Tribe evolve to thrive under sea

Henrietta Brewer
April 20, 2018

While the Bajau divers today use equipment to help with their breathing when they search for the sea-critters, their ancestors have engaged in the breath-holding technique for thousands of years. Members of the Bajau can dive up to 70 meters with nothing more than a set of weights and a pair of wooden goggles.

A tribe of "superhuman" Indonesians have evolved larger spleens that allow them to dive more than 200ft. For generations, the Bajau people have built their homes above the water and spend most of their time at sea. One of the tribe told Ms Ilardo that he had once dived for 13 minutes. Dr. Ilardo also stated that she was excited to share her findings with the Bajau people as she felt that they understood the "science" behind her research and to satisfy their curiosity about the world and themselves.

"If natural selection had acted in seals to give them larger spleens, then it might've done the same thing in humans", Ilardo said.

Bajaus are particularly known for their exceptional diving abilities. This is the first time that a study has shown that genetics and evolution plays a role in spleen size of the divers. The heart rate slows down, blood vessels in the extremities shrink to preserve blood for vital organs, and the spleen contracts. Now researchers have found that they have developed extra large spleens over the years which enable them to dive to a depth of 200 feet.

The Bajau people have spleens which are 50 percent larger than that of their land-dwelling counterparts the Saluan.

The Bajau having larger spleens than their non-diving neighbors suggested that their diving culture had shaped their physiology.

Variations in one gene in particular, PDE10A, may cause changes in thyroid hormone levels that in turn lead to an enlarged spleen, researchers speculate.

Among the Bajau, there wasn't much difference in spleen size between divers and non-divers. Ilardo explained that thyroid and spleen sizes have been linked to each other before in mice. "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".

Indeed, this isn't the first time that scientists have documented natural selection at work on modern humans. "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".

"Free diving is extremely unsafe, and so even highly trained free divers often die because they lose consciousness on ascent and they drown", said Ilardo, who has been free diving recreationally since she was 4 years old.

"She said she wanted to do it anyway and that paid off. Melissa was right and our concerns were wrong".

Beyond its small size, another limitation of the study is that it wasn't a controlled experiment created to prove whether or how generations of divers might evolve to adapt an underwater lifestyle.

A new study has been conducted on the Bajau people of Malaysia and the Philippines. "It was very exciting to find, and it just opens up so many possibilities", says Ilardo.

As humans, we sometimes like to think we're immune to natural selection, but this study shows that's simply not the case.

While both of these studies chronicle respiratory adaptations among living humans, the nature of the adaptations are qualitatively different.

Understanding how the human body responds to a lack of oxygen, for instance, is important in a lot of medical contexts, from chronic obstructive pulmonary disease to surgery.

This article has been republished from materials provided by St John's College Cambridge.

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