EVOLUTIONARY DIVERS
The sea-dwelling Bajau have bodies remodeled for a life at sea.
WE are the products of evolution, and not just evolution that occurred billions of years ago. As scientists peer deeper into our genes, they are discovering instances of human evolution in just the past few thousand years. People in Tibet and Ethiopian highlands have adapted to living at high altitudes, for example. Cattle-herding people in East Africa and northern Europe have gained a mutation that helps them digest milk as adults.
Last Thursday in the journal Cell, a team of researchers reported a new kind of adaptation — not to air or food, but to the ocean. A group of sea-dwelling people in Southeast Asia have evolved into better divers.
The Bajau, as these people are known, number in the hundreds of thousands, scattered in communities in Indonesia, Malaysia and the Philippines. They have traditionally lived on houseboats; in recent times, they’ve also built houses on stilts in coastal waters.
“They are simply a stranger to the land,” said Rodney C. Jubilado, a University of Hawaii anthropologist who studies the Bajau but was not involved in the new study.
Jubilado first encountered the Bajau while growing up on Samal Island in the Philippines. They made a living as divers, spearfishing or harvesting shellfish. “We were so fascinated that they could stay underwater much longer than us local islanders,” 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 have 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,” Ilardo said.
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 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.
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 she 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, Ilardo found. It was likely the Bajau are born that way, thanks to their genes.
On her visit to Sulawesi, 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 with 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 Ba-