A life aquatic
For over a thousand years, the Sama-Bajau people have lived a nomadic life on the waters of Southeast Asia. Their communities are traditionally built on houseboats, called lepa-lepa, or in stilt villages over shallow seas. This maritime existence means their diet consists almost entirely of what they can harvest from the ocean. They are expert free-divers, spending as much as 60% of their working day underwater, hunting fish and gathering crustaceans with handmade wooden goggles and spearguns.
This constant interaction with the marine environment has driven remarkable human adaptations. The pressures of a life spent diving for sustenance have shaped Bajau physiology in ways not seen in land-dwelling populations. They can dive to depths of 70 meters (230 feet) with no modern equipment. Some divers have been anecdotally reported to hold their breath for as long as 13 minutes, a feat that mystified scientists for centuries. The explanation for these abilities shows natural selection acting on a human population.
Evolution in action
Scientific studies have revealed that the Bajau possess unique genetic traits directly linked to their diving prowess. The main adaptation is the size of their spleens, which are, on average, 50% larger than those of their land-dwelling neighbors, the Saluan people. The spleen acts as a biological scuba tank by storing oxygenated red blood cells. During a dive, the spleen contracts, releasing this oxygen-rich blood into the circulatory system. This adaptation is linked to a specific genetic variant in a gene known as PDE10A, which influences thyroid hormone levels and, consequently, spleen size. The trait is genetic, not a result of training, as even non-diving Bajau individuals have enlarged spleens.
Other genetic adaptations contribute to their underwater abilities. The Bajau have variants of the BDKRB2 gene, which is related to the peripheral vasoconstriction of blood vessels, an important part of the human diving reflex that preserves oxygen for important organs. They also possess unusually keen underwater eyesight. This was once thought to be a genetic trait, studies suggest it is a plastic response to a lifetime of training from a young age. Many Bajau children intentionally rupture their eardrums to make deep-water diving more comfortable and to avoid pain from pressure changes. These combined factors allow them to spend hours a day hunting beneath the waves.
Today, this traditional way of life is under threat. Government settlement programs, overfishing by industrial vessels, and climate change are forcing many Bajau to abandon their nomadic traditions.