When you dive into water, your body activates what’s called the diving reflex – your heart slows down, peripheral blood vessels are constricted, blood is redirected to the vital organs and the spleen contracts and releases oxygenated red blood cells (RBCs) that it stores.
An average human being can last around a minute holding her breath underwater, a trained swimmer a little more than that. But the Bajau people of Jaya Bakti village in Indonesia, can last up to 13 minutes underwater, while diving to depths of over 70 metres.
A study conducted by Melissa Ilardo and her colleagues from the University of Copenhagen, has discovered that natural selection can act on the diving reflex in humans, and this has enabled the Bajau to hold their breaths for much longer than the average human can.
Ilardo first heard of the Bajau people on a visit to Thailand. Intrigued by their diving skills, she went back to Indonesia, went diving with them and talked to them to understand who they were and how they worked. The Bajau, also called ‘sea nomads’, are a group of people who traditionally live near the seas, very rarely coming back to land. Though many have abandoned their traditional nomadic lives, a good number continue to spend their lives on and in the waters.
The Bajau live in houseboats and spend about eight hours working everyday, catching fish, octopuses and other sea creatures with their bare hands, or sometimes using a sharp spear that they fashion out of wood. In the course of their work day, they can spend over five hours in the water, diving deep to catch fish as well as collect black coral for jewellery. Apart from the spear, the only equipment they need for their underwater hunt is a pair of wooden goggles.
Ilardo recollected the time she spent diving with the Bajau to The Atlantic: “Underwater, the Bajau are as comfortable as most people are on land. They walk on the seafloor. They have complete control of their breath and body. They spear fish, no problem, first try.”
Looking at how much control the Bajau people had over their breath and body, and how at home they were inside water, Ilardo was tempted to predict that they might have developed genetic changes that allowed them to adapt to their ways of life. Egged on by evidence of genetic differences in people who live at extremely high altitudes in Tibet and Ethiopia, Ilardo predicted that similar changes may have occurred in the Bajau as well.
The next time she went to visit them, she was armed with a portable ultrasound machine to look at their spleens and see if they were any different. Since the spleen is a reserve of oxygenated RBCs, it is one of the more important players in the diving reflex in humans. Previous studies have also showed that the spleen contracted and released more oxygenated RBCs in the case of people who had climbed Mount Everest, under hypoxic conditions similar to that experienced while diving.
Enlarged spleen, genetic differences
True to her expectation, Ilardo saw that the spleen in the Bajau people was 50% larger than the closely related Saluan people of the Indonesian mainland, who rarely ventured into the sea. And this wasn’t just the result of training – changes accumulated over an individual’s lifetime that cannot be passed on – became obvious when looking at Bajau who had shunned the aquatic way of life and settled on land.
“The fact that both Bajau divers and non-divers have the large spleen points to the fact that this is something they have from birth rather than something they acquired through experience over time,” Fernando Racimo, an assistant professor at the University of Copenhagen who was a part of this study, told the Washington Post.
On testing genetic samples from the Bajau and the Saluan, Ilardo saw that the Bajau differed from the Saluan and a bigger sample of the Han Chinese at 25 different genes. One of them was the PDE10A gene, which is responsible for regulating thyroid activity, by affecting T4 hormone levels. A prior study in mice had implicated a connection between thyroid hormones and spleen size. “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,” Illardo told Newsweek.
Other genes that also showed signs of adaptation were the BDKRB2, which controls the constriction of blood vessels in the periphery and FAM178B, which regulates the level of carbon dioxide in the blood – another important factor that determines how long a person can stay underwater.
These results were published in the journal Cell in April 2018.
Too early to conclude
The responses triggered in the diving reflex are very similar to a clinical condition called acute hypoxia, where blood oxygen levels are low. Scientists are convinced that the findings from Ilardo’s study and others that will follow from it should be helpful in tackling hypoxia, which, according to the National Geographic magazine, is often a cause of death in emergency rooms.
Rasmus Nielsen, a professor at the University of Copenhagen and the University of California, Berkeley, who supervised Ilardo during the study, said in a statement, “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.”
A unique feature of this study is how it has demonstrated natural selection in a human population, especially over such a short period: the Bajau have been living in the region and practising diving in the sea for only over a thousand years.
But others are a little more skeptical, saying that Ilardo and the team have not understood what is going on with the Bajau completely yet. Cynthia Beall, an anthropologist at the Case Western Reserve University, Ohio, pointed out that a larger spleen need not result a greater expulsion of RBCs. Edward Gilbert-Kawai, a physiologist at the University of London, told Science magazine that “it is highly unlikely that spleen size is controlled by only one gene.”
Richard Moon, who studies cardiorespiratory function in humans at the Duke University School of Medicine, North Carolina, thought that it was likely that there were other adaptations in the Bajau that, in addition to the enlarged spleen, helped them with their underwater skills.
The traditional way of life of the amphibious Bajau is increasingly under threat. The light wooden trees from which they make their floating home are now endangered. They are now forced to make their boats with heavier wood, which then demand a motor and fuel, both of which in turn are expensive. Ilardo said, “They’re slowly becoming connected to the land, but some of them still build houses on stilts to maintain a connection to the sea.”