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 the 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.
Recently 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 has 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.
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.
After visiting a Bajau village in Indonesia, she returned 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. 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.
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 François-Xavier Ricaut, an anthropologist at the University of Toulouse.