It’s wintertime. The water is cold, and the air even more frigid. The waves might be crashing. The wind could be blowing. And still, short of a storm, Haenyeo will be out in the water. The aging, all-women group of free divers embody a many-generation tradition on the Korean island of Jeju. Like the centuries of women who came before them, the Haenyeo, which translates to “sea women,” spend whole days holding their breath below the surface–a minute or two at a time– to harvest abalone, urchin, seaweed, and mollusks by hand.
Subsistence freediving can be traced back thousands of years in the region. Spending so much time underwater over many millennia has equipped the Haenyeo and their relatives with certain genetic advantages, according to some new research. Divers and other Jeju natives share gene variants related to cold tolerance and reduced diving blood pressure, according to the study published May 2 in the journal Cell Reports. These variants gained evolutionary ground because of the intense selective pressures diving brings, the study authors suggest.
Many human populations carry traits evolved in response to their lifestyle or environment. In mountainous regions around the world, people often share genes linked to high-elevation survival. The result is traits like altered hemoglobin boosts Tibetans’ and Andeans’ ability to thrive in the thin air at altitude. Lactose tolerance popped up among subsets of historically cow-herding people, to enable more comfortable and less risky dairy drinking. And diving, too, can drive adaptation.
“Diving is really dangerous,” Melissa Llardo, senior study author and an evolutionary geneticist at the University of Utah, tells Popular Science. Even the most experienced swimmers can pass out underwater, get stuck, or end up dragged along by a current. Over time, it makes sense that people living in diving cultures would evolve ways to minimize the threats.
Past work from Llardo has shown that nomadic Bajau divers in Indonesia, Malaysia, and the Philippines have especially large spleens. Even Bajau people who don’t dive seem to share this trait, suggesting it’s the product of genetics, not training.
When humans submerge under water, our mammalian dive reflex kicks in and a suite of physiological changes occur. Heart rate slows, peripheral blood vessels contract, blood pressure rises, and organs adjust their operations. The spleen, which plays a role in blood storage and filtering, contracts to help circulate more oxygenated blood as part of the reflex. A larger spleen means a bigger oxygen boost while diving.
Given those 2018 findings, Llardo was eager to see if other diving populations have developed the same spleen strategy. “That really opened up this whole world. [We thought], ‘there are other diving populations out there, and maybe the Bajau aren’t the only ones who have evolved to be better divers.’”
[ Related: The world’s best freedivers seem to have evolved super-sized spleens. ]
To find out, she and her colleagues sequenced full genome samples from about 30 Haenyeo divers, 30 non-divers from Jeju, and 30 women from Seoul, as a genetic control. Then, they surveyed the DNA for genes under selection.They did this by looking for variants that appeared in higher frequency in the Jeju population than the Seoul group, and comparing it to wider, previously collected population-level samples.
The researchers also collected physiological data on the participants at a normal baseline, and during “simulated dives” where subjects held their breath while submerging their faces in cold water. They used this information to track physical differences between the groups.
“We were sort of expecting that they would share the same variants as the people from Indonesia,” says Diana Aguilar-Gómez, lead study author and an evolutionary biologist and geneticist at UCLA. But, ultimately, spleen size wasn’t what set the Jeju and Haenyeo population apart.
Instead, the scientists found that blood pressure was the big divider. Divers and non-divers alike from the island population maintained a much lower diastolic blood pressure (about 10 mmHg lower) during dive tests than the Seoul population. In their genetic survey, they found that a gene variant linked with lower diastolic blood pressure was more frequent in the Jeju group.It was present in about one third of Jeju natives, compared with only a single digit percentage of mainland Koreans. At least two other gene variants under significant selection were also present: one variant linked with red blood cell count and another related to pain and cold sensitivity.
The cold sensitivity factor is self-explanatory. Haenyeo were diving in thin cotton bodysuits until wetsuits came on the scene in the 1980s. “I can’t imagine,” says Llardo. “It must’ve provided no thermal protection at all.” And the red blood cell variant is likely to be related to spleen function and blood oxygen levels.
However, the blood pressure finding was a surprise. It wasn’t until the scientists were presenting early findings at a conference, that an audience member helped them piece together a possible explanation. “He said, ‘I’m a maternal health specialist, and this makes complete sense to me,’” recounts Llardo. Now, “we think this adaptation is actually protecting divers and their unborn children as they’re diving through pregnancy,” she explains.
Diving while pregnant has long been common practice among Haenyeo–or at least it was, before their average age was upwards of 70. Women would continue diving up until the day they gave birth. But during pregnancy, blood pressure disorders like preeclampsia pose a particularly acute risk for both mother and child.
For Haenyeo, genetic shifts may have emerged to have mitigated some of the challenge. “If you’re a diver and you’re having problems with hypertension, you’re probably going to have fewer children,” says Aguilar-Gómez. “Maybe you’ll have one kid instead of three,” she adds– or maybe, pregnancy complications brought on by dive-induced hypertension have been historically deadly.
In this way, “diving during pregnancy could have shaped the genetics of a whole island of people,” Llardo says.
Though more research is needed to know for sure. The relatively small number of study participants means that the team couldn’t suss out every gene variant that may or may not be under selection. And right now, the pregnancy hypothesis isn’t quite set. “I think the study raises more questions than it answers… but it’s all exciting questions,” she says. Down the line, it’s possible answering some of those questions could offer insights for better treatments of blood pressure-related pregnancy complications.
One thing the study proves for sure, though: genes are far from the whole story. During the dive tests, Haenyeo had much lower heart rates, even compared with non-divers from Jeju who share the same ancestry. There, skill and experience offer the most likely explanation, says Aguilar-Gómez. “It’s hard work and it’s their long life of training and tradition and culture.”
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