“We were so fascinated that they could stay underwater much longer than us local islanders,” Dr. Jubilado said. “I could see them literally walking under the sea.”
Even as anthropologists study Bajau culture, biologists have grown curious about them, too. Bajau divers been observed plunging more than 200 feet underwater, their only protection a pair of wooden goggles — a physiological marvel.
In 2015, Melissa Ilardo, then a graduate student in genetics at the University of Copenhagen, heard about the Bajau. She wondered if centuries of diving could have led to the evolution of traits that made the task easier for them.
“It seemed like the perfect opportunity for natural selection to act on a population,” said Dr. Ilardo.
Her first step was to travel to Sulawesi, Indonesia, and then to a coral reef island where she reached a Bajau village. After she proposed her study, they agreed to the plan. She returned a few months later, this time with a portable ultrasound machine to measure the size of the Bajau people’s spleens.
When people plunge into water, they respond with the so-called diving reflex: the heart rate slows and blood vessels constrict as a way to shunt blood to vital organs. The spleen also contracts, squirting a supply of oxygen-rich red blood cells into the circulation.
All mammals have a diving reflex, but marine mammals like seals have a particularly strong one. Scientists suspect that the reflex helps them dive deeper — as it turns out, seals with bigger spleens can dive deepest. An enlarged spleen seems to function like a bigger scuba tank.
Dr. Ilardo scanned the abdomens of the Bajau villagers and then traveled about 15 miles inland to a village occupied by farmers known as the Saluan. She scanned them, too.
When Dr. Ilardo compared scans from the two villages, she found a stark difference. The Bajau had spleens about 50 percent bigger on average than those of the Saluan.
Yet even such a remarkable difference might not be the result of evolution. Diving itself might somehow enlarge the spleen. There are plenty of examples of experience changing the body, from calloused feet to bulging biceps.
Only some Bajau are full-time divers. Others, such as teachers and shopkeepers, have never dived. But they, too, had large spleens, Dr. Ilardo found. It was likely the Bajau are born that way, thanks to their genes.
On her visit to Sulawesi, Dr. Ilardo also took mouth swabs from the Bajau and Saluan from which she extracted DNA. She looked at the genetic variations in each village and compared them to people from neighboring countries, such as New Guinea and China.
A number of genetic variants have become unusually common in the Bajau, she found. The only plausible way for this to happen is natural selection: the Bajau with those variants had more descendants than those who lacked them.
One variant of a gene called PDE10A influenced the size of spleens in the Bajau. People with one copy of the mutant gene had bigger spleens than those with none. People with two copies had even bigger spleens.
Scientists had never found a special role for PDE10A in the spleen. “This connection was a bit bizarre,” Dr. Ilardo said.
But there’s one possible link. PDE10A has been shown to control the level of thyroid hormone in the body. And scientists have found that injecting thyroid into mice with stunted spleens can make the organs grow larger.
Still, that wouldn’t pin down exactly how PDE10A became so common in the Bajau. “It’s the question that’s harder than others,” said Rasmus Nielsen, a geneticist at the University of California, Berkeley, who collaborated with Dr. Ilardo.
For her own part, Dr. Ilardo suspects that natural selection favored the Bajau variant of PDE10A because deep diving is so risky. “I would think, as morbid as it is, that if they didn’t have this, it would kill them,” she said.
François-Xavier Ricaut, an anthropologist at the University of Toulouse who was not involved in the study, said that it wasn’t clear yet how quickly this evolutionary change happened.
Some researchers suspect the Bajau only began diving to great depths when a market for sea cucumbers opened up in China in the 1600s. Or perhaps the adaptation began thousands of years earlier, at the end of the Ice Age, when rising sea levels turned the region around Indonesia into islands.
“This study acts as a cornerstone for exciting questions to follow,” said Dr. Ricaut.
Dr. Ilardo said there were likely a number of other genes that help the Bajau dive. She and her colleagues also found evidence for natural selection on a gene called BDKRB2.
In a study published last year, Russian scientists discovered that it plays a role in the diving reflex. In people with variants of BDKRB2, blood vessels are more tightly constricted when they plunge their faces into cold water.
To see if that’s the case with the Bajau, Dr. Ilardo will need to take another trip to beautiful Sulawesi. “I would be happy doing this as long as I can,” she said.
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