Legendary Female Free-Divers Reveal Evolution in Action on South Korean Island
We see it at high altitudes, where the human body has adapted to lower oxygen levels in the air. Now, scientists have found the genetic tracers for adaptation on South Korea's Jeju Island, written in the genetic code of the Haenyeo: women who free-dive into frigid waters, year-round, to harvest food from the seafloor.
For centuries, Jeju Island relied on the work of the Haenyeo. Now, the need for their services is on the wane, and most of the women are older, with an average age around 70 – possibly the last generation to exercise the tradition.
"They're absolutely extraordinary women," says geneticist Melissa Ilardo of the University of Utah. "Every day, they head out and get in the water, and that's where they work all day. I saw women over 80 diving off a boat before it even stopped moving."
Ilardo has spent years studying human adaptations to extreme environments. In 2018, she and her colleagues published a paper describing the first population found to have evolved genetic adaptations to ocean free-diving – the Bajau Laut people of Southeast Asia.
She and her colleagues wanted to know if similar adaptations improve the abilities of the Haenyeo as they dive to depths of around 10 meters (33 feet) to collect ingredients like abalone and seaweed from beneath the cold, heavy waters.
Their study involved 91 participants: 30 Haenyeo divers; 30 Jeju locals who are not divers; and 31 people from Seoul on mainland Korea, with an average age of 65 for all participants. DNA samples were taken, and participants also had their heart rate and blood pressure tested, both at rest, and during a simulated dive in which they submerged their faces in a bowl of cold water.
"If you hold your breath and put your face in a bowl full of cold water, your body responds as if you're diving," Ilardo explains. "A lot of the same processes happen in your body that would happen if you were to jump in the ocean, but it's done in a way that's safe for people with no diving experience."
Interestingly, there was no genetic difference between the Haenyeo divers of Jeju Island and the other locals on the island. But the Jeju populations were more than four times more likely than Seoul residents to have a genetic variation associated with lower blood pressure.
This is likely because holding one's breath, as is necessary for a free-dive, raises one's blood pressure. The researchers speculate that naturally lower blood pressure is vital for Haenyeo divers, who work even while pregnant. Heightened blood pressure during pregnancy can result in conditions such as preeclampsia, which can be quite dangerous for both mother and fetus.
"This association may reflect natural selection to mitigate the complications of diastolic hypertension experienced by female divers while diving through pregnancy," Ilardo says. "Since Bajau women also dive while they're pregnant, we wonder whether pregnancy is actually driving a lot of the genetic changes in these diving populations."
Interestingly, the stroke mortality rate on Jeju is lower than most of Korea, suggesting that this adaptation may have some side benefits, since stroke is associated with high blood pressure.
The other genetic difference between the Jeju and mainland populations that the researchers observed has to do with the body's pain tolerance for cold. This, they believe, may help make the divers less susceptible to hypothermia, since they dive year-round, even in winter when temperatures plunge to freezing levels.
"While we did not measure thermoregulatory physiology in our study, this represents an avenue for future research," the authors write.
There was one very intriguing difference between the Haenyeo and the non-divers of Jeju Island. When placed in the simulated dive scenario, the heart rates of the Haenyeo slowed much more than the heart rates of either control group. This suggests that the heart rate adjustment is a learned response, rather than a genetic one – the product of decades of experience.
The study results, the researchers say, could help scientists better understand the workings of the human body, how we respond to environmental pressure, and the effects those changes have on other health outcomes.
"We're really excited to learn more about how these genetic changes may be affecting the health of the broader population of Jeju," Ilardo says. "If we can more deeply characterize how those changes affect physiology, it could inspire the development of therapeutics to treat different conditions, such as hypertensive disorders of pregnancy and stroke."
The research has been published in Cell Reports.
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