A tribe of "fish people" in Indonesia have developed a genetic mutation that enables them to dive much better than the average human. The results were sequenced at the University of Copenhagen and clearly showed the Bajau have a median spleen size 50% larger than the Saluan.
This is not the first time that examples of continuing human evolution among specific groups of people have been discovered.
Melissa Ilardo, from Cambridge University, started the project as a PhD candidate.
Contraction of the spleen is a key way in which mammals respond to diving and some deep-diving seals, such as Weddell seals, have enlarged spleens.
"The Bajau and other Sea Nomads are simply extraordinary and I wanted to be able to prove that to the world", said Ilardo, now a US National Institutes of Health postdoctoral scholar at the department of Molecular Medicine at the University of Utah. There are about one million Bajau people who live across the Philippines, Indonesia and Malaysia. Instead they rely on weights, handmade wooden goggles - and a single breath of air.
The spleen holds oxygenated red blood cells, so presumably an enlarged spleen - those of the sea nomads, or Bajau people, are about 50 percent larger than the spleens of unrelated, non-diving neighboring groups - injects more blood cells into the circulation and makes more oxygen available for basic body functions during prolonged dives. During a dive, the spleen contracts and pushes these extra red cells into the circulating blood, increasing its capacity to carry oxygen. Also, a larger spleen doesn't necessarily mean a bigger expulsion of red blood cells, which is what actually helps the divers, she notes. Thus, diving leads to a slowing of the heartbeat (bradycardia), which reduces oxygen consumption, as well as a narrowing of the peripheral blood vessels (vasoconstriction), which selectively redistributes the blood flow to the organs most sensitive to hypoxia. Both groups may have evolved the changes because hypoxia was common enough from living at higher altitudes or breath-holding under water that having the mutated genes gave them a significant advantage.
What makes the Bajau different, Ilardo said, is that their historically seafaring lifestyle and fishing practices may have driven the evolutionary process. As they never dive competitively it is uncertain exactly how long the Bajau can remain underwater, but one of them told researcher Melissa Ilardo that he had once dived for 13 minutes consecutively.
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According to the Economist, DNA analysis also showed a mutation of a gene allowing oxygen to be preferentially sent to oxygen-hungry vital organs.
It's often hard to show that a gene has undergone recent evolutionary adaptations in living people, but in this case, there is strong evidence, even though the Bajau have been living the sea life for just a few thousand years, says study co-author Eske Willerslev, an evolutionary genomicist at the University of Copenhagen. The researchers found that Bajau people of South-East Asia have spent a long time of their day in diving and collecting shellfish from the sea floor to survive. After they agreed to the study, she returned with an ultrasound machine.
The researchers are excited to share their findings with the Bajau study participants.
According to study co-author Rasmus Nielsen, "This is the first time that we really have a system like that in humans to study".
Studying free-divers such as the Bajau could help improve the understanding of acute hypoxia.
Willerslev admitted he initially urged Ilardo not to pursue the research for her PhD thesis, believing it was too risky and that she may find nothing.