What you need to know

• A small island off the Korean coast is home to a genetically distinct population of humans.
• Haenyeo — literally "sea women" — are Korean cultural divers who have developed adaptations allowing them to dive for longer periods than other people.
• The island's inhabitants appear to have unique genes that protect against blood pressure changes that can affect pregnancy.

A sea culture tradition spanning generations may have given the people of an isolated South Korean island a unique set of genetic "superpowers" — lower blood pressure and cold tolerance mutations that could help science develop new medical treatments in the future.

The Haenyeo — literally "sea women" — divers live on Jeju Island, 53 miles (85 km) south of the South Korean mainland.

From adolescence, they train to deep dive to harvest food from the seabed.

A transatlantic research team has found the Haenyeo's generations-long cultural practice has helped them develop adaptations enabling them to dive longer than other humans.

But they have also evolved genetically to have lower blood pressure and enhanced cold-water tolerance.

If the researchers' findings are correct, this means the Haenyeo are one of only two groups of people known to have, quite literally, evolved to dive.

The Haenyeo difference

Genomic analysis found Haenyeo had developed at least three distinctive traits.

The first is a non-genetic adaptation — bradycardia: a slow heart rate, below 60 beats per minute.

While this phenomenon can occur due to medical conditions, it can also result from cardiovascular training. Bradycardia also manifests during their deep dives as part of an evolutionary response called the mammalian dive reflex.

"When anyone dives, the combination of holding your breath and submerging yourself in water triggers the mammalian dive reflex," said Melissa Ann Ilardo, a specialist in human biological adaptations at the University of Utah, US, who led the research.

"A component of that, is that your heart rate slows down."

This helps the mammal — humans in this case — conserve oxygen, giving more time before another breath is required.

"Compared to people with the same genetic background, the trained divers had a greater heart rate response," Ilardo said.

There is no genetic basis for this adaptation and the researchers think any human being could develop the same ability given a lifetime of training.

Other adaptations may be connected to generations of genetic evolution. 

At depth, the pressure of the ocean causes blood vessels to constrict, limiting the flow of oxygen to the brain, lungs and heart.

"So, your body says ‘OK, there's not a lot of oxygen coming, let's keep it in the organs that need it the most',” explained Ilardo, adding that blood pressure rises during deep dives to ensure an adequate oxygen supply to crucial organs.

Rising blood pressure isn't usually an issue for divers, but the Haenyeo continue their deep-sea practice even while pregnant — high blood pressure can be dangerous in pregnancy.

Usually studied among women with sleep apnea, breath-holding has been shown to have associations with pregnancy complications. For pregnant women with sleep apnea, Ilardo said it's as if they unintentionally "dive in their sleep."

But it appears the Haenyeo resist this phenomenon thanks to genetic mutations. Ilardo's group compared the genomes of Jeju Island inhabitants to non-Jeju populations.

Their study found Jeju peoples have descended from genetically distinct populations, and that two mutations — the first is associated with cold tolerance, the second with decreasing diastolic blood pressure — have also evolved among the community.

"We think … evolution acted to protect these pregnant women while they were diving," Ilardo said. "Their diastolic blood pressure did not increase as much as people with a different genetic background. And that was regardless of training."

Natural selection on an isolated island

Charles Darwin, the British naturalist, was the first Western person to observe evolutionary adaptations in unique environments among the finches of the Galapagos Islands.

Darwin noted the birds spread across these islands had different beak shapes and sizes. It suggested the various populations had evolved specific adaptations over time to help them feed on food sources specific to their island. 

In a similar vein, the Jeju people show how humans can also evolve genetic adaptations, in this case as a response to long-standing marine-based cultural practices.

"At one point diving was the largest economic force defining the island of Jeju," said Ilardo. "We think that at some point in the past, essentially everyone in Jeju was diving."

A similar study Ilardo worked on found the Bajau people of Southeast Asia evolved larger spleens to hold more oxygen-rich blood, helping them to hold their breath underwater for prolonged periods as well.

Other research has studied cultures in places like Tibet, Ethiopia and the Andes, where people have evolved unique genetic mutations to enable survival at high altitudes. The Quechua peoples of Peru and Tibetans, for example, possess different mutations to a single gene, but both seem to bestow a greater capacity for altitude tolerance where oxygen levels are lower.

Ilardo hopes to expand this research to other diving cultures around the world and potentially compare these with people living at high altitudes. This could also lead to new medical therapies.

"Jeju has one of the lowest rates of stroke mortality in all of Korea, so it's interesting to think about the gene that's been evolving, influences the actual physiology of the blood vessels.

"Maybe that [gene mutation] has a protective effect that could be used, in theory, to develop therapeutics to treat stroke in people all over the world."

Sources

• Functional EPAS1/HIF2A missense variant is associated with hematocrit in Andean highlanders
• Genetic and Training Adaptations in the Haenyeo Divers of Jeju, Korea

Edited by Zulfikar Abbany