The indigenous Bajau people of Southeast Asia spend their whole lives at sea, working 8-hr diving shifts with traditional equipment and short breaks to catch fish and shellfish. In a new study published in the journal Cell, an international team of scientists led by the Universities of California at Berkeley, Copenhagen and Cambridge report that Bajau’s diving abilities may be thanks in part to genetically enlarged spleens. The findings have implications for hypoxia research, a pertinent medical issue.
The Bajau can spend as much as 60% of their day doing repeated dives to catch fish, octopus and sea cucumbers. Over the centuries, they appear to have adapted to deep diving by developing larger spleens that hold more oxygenated blood to sustain them during these dives. This could be due to a genetic mutation that boosts thyroid hormone levels, which have been linked to larger spleens in mice. Image credit: Melissa Ilardo.
The Bajau people, groups of whom are spread among the islands of Indonesia, Malaysia and the Philippines, are often called ‘Sea Nomads,’ and traditionally lived on boats and harvested nearly everything they ate from the sea.
Some have been shown to spend as much as 60% of their work day diving for food — spearing fish and octopus and gathering crustaceans and sea cucumbers — at depths greater than 230 feet (70 m), using only a wooden mask.
Linguistic analysis suggests that they have lived this way for more than 1,000 years. A chronicler of one of Ferdinand Magellan’s voyages recorded their unusual lifestyle in 1521.
“I suspected that the Bajau could have genetically adapted spleens as a result of their marine hunter-gatherer lifestyle, based on findings in other mammals,” said study first author Melissa Ilardo, a doctoral student in the Centre for GeoGenetics at the University of Copenhagen.
“There’s not a lot of information out there about human spleens in terms of physiology and genetics, but we know that deep diving seals, like the Weddell seal, have disproportionately large spleens. I thought that if selection acted on the seals to give them larger spleens, it could potentially do the same in humans.”
The spleen plays a central role in prolonging free diving time as it forms part of what is known as the human dive response.
When the human body is submerged under cold water, even for brief amounts of time, this response is triggered as a method of assisting the body to survive in an oxygen-deprived environment. The heart rate slows down, blood vessels in the extremities shrink to preserve blood for vital organs, and the spleen contracts.
This contraction of the spleen creates an oxygen boost by ejecting oxygenated red blood cells into circulation and has been found to provide up to a 9% increase in oxygen, thereby prolonging dive time.
In order to gain evidence for this study, Ilardo and colleagues spent several months in Jaya Bakti, Indonesia taking genetic samples and performing ultrasound scans of the spleens from both the Bajau and their land-dwelling neighbors, the Saluan.
The results clearly showed the Bajau have a median spleen size 50% larger than the Saluan. Enlarged spleens were also visible in non-diving Bajau individuals as well as those who regularly free dive.
The study authors therefore eliminated the possibility that larger spleens were simply a plastic response to diving and began to investigate the Bajau’s genetic data.
They discovered that members of the Bajau have a gene called PDE10A which the Saluan do not. It is thought that the PDE10A gene controls the levels of thyroid hormone T4.
“We believe that in the Bajau they have an adaptation that increases Thyroid hormone levels and therefore increases their spleen size,” Ilardo said.
“It’s been shown in mice that thyroid hormones and spleen size are connected. If you genetically alter mice to have an absence of the thyroid hormone T4, their spleen size is drastically reduced, but this effect is actually reversible with an injection of T4.”
This is the first time a genetic adaptation to diving has been tracked in humans.
“Until now it has been entirely unknown whether Sea Nomad populations genetically adapt to their extreme lifestyle,” Ilardo said.
“The only trait previously studied is the superior underwater vision of Thai Sea Nomad children, however this was shown to be a plastic response to training, and was replicable in a European cohort.”
“The physical and genetic changes that have enabled the Bajau to dive longer and deeper is yet another example of the immense variety of human adaptation to extreme environments, in this case, environments with low levels of oxygen,” added co-lead author Professor Rasmus Nielsen, from the University of California, Berkeley.
“These examples can be key to understanding human physiology and human genetics.”
The study also has implications for the world of medical research.
The human dive response simulates the conditions of acute hypoxia in which body tissue experiences a rapid depletion of oxygen. It is a leading cause for complications in emergency care and as a result is already the subject of several genetics studies, specifically in relation to people groups who live at high altitudes.
Studying marine dwellers such as the Bajau has great potential for researching acute hypoxia in a new way.
“This is the first time that we really have a system like that in humans to study,” Professor Nielsen said.
“It will help us make the link between the genetics and the physiological response to acute hypoxia. It’s a hypoxia experiment that nature has made for us and allows us to study humans in a way that we can’t in a laboratory.”
These findings open up the possibility of further research on other Sea Nomad populations such as the Thai Moken population and the Haenyeo diving women of Jeju in South Korea.
Studying similar people groups could shed more light on the nature of the connection between human physiology and genetic adaptations to extreme lifestyles, and clarify whether these genetic adaptations have developed separately.
“This really tells us how valuable and important indigenous populations are around the world that are living extreme lifestyles, in terms of understanding the function of various genetic traits and finding the underlying genetic background for various physiological traits,” said co-lead author Professor Eske Willerslev, from St John’s College, Cambridge and the University of Copenhagen.
“Most of these populations are completely understudied, and I think there is a huge benefit, not only potentially to them, but also to the rest of mankind by actually paying some attention to them.”
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Melissa A. Ilardo et al. 2018. Physiological and Genetic Adaptations to Diving in Sea Nomads. Cell 173 (3): 569-580; doi: 10.1016/j.cell.2018.03.054
