An international team of scientists discovered how a group of Southeast Asian "sea nomads" gains their deep-diving prowess over hundreds of years. They have evolved with larger spleens.
A study published Thursday in the journal Cell has shown that the spleens of the sea nomads, or Bajau people, are about 50 percent larger than those of unrelated, non-diving neighboring groups.
Those spleens inject more oxygenated red blood cells into the circulation and make more oxygen available for basic body functions during prolonged dives.
The physical and genetic changes that have enabled the Bajau to dive longer and deeper are another example of the immense variety of human adaption to extreme environments, in this case, environments with low levels of oxygen, said Rasmus Nielsen, a professor of integrative biology at the University of California, Berkeley.
"We can't really make experiments in humans, where we expose people to new conditions and have controlled genetic experiments in the same way we can do in fruit flies and mice," Nielsen said.
"But nature has made experiments for us that tell us how humans react and adapt genetically to a whole new set of physiological conditions, so that we can explore and learn much more about the interaction between genetics and physiology."
The surprise finding led researchers from the University of Copenhagen and UC Berkeley to a genetic mutation that appears to have spread throughout the population to increase spleen size.
This genetic variant upregulates thyroid hormone, which in mice has been linked to larger spleen size.
The Bajau people, groups of whom spread among the islands of Indonesia, Malaysia and the Philippines, are often called sea nomads. Some have been shown to spend as much as 60 percent of their work day diving for food at depths greater than 70 meters, using only a wooden mask.
Previously, Nielsen showed that Tibetans adapted to living at oxygen-poor high altitudes by selecting for a specific genetic mutation that prevented chronic high red blood cell levels, which can cause medical problems.
The researchers measured the spleen size in 59 Bajau in addition to 34 people from a neighboring village, Koyoan, inhabited by the unrelated Saluan people, who do not dive.
They found that, on average, the Bajau have spleens about 50 percent larger than those of Saluans.
According to Nielsen, this means they could mobilize perhaps 10 percent more red blood cells during a dive than those not adapted to the low-oxygen conditions of breath-hold diving.
There was no difference in spleen size between Bajau who dived and Bajau who did not dive.
"The Bajau have a different solution to low-oxygen conditions than the Tibetans," Nielsen said.
"While it is unhealthy to have high concentrations of red blood cells all the time, it is really good for you if you have high red blood cells when you really need them. They have increased the storage capacity in the spleen for when they need it, but they don't have any negative effects of constantly having too high red blood cells."
A second genetic adaptation associated with the Indonesian population of Bajau involves the constriction of blood vessels in the extremities to preserve oxygen for vital organs, a key part of the human "diving reflex."
Contraction of the spleen to release oxygenated red blood cells into the circulation is a key part of humans' and all mammals' innate diving reflex, which is initiated when you hold your breath and at the same time immerse your face in cold water.
"We all know that if you dive down into cold water, you feel refreshed," Nielsen said. "That is not just in your brain, it's in your body. In addition to contraction of the spleen, you also have bradycardia, a lowering of the heart rate, and the blood is shunted into the central vital organs and away from the skin."
The researchers further collected spit samples from the Bajau villagers and identified several gene variants that were at much higher frequency in the genomes of the Bajau versus the Saluans.
One, PDE10A, codes for an enzyme called phosphodiesterase, which mediates contraction of smooth muscle but is also associated with thyroid hormone release. The researchers collaborated with a group in the Netherlands, which confirmed that higher PDE10A levels are associated with an increased release of thyroid hormone.
"We think the way it works is that the expression of this variant gene changes thyroid hormone release, which then has an effect on spleen size," Nielsen said. "Nothing is really known about the genetic basis of spleen size in humans, so it is hard to validate without further research."