The Bajau people of Malaysia and the Philippines are renowned for their free-diving abilities, often working eight-hour shifts in search of fish and other sea critters. Underwater sessions can last upwards of two minutes, with accumulated daily totals of breath-holding often reaching five hours. New research suggests these impressive feats aren't the result of training, but rather, an example of natural selection at work - which, in this case, has endowed Bajau individuals with abnormally large spleens.
The Bajau are a small community from Southeast Asia, living in Malaysia, the Sulu Archipelago in the Philippines, Mindanao, Borneo, and the eastern Indonesian islands. Some pockets of Bajau still live a traditional lifestyle, living in houseboats or wooden huts propped over the water on wooden stilts. Often referred to as "sea nomads," the Bajau are formidable free divers who have engaged in breath-hold diving for thousands of years. Today, Bajau free divers use rudimentary equipment, like masks and a spears, while searching for fish, lobster, and octopus.
Bajau homes on the water. Photo: Melissa Ilardo
Their ability to stay submerged for extended periods is well documented, prompting researchers from the University of Copenhagen and the University of California-Berkeley to launch a scientific investigation. The results of the study, published today in Cell Press, suggests their unusually large spleens are the result of a genetic mutation, and not a lifestyle-related phenomenon. It's a rare example of natural selection at work on modern humans, and the new insights gleaned from this research could lead to new treatments for respiratory disorders.
A Bajua diver hunts fish. Photo: Melissa Ilardo
At first glance, the spleen doesn't seem a likely organ to help us hold our breath. Its primary functions are to filter blood as part of the immune system, fight bacteria, and to recycle red blood cells. But it also plays an important role during acute oxygen shortage, i.e. when we hold our breath for an extended period of time. When breathing stops, our bodies trigger a series of physiological changes: our heart rate slows down, the blood vessels in our extremities constrict, and our spleen shrinks down in size. When the spleen contracts like this, it releases oxygenated red blood cells, which provides an extra supply of oxygen to the bloodstream. The bigger the spleen, the greater amounts of freshly oxygenated blood.
Figuring this was a clue to extraordinary breath-holding ability, lead researcher Melissa Ilardo brought a portable ultrasound machine to southeast Asia to measure the size of Bajau spleens. Which, as the researchers themselves admit, was a bit werid.
"I basically just showed up at the house of the chief of the village, this bizarre, foreign girl [referring to herself] with an ultrasound machine asking about spleens," said Ilardo in a statement, adding that "They're the most welcoming people I've ever met."
Results of the ultrasounds showed that Bajau individuals do indeed have larger spleens, and they're larger that those found in unrelated neighbouring populations. At first, this observation was attributed to differences in physical conditioning or physiological responses — but spleen sizes among diving and non-diving Bajau individuals did not vary in size, which suggested something else was going on. Something a bit more genetic.
Villagers and curious children look on as researcher Melissa Ilardo takes spleen measurements using a portable ultrasound in a Bajau home.Photo: Peter Damgaard
Accordingly, and for the next stage of the study, the researchers conducted a genetic analysis. They uncovered over two dozen genetic mutations, or variants, among the Bajau people that were distinct when compared to two other populations, the Saluan and the Han Chinese. One marker, a gene known as PDE10A, was associated with the enlarged spleen. Scientists who work on mice are quite familiar with this gene, as it regulates the thyroid hormone that controls the size of, you guessed it, the spleen.
Importantly, spleen size isn't the be-all and end-all of extended breath-holding abilities. Divers use many techniques to prolong their stints underwater, which could also explain the remarkable abilities of Bajua divers. Still, it's an intriguing finding.
"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," conclude the authors in their study.
Indeed, this isn't the first time that scientists have documented natural selection at work on modern humans. In 2017, for example, researchers from the University of Texas discovered genetic mutations among Tibetan individuals that help them to live and work in the Qinghai-Tibetan Plateau, which features elevations as high as 4,511.04m. Fascinatingly, one of these variants was inherited from an extinct group of humans known as the Denisovans, a sister species to the Neanderthals. So while the adoption of advantageous new traits is the result of mutation and natural selection (as was likely the case among the Bajau), new traits can also be introduced by interbreeding, and the subsequent retention (still via natural selection) of a beneficial new characteristic.
While both of these studies chronicle respiratory adaptations among living humans, the nature of the adaptations are qualitatively different. Where hypoxia affects people living in low-oxygen environments (i.e. high altitudes), Bajau individuals work in environments where there is zero breathable oxygen, even if it is for relatively brief periods of time. For Bajau free divers, the lack of oxygen is more acute, so it's less like chronic hypoxia and more like sleep apnea. And in fact, the researchers are hoping that their findings can be used to develop new treatments for both high-altitude hypoxia and sleep disorders like sleep apnea.
As humans, we sometimes like to think we're immune to natural selection, but this study shows that's simply not the case.
"I think it's fascinating to see just how extraordinary this population is, to think that they're almost like superhumans living among us with these really extraordinary capabilities," said Ilardo. "But I also think natural selection is a lot more powerful than we sometimes give it credit for, and maybe we should be looking for it in more places than we thought."
The researchers are excited to share their findings with the Bajau study participants.
"I wanted to make sure that they understood the science behind what I was doing, so that it wasn't just me taking measurements from them without giving back. And we do have a trip planned to return to the community to explain the results to them," said Illardo. "They're explorers, so I think they're inherently curious and want to know more about the world, including about their own biology."