Researchers have found that a community of people known as "sea-nomads" in Southeast Asia have enhanced diving abilities thanks to natural selection working to enlarge their spleens. The spleen is normally considered important for immune function, but in these deep-diving people their enlarged spleens are acting as internal scuba tanks, providing a little extra oxygen for deep dives.

Researcher Dr. Melissa Ilardo made extended visits to one community of sea nomads known as the Bajau people. She used portable ultrasound equipment to measure the size of their spleens, and took DNA samples that the researchers used to identify genetic variants in the Bajau responsible for their enlarged spleens.

The Bajau and other sea nomads have a unique aquatic lifestyle.

"Traditionally they're spending their whole lives at sea, in these houseboats, only visiting land occasionally," said Ilardo. "It's actually said many Bajau children learn to swim before they learn to walk."

In modern times they've become slightly more sedentary, building stilt-houses on coral atolls, but still living almost entirely from the sea.

A Bajau diver spear-fishes on a coral reef. The Bajau people have evolved an oversized spleen that carries oxygen-rich red blood cells to help them dive. (M. Ilardo)

What stimulated Ilardo's curiosity about the Bajau was their remarkable diving abilities. "It's really like nothing I've ever experienced. The level of comfort and ease they show in the water — it's remarkable."

Bajau traditional harvesting practices involve hunting fish and octopus underwater using spears, and gathering other sea creatures like crustaceans and sea cucumbers from the ocean floor.

They're able to dive to great depths — more than 70 metres — and can stay submerged for 10 minutes at a time. They also recover from dives quickly, so that they're able to spend many hours every day underwater.

Ilardo saw the Bajau as a likely group of people to study for signs of recent evolutionary changes. "With such a dangerous activity, if they've been living like this for thousands of years, and it's such a big part of their life, then it's a great chance for natural selection to act on a population."

Those individuals with biological advantages that adapted them to the risks of diving would be more likely to survive their aquatic lifestyle and have offspring.

Bajau divers can dive to 70 meters and stay underwater for 10 minutes, often using homemade goggles or masks. (M. Ilardo) The team focused on the spleen because it's known to be involved in the diving reflex.

When humans hold their breath and dunk themselves in water, three things happen that help the body use oxygen more efficiently: the heart rate slows, the body pulls blood from the extremities to the core, and the spleen contracts, pushing oxygen-rich red blood cells into the bloodstream.

Research on deep-diving marine mammals, like seals, has shown that some have enlarged spleens to take advantage of this reflex. The spleen acts like an internal scuba-tank, storing oxygenated blood cells that are then released into the bloodstream during dives.

How do their bodies do it?

Ilardo and her team wondered if the Bajau also had enlarged spleens. Their first step was to visit a Bajau village on an island off the coast of Indonesia.

They were greeted with enormous hospitality, says Ilardo. "We went to the house of the chief of the village and his wife, they had no idea we were coming, and they just welcomed us with open arms," she said.

Ilardo explained the project, which interested the villagers, then brought out her portable ultrasound. She had no shortage of volunteers willing to participate. "They were really curious," she says. "I mean, I don't know how I would feel if someone showed up out of nowhere and asked if they could take a picture of my spleen."

The Bajau village of Jaya Bakti is perched on an atoll off the coast of Indonesia, and gives the people easy access to the water. (M. Ilardo)

Analysis of the ultrasound spleen images showed two important things. First of all, the Bajau did, indeed, have large spleens. They were 50 per cent larger than the comparison group — farmers from a nearby village.

Secondly all the Bajau — those who dived and those who didn't — had large spleens. This suggested that this was not a "training" effect associated with diving, like a muscle growing larger with use, but was more likely due to genetic changes common to all Bajau.

DNA analysis supported this conclusion. Ilardo's team found a gene that showed signs of recent natural selection called PDE10A, which has been associated with spleen size. They also found another genetic variation that they linked to blood vessel constriction — another part of the dive reflex that might strengthen Bajau's diving ability.

In one sense, this is an example of recent human evolution that is unique. There are other examples of recent genetic changes to human populations. Genes arose in Europeans that allowed them to process lactose just a few thousand years ago, and the peoples of the high Andes and Himalayas have acquired adaptations that allow them to thrive in the thin air of their mountain homes.

The fascinating difference with the Bajau, according to Ilardo, is that they seem to have driven their own evolution. "Rather than adapting to a particular environment, the Bajau seem to have adapted to a lifestyle. So these are conditions they have imposed on themselves."