Evolution is such a slow process that it can be easy to be fooled into thinking it's not still happening. But certain populations of fish and geckos have adapted to changing environments in a span of mere years, and a recent, large genetic study has shown that humans are gradually weeding out harmful genes. Now, researchers have discovered one of the most striking examples of natural selection at work in humans: a seafaring population of people with a mutation that allows them to dive deeper and hold their breath longer.

The Sama-Bajau (or Bajau) people live in parts of Southeast Asia, and are known to spend most of their lives at sea. They often live in flotillas of house-boats – earning them the nickname "Sea Nomads" – and their diet is mostly made up of fish and shellfish caught by free-diving. In doing so, the Bajau have been observed diving to depths of over 70 m (230 ft), just holding their breath.

The researchers on the current study, led by Melissa Ilardo, set out to determine if this ability was a learned skill or a product of natural selection, acting on the Bajau genes after 1,000 years of this lifestyle.

"Humans are pretty plastic beings," says Ilardo, first author of the study. "We can adapt to a number of different extreme environments just through our lifestyle changes or our behavioral changes, so it wasn't necessarily likely that we would find an actual genetic adaptation to diving. The first sign that we were maybe onto something was when we saw that both the Bajau divers and non-divers had larger spleens than the Saluan, a nearby, non-diving population."

The researchers tested 59 Bajau and 34 Saluan people, measuring the size of their spleens with a portable ultrasound machine and collecting DNA samples from their saliva. After accounting for factors like age, height and sex, the team found that the Bajau people had larger spleens on average.

Why the focus on spleens? The organ has been found to play a key role in the human dive response. When we submerge our faces in water and hold our breath, the body responds by trying to make our oxygen reserves last as long as possible, by slowing our heart rate, constricting blood vessels in the extremities and squeezing the spleen. This last step releases more oxygenated red blood cells into the body, boosting how long a person can hold their breath.

Having bigger spleens helps the Bajau people stay underwater for longer, and interestingly, the researchers found that the organ was enlarged whether the individual regularly dives or not. That shows that it isn't just a bodily response to a life of diving, but has a genetic root in the population.

The team then compared the Bajau genome to two comparison populations, the Saluan and the Han Chinese. Among the 25 sites where the Bajau differed significantly from the others, the researchers found one particular mutation on a gene called PDE10A, which has been linked to spleen size in mice.

"The chance of finding evidence of population-specific natural selection, even in a population as extreme as the Bajau, was pretty slim," says Ilardo. "It was very exciting to find, and it just opens up so many possibilities. I think it's fascinating to see just how extraordinary this population is, to think that they're almost like superhumans living among us with these really extraordinary capabilities. But I also think natural selection is a lot more powerful than we sometimes give it credit for, and maybe we should be looking for it in more places than we thought."

This isn't the first time a genetically-isolated human population has been found to have beneficial mutations. In November, a team from Northwestern University found a helpful anti-aging mutation in an Amish community.

The researchers believe the Bajau study could help improve our understanding of what happens to the body when it's deprived of oxygen, which may lead to better treatments for hypoxia.

The research was published in the journal Cell.