For generations, many Bajau communities in Southeast Asia have depended heavily on the sea for food, travel and work. Often described as “Sea Nomads,” some Bajau traditionally practice repeated breath-hold diving while fishing and gathering marine resources. Their diving ability has attracted the attention of scientists because certain experienced divers can spend long periods working in the water without conventional scuba equipment. Rather than relying on compressed-air tanks, they use breath-hold techniques developed through practice and tradition.
Researchers wanted to know whether this demanding lifestyle might also be associated with biological adaptations. A major study published in 2018 provided evidence that at least part of the answer may involve the spleen and genetic differences shaped by natural selection. The study was led by researcher Melissa Ilardo and examined Bajau individuals in Indonesia alongside members of a nearby population with a different lifestyle. Researchers used ultrasound measurements to compare spleen size and also collected genetic data.
They found that the Bajau participants had significantly larger spleens on average than individuals in the neighboring comparison group. Importantly, the difference was observed not only among active Bajau divers but also among Bajau participants who did not regularly dive. That finding suggested that training alone might not fully explain the difference. The researchers therefore investigated whether inherited genetic factors could be contributing to the larger average spleen size. The spleen is relevant to breath-hold diving because it can act as a reservoir for red blood cells.
During the human diving response, several physiological changes occur when a person holds their breath and is submerged. Heart rate can slow, blood vessels in the limbs can constrict, and the body prioritizes oxygen delivery to critical organs. The spleen can also contract during apnea, releasing additional red blood cells into circulation. Since red blood cells transport oxygen, this response can temporarily increase the amount of oxygen available in the bloodstream. A larger spleen could therefore potentially provide an advantage during repeated breath-hold dives, although it is only one part of a much more complicated physiological response.
Genetic analysis in the 2018 research identified evidence of natural selection in several regions of the Bajau genome. One of the strongest signals was associated with a genetic variant near PDE10A, a gene involved in biological pathways connected with thyroid hormone regulation. Previous animal research had linked thyroid hormones with spleen development, providing a possible mechanism through which genetic variation could influence spleen size. The researchers proposed that natural selection may have favored variants contributing to larger spleens in a population with a long history of breath-hold diving. This is very different from saying that scientists discovered a completely new or unique “mutant gene.” The evidence concerns differences in the frequencies of genetic variants already found within humans.
The research is especially interesting because the Bajau have maintained a strongly maritime lifestyle over many generations. University of Copenhagen research describing the work notes that Bajau and other Southeast Asian Sea Nomad populations have depended on marine resources for a very long period. Traditional diving can involve repeated underwater work throughout the day rather than a single prolonged dive. Experienced divers may descend considerable distances while collecting fish, shellfish and other marine resources. Researchers studying this lifestyle became interested in whether generations of exposure to repeated low-oxygen conditions could leave detectable evolutionary signatures. The 2018 results provided evidence that cultural practices and biological evolution may sometimes interact over long periods.
