 The Bajo of Southeast Asia, sometimes called synomads, are an extraordinary group of people. For thousands of years, they have existed as marine hunter-gatherers making their living and surviving entirely off the sea. They survive in part through breath hold diving, and they are exceptionally good at this. It was entirely unknown whether the synomads' abilities had a genetic basis. Using ultrasound measurements in a comparative hold genome study, we demonstrated that natural selection has acted on the Bajo's diving abilities through a number of traits related to the human dive response. One component of the human dive response is contraction of the spleen. This pushes a supply of oxygenated red blood cells into the system, providing an oxygen boost. Seals with high dive capacity, like the Waddell seal, have disproportionately large spleens. We found that the Bajo, both divers and non-divers, have significantly larger spleens than their close geographic and genetic neighbors, the Salawan. Through a selection scan, we identified a genetic variant under selection in the Bajo in the gene PDE10A that is associated with spleen size, which we confirmed in a European cohort. PDE10A is involved in the regulation of thyroid hormone levels. In mice, it has been shown that low levels of the thyroid hormone T4 decrease spleen size, in a fact which is reversible through an injection of T4. So it seems that the large spleens of the Bajo might be the result of increased thyroid hormone levels. We found a number of other candidates under selection in the Bajo with potential relevance for diving, including in the gene BDK-RB2. This gene is associated with another component of the human dive response, peripheral basal constriction. A decrease in the size of the blood vessels in your extremities helps to preserve oxygen for your vital organs. Our results indicate that the Bajo and likely other diving populations represent an entirely new system for studying hypoxia.