Amazing Places

These "Sea Nomads" Have Supersized Spleens for Diving

Of all of the organs, the spleen might be the least understood by the general public. What does it do, exactly? It is even necessary? Well, a new discovery brilliantly illustrates one of the spleen's little-known superpowers. The Bajau, a nomadic people indigenous to Indonesia known for their astonishing diving ability, have super-sized spleens. Why? It helps them hold their breath for longer.

Down Periscope

While they can stay underwater for up to 13 minutes at a time, the Bajau aren't the only impressive seafaring people in their neck of the woods. South Korean haenyo are elderly women who regularly free dive to collect shellfish from the ocean floor, and children in the Moken tribe around Thailand can see underwater twice as well as land dwellers. But every adaptation to diving that we've seen in humans has happened in the individuals' lifetimes — European children were able to adapt similar vision to the Moken children with enough training, for example.

That's why the latest discovery is so exciting: the enlarged spleens of the Bajau aren't a result of "plastic" adaptations, but of gene evolution. Melissa Ilardo of the University of Copenhagen had heard about the Bajau people and had a hunch that they might have large spleens like Weddell seals and other deep-diving marine mammals do.

The spleen plays a pivotal role in what's known as the "mammalian dive reflex," or simply the human dive response. When you jump into cold water (or even splash water on your face), your heart rate immediately slows. At the same time, blood vessels in your arms and legs constrict to help shuttle oxygen-rich blood to your heart and brain. And whereas your spleen does most of its work filtering your blood, it also holds oxygen-rich red blood cells in storage that it can deploy in case of emergency.

"In seals, the organ is basically a natural scuba tank: it can hold more than 20 liters of red blood cells, and during dives it contrats like a wrung-out sponge, shrinking by more than 85 percent as it pushes the blood into circulation," writes Alex Hutchinson, recent guest of the Curiosity Podcast, in his book "Endure: Mind, Body, and the Curiously Elastic Limits of Human Performance." Humans don't have quite the spleen powers of seals, but we too get a burst of oxygenated blood from this underestimated organ.

Diving DNA

For the new research, Ilardo and her team took ultrasound scans and saliva samples from both the Bajau and their land-dwelling neighbors, the Saluan. They found that the Bajau have spleens that are 50 percent larger than the Saluan — even the spleens of Bajau who never dive, pointing to a genetic origin. And when the researchers sequenced the DNA samples, they found that the Bajau have a gene that the Saluan don't: one called PDE10A, which is thought to control levels of a thyroid hormone shown to influence spleen size in mice.

This is the first time genetic adaptations to diving have been seen in humans. Not only that, but it could give researchers a new way to study something seemingly unrelated: acute hypoxia, the rapid depletion of oxygen in the body caused by complications of lung disease, heart problems, and a number of other issues. Holding your breath underwater is a type of hypoxia, after all. "This is the first time that we really have a system like that in humans to study," said Dr. Rasmus Nielsen, Ilardo's Ph.D. supervisor, in a statement. "It will help us make the link between the genetics and the physiological response to acute hypoxia. It's a hypoxia experiment that nature has made for us and allows us to study humans in a way that we can't in a laboratory."

It also brings new urgency to the preservation of this dwindling nomadic culture and others like it. Researchers want to do the same studies on the haenyo, who may be in their last generation of free divers. "A lot of [these cultures] are threatened and this is not just a loss culturally and linguistically, but for genetics, medicine, and sciences in general," said Ilardo's other Ph.D. supervisor, Professor Eske Willerslev. "There's still a lot of information to be gathered from these understudied populations."

The human body is capable of a lot more than we give it credit for. Hear or read Alex Hutchinson explain all about it on the Curiosity Podcast or in his book "Endure: Mind, Body, and the Curiously Elastic Limits of Human Performance." We handpick reading recommendations we think you may like. If you choose to make a purchase, Curiosity will get a share of the sale.

Written by Ashley Hamer May 14, 2018