When a human is submerged in water, within seconds the body begins to reflexively adjust. The heart rate slows; blood vessels in the extremities tighten, diverting blood flow to vital organs. And, crucially, the spleen constricts, expelling a precious reserve of oxygenated red blood cells into the bloodstream. All of this extends the time we can go without gasping.
Now a new study suggests some seafaring people may have evolved over thousands of years to push the limits of typical dive responses even further. Genetic changes have allowed one population in Southeast Asia to grow plus-size spleens that may enhance their breath-holding capabilities, according to an international research teams analysis. Some scientists have likened these evolutionary adaptations to the ones that have allowed Tibetans to thrive at high elevations.
The new study dealt with people who are often locally called Sea Nomads and live among the islands and coastlines of Southeast Asia. Traditionally, they live on houseboats and come to land only occasionally, says Melissa Ilardo, a postdoctoral researcher at The University of Utah and first author on the study. They have a reputation for being incredible divers, and for their connection to the sea. I went diving with them, and their abilities are just unreal.
Among the Bajauone group of people who live on houseboats in the waterways around and between the Philippines, Malaysia and Indonesiadivers have been recorded holding their breath for over five minutes while hunting for fish or shellfish. In comparison, average people might be able to stay underwater for one to two minutes, and world-class free divers can hold their breath in competitive settings for up to three or close to four and a half minutes.
Previously, a crew filming for the BBC documentary series Human Planet recorded a Bajau hunter during dives, and noted his heart rate plummeted to a mere 30 beats per minute. (The diving reflex in most humans only drops the heart rate to perhaps 50 beats per minute in a healthy adult.) Theyve been observed diving over 70 meters with only a weight belt and a set of goggles, Ilardo says. If theyre just collecting shellfish at 10 meters, they could spend all day doing these shallow dives. We were diving at one point and [a Bajau friend] looked down and saw a large clam. He dropped another 15 meters in an instant and grabbed it. Its pretty remarkable.
Ilardo, an evolutionary geneticist, wanted to know if the Bajaus abilities were a result of being trained from birth or if they evolved to be elite divers over generations of marine living. So she asked the Bajau, along with the Saluana genetically similar group of farmersto let her sequence their genomes and measure the size of their spleens. A larger spleen may store greater amounts of oxygenated red blood cells, allowing divers to remain submerged longer. I had a portable ultrasound machine. I brought it to the villages, and people would come and let me measure their spleens, she says. 43 Bajau and 33 Saluan participated in the study, which was published in Cell on Thursday.
The Bajau had significantly larger spleens than the nondiving Saluan, says Rasmus Nielsen, a computational biologist at the University of California, Berkeley, and a senior author on the study. We found the Bajau had 50 percent larger spleen size, he says. But among the Bajau themselves, divers had only slightly larger spleens (roughly 10 percent larger) than those who eschewed the traditional diving lifestyle, he notes. That surprising finding raised the possibility that the reason for the Bajaus larger spleens was geneticnot from a lifetime of underwater training.
Next, the researchers scoured the Bajau genome for signs of natural selection and found 25 gene variants that seemed unique to this population. When Ilardo and colleagues cross-referenced what the involved genes do, they discovered a few of them seem to be related to breath-holding and oxygen deprivation. [That] was absolutely thrilling when we saw all of these genes that were under selection that had potential relevance for diving, Ilardo says.
One of the genes the team identified is called PDE10A. We know that this gene controls thyroid hormone levels, and we know that controls spleen size, Nielsen says. Just under half of the Bajau carry the version of this gene that is associated with larger spleen, compared with 6 percent of the Saluan and 3 percent of Han Chinese (a population chosen for comparison because they are not closely related to either group), he says. Two other genes that the analysis suggested had evolved in the Bajau were BDKRB2, which controls blood vessel constriction in the extremities, and FAM178B, which helps regulate carbon dioxide balance in the blood. Both could be important for oxygen conservation and breath-holding ability underwater, according to the researchers.