What do you think of this fact?
Most people can hold their breath for about a minute. The Bajau, meanwhile, spend a huge chunk of their workday underwater. And their bodies seem to have adjusted to it, all the way down to their genes.
A life at sea
The Bajau are often called “sea nomads.” For about 1000 years they’ve lived in the waters between the Philippines, Malaysia and Indonesia. Traditionally they dive for fish and other seafood, in long shifts and with simple gear. By some estimates, some of them spend around 60 percent of their workday underwater.
Geneticist Melissa Ilardo wanted to know how. She traveled to Indonesia, got to know the community, and then used a portable ultrasound machine to measure the spleens of Bajau people and of the Saluan, a related group that lives on land.
The spleen as an oxygen tank
Why the spleen? During a dive, a reflex kicks in: the spleen contracts and pushes extra oxygen-rich red blood cells into circulation. A bigger spleen means a bigger reserve. Deep-diving seals have unusually large spleens too.
The result, published in 2018 in the journal Cell: Bajau spleens are about 50 percent bigger on average than those of the Saluan. The key clue: even Bajau who don’t dive had larger spleens. So training alone can hardly explain it.
The trail leads to a gene
In the genome, the researchers found 25 sites where the Bajau differed significantly from the comparison groups. One of them is in a gene called PDE10A. Variants there are linked to a larger spleen. In mice, the gene influences a thyroid hormone that controls spleen size.
A follow-up study in 2021 found that mice with less PDE10A protein really did develop larger spleens, matching what had been seen in the Bajau.
The case isn’t completely closed. Experts point out that more data is needed to firmly prove a causal link between the gene variant, the big spleen and the diving skills. Still, it counts as one of the rare examples where you can see natural selection in modern humans this concretely.
Evolution doesn’t just happen in textbooks. Sometimes it happens in the middle of the ocean.