The Bajau people of Southeast Asia reside in homes that precariously balance above the Pacific Ocean’s blue seas. These “sea nomads” blur the line between marine and terrestrial species by utilizing only a wooden mask to breathe underwater for long enough to dive to depths of more than 230 feet. According to researchers in a recent Cell study, this superhuman skill that enables them to spend up to 60% of their day diving is the result of the evolution of one very critical organ.
Over a thousand years, the Bajau have lived in a largely solitary environment. Over the years, the population has adjusted in many ways to its particular habitat and way of life, but perhaps most significantly, as the new article reveals, natural selection has lead many members of the species to carry a gene for large spleens. This fist-shaped, purple organ is essential for storing oxygenated red blood cells, which are needed when you’re holding your breath. It typically sits to the left of the stomach. The spleen is typically four inches long in humans. However, the Bajau’s (pronounced “be joe”) spleens are around 50% bigger than those of nearby non-divers.
The larger spleens of the Bajau are proof that small-scale natural selection is still guiding human development. They attribute their enlarged size to a genetic mutation that has been naturally selected for within their population over many generations, according to the research team lead by biologist Rasmus Nielsen, Ph.D., of the University of California Berkeley. This gene variation is hypothesized to enlarge the spleen by raising the body’s thyroid hormone levels, which have been associated with increased spleen size in mice.
The discovery surprised Nielsen and his colleagues, which also featured University of Copenhagen PhD candidate Melissa Ilardo as the first author. Even with a community as extreme as the Bajau, the likelihood of discovering proof of population-specific natural selection was “very modest,” said Ilardo in a statement released.
Ilardo visited Bajau settlements in Indonesia and used portable ultrasonography equipment to measure the spleens of 59 Bajau and 34 Saluan residents of a nearby hamlet who did not dive. The team discovered that the predominance of the thyroid hormone-stimulating gene known as PDE10A in the Bajau was associated with their larger spleens by comparing that information with each person’s genome sequence, which was determined from spit samples. With normal-sized spleens, the Saluans lacked that gene variation.
According to Nielsen, the Bajau can carry 10% more oxygenated blood cells during a dive than non-divers due to their enlarged spleens. Our blood vessels constrict and our heart rate slows in an effort to conserve oxygen, and the spleen, acting like a battery, begins releasing its reserves of oxygen-rich blood cells to keep us going. This reflex occurs when we are submerged in water and forced to hold our breath. The Bajau are a genuinely unique example of human adaptability since they never stop moving, much like Energizer rabbits.
“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,” says 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.”
