and 33 Saluan.

The imaging confirmed that the Bajau have larger spleens than the non-diving Saluan, meaning that their bodies can store more dive-time boosting RBCs. Moreover, even Bajau individuals who did not work as divers had larger spleens, supporting the theory that these people are born to swim.

When subsequently attempting to pinpoint the genetic origin of the enlarged spleens, the team found that many of the Bajau individuals have a unique variation in a gene related to thyroid function, PDE10. Though the link remains murky, other genetic studies have suggested that thyroid hormone levels affect RBC production during early childhood development, and consequently, could lead to a bigger spleen.

“Regardless,” they write, “the resulting physiological change seems to have provided a functional adaptation to the conditions of acute hypoxia that is characteristic of breath-hold diving.”

By comparing the PDE10 sequences to those of the Saluan and other Asian populations, the authors found evidence confirming that this adaption arose from selection, rather than repeated random mutations. It appears that the variations were present at random frequency in the Bajau's ancestral groups and became increasingly common after the Bajau split off from the Han Chinese group about 15,400 years ago, possibly corresponding to when the people began adopting a more sea-faring lifestyle.

In addition, the Bajau DNA revealed that the group carries a high frequency of variations in the BDKRB2, the only gene known to associated with the physiological dive response in humans. People with variations in BDKRB2 have boosted peripheral vasoconstriction, allowing for longer dive times.

“Overall, our results suggest that the Bajau have undergone unique adaptations associated with spleen size and the diving response, adding new examples to the list of remarkable genetic adaptations humans have experienced in recent evolutionary history.”