Scientists have found evidence that some humans have genetically adapted to diving by studying the Bajau, a group of people indigenous to parts of Indonesia. The Bajau, also known as “sea nomads”, have an enlarged spleen that allows them to dive to depths of up to 70 m (230 ft).
The genetics of the perfect dive
Melissa Ilardo spent several months in Jaya Bakti, Indonesia, taking genetic samples and performing spleen ultrasounds from the Bajau and their land neighbors, the Saluan. She had a hunch that the Bajau had genetically adapted so they could dive to staggering depths. Carrying only a spear, weights and wooden goggles, some Bajau divers can stay underwater regularly for 15 minutes. They have been living this hunter-gatherer lifestyle for thousands of years.
The spleen is directly relative to how long a person can dive. When the human body is submerged under cold water, even for a few seconds, it triggers the so-called “human dive response”. During this state, the heart rate slows, blood vessels in the extremities narrow, and the spleen constricts. This last action of the spleen gives the body an oxygen boost – up to 9% extra O2 – by ejecting oxygenated red blood cells into the bloodstream, thus extending the dive time.
“There isn’t a lot of information about the human spleen in terms of physiology and genetics,” Ilardo said, “but we do know that deep-diving seals, like the Weddell seal, have a disproportionate spleen. I thought that if selection acted on seals to give them bigger spleens, it could potentially do the same thing in humans.
Ilardo undertook this study for his doctorate. at the University of Copenhagen, where her supervisors, Professor Eske Willerslev and Professor Rasmus Nielsen advised against her. The two professors were of the opinion that Ilardo might not find anything. “She said she wanted to do it anyway and it paid off. Melissa was right and our concerns were wrong,” Professor Willerslev said.
The study first showed that the Bajau have a median spleen size 50% larger than the Saluan. Enlarged spleens were seen in non-diving Bajau individuals as well as those who regularly snorkeled, eliminating the objection that this was merely a plastic response to diving.
Genetic sequencing of Bajau individuals revealed that they carry a gene called PDE10A what the Saluan don’t do. Ilardo and his colleagues say that the PDE10A gene controls levels of the thyroid hormone T4. It is these high levels of thyroid hormone that cause the spleen to grow much larger than that of probably every other person in the rest of the world.
“Thyroid hormones and spleen size have been shown in mice to be related. If you genetically engineer mice to have an absence of the thyroid hormone T4, their spleen size is dramatically reduced, but this effect is actually reversible with an injection of T4,” Ilardo said.
Until now, scientists weren’t sure if sea nomads were genetically adapted to their extreme lifestyle. The new study now provides the first evidence that such genetic adaptation has been tracked in humans. Previously, researchers also studied the superior underwater vision of Thai Sea Nomad children, but found that it was a plastic response to training rather than a genetic adaptation.
The findings could have important implications for medical research. It is well established that the human diving response essentially simulates acute hypoxia, which is the body’s response to rapid oxygen depletion. Acute hypoxia is one of the main causes of complications in emergency care, so any other means of treat it could save human lives. For example, the new study may one day lead to an association between genetics and the physiological response to acute hypoxia.
Perhaps studies of other prolific divers could to bring even more clues. Besides the sea nomads, other populations with similar diving prowess include the Thai Moken population and the Haenyeo female divers of Jeju in South Korea.
“This study is a wonderful example of the value of studying these small populations living in extreme conditions,” said Professor Eske Willerslev. “Many of them are threatened and it is not just a cultural and linguistic loss, but for genetics, medicine and science in general. There is still a lot of information to be gathered from these understudied populations. .
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