The first evidence of a genetic adaptation of the human being to deep diving, namely the exceptional development of the Bajau people’s spleen in Indonesia, was published in the journal Cell on Thursday 19 April 2018.
The contraction of the spleen makes it possible to reoxygenate the blood
The human body, like that of other diving mammals, responds to apnea and facial immersion in cold water by different physiological effects. 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. Finally, the contraction of the spleen (or spleno- contraction) makes it possible to inject a surplus of oxygenated red blood cells into the circulation. A single contraction of the spleen expels approximately 160 mL of red blood cells, causing an increase of 2.8% to 9.6% in the oxygen content of the blood, according to the authors of the publication.
The Bajau’s spleen is 50% larger than that of the neighboring non-diving people
Researchers have shown in 1990 that the spleen of Japanese divers contracted more than normal, which increased the oxygen level in the blood up to 9% during dives.
Intrigued by the work on genes that had helped Tibetans, Ethiopians and South Americans adapt to chronic high altitude oxygen deprivation, Melissa Ilardo, now a postdoc at the University of Utah in Salt Lake City, proposed to her colleagues to do similar studies on the Indonesian Bajau people.
Nicknamed the “nomads of the sea”, the Bajau people fish down to 70 meters deep with only weight equipment and a wooden mask. They spend up to 60% of their 8 hour workday diving for fish, octopus and other crustaceans.
The goal of Melissa Ilardo was to check if the size of their spleen was correlated with their breathing capacity, and to check if it was a genetic adaptation.
She spent several months in Indonesia with the Bajau and another people who do not dive, the Saluan. By echographing 59 Bajau and 34 Saluan, she showed that the Bajau’s spleen, whether they were divers or not, was about 50% larger than that of the Saluan! By comparing the Bajau genome to two different populations (Saluan and Chinese Han), scientists found 25 genomic sites with significant differences. One of them is the PDE10A gene, which affects thyroid activity in mice, which in turn affects the size of the spleen. In humans, these thyroid hormones regulate the production of red blood cells in the newborn, explain the authors of the publication. The width of the Bajau’s spleen thus indicate a higher volume of red blood cells.
Their genetics could thus provide a “double benefit”: both “an increased amount of oxygenated cells” and “a larger reservoir in which to store them,” the researchers write.
Is evolution at work?
The major piece of information to be learned from this work “is that natural selection is still at work on human populations,” says Mark Aldenderfer, an archaeologist at the University of California at Merced who was not involved in the work.
But not everyone is convinced by these works. Thus, according to anthropologist Cynthia Beall of Case Western Reserve University in Cleveland, although the Bajau dive more frequently than most people, each of these dives is not necessarily very long. In addition, she adds, a larger spleen does not necessarily mean greater expulsion of red blood cells. Finally, Edward Gilbert-Kawai, physician-physiologist at the University of London, also notes that “it is highly unlikely that the size of the spleen is controlled by a single gene”. Further research is needed to determine how the PDE10A gene affects human spleen size and Bajau oxygenation.
If true, this discovery could accelerate medical research on how the body responds to oxygen deficiency under different circumstances, such as diving but also altitude, surgery or lung disease.