Imagine a muscle you have never consciously engaged, capable of growing by half simply because your ancestors used to dive to find food. That is precisely what happened to the Bajau, the sea-nomad communities of Indonesia. Their spleen, that quiet organ tucked beneath the left ribs, literally changed size under the pressure of evolution.
- The Bajau, sea-nomads who have dived for air for generations, possess a spleen that is on average 50% larger than that of their non-diving neighbors, the Saluan.
- This enlarged spleen allows more oxygen-rich red blood cells to be released during apnea, and its size does not depend on individual diving practice but on a hereditary genetic adaptation linked to the PDE10A gene.
- A second gene, BDKRB2, associated with the diving reflex (slowing the heart rate), has also undergone strong natural selection in this people.
- A people who breathe underwater like no one else
- A spleen 50% larger: evolution’s strange gift
- PDE10A, the gene that rewrote the Bajau anatomy
A People Who Breathe Underwater Like No One Else
The Bajau have lived for centuries on or near the water. Nestled in the Indonesian archipelago, these communities spend a large part of their lives aboard boats or in stilt houses, and especially beneath the surface. Their daily life consists of freediving for fish, sometimes beyond ten meters, without sophisticated bottles or masks—just wooden goggles and a spear.
This lifestyle is not a mere quirk. It entails repeated dives, day after day, generation after generation, for hundreds of years. An intense training regime, certainly, but not enough to fully account for everything researchers have uncovered about this people.
That is what intrigued Melissa Ilardo when she was a doctoral student at the University of Copenhagen. With her team, she aimed to understand whether the Bajau body had biologically adapted to this underwater life, not merely shaped by habit or repeated practice.
A Spleen 50% Larger: Evolution’s Strange Gift
To find a definite answer, Melissa Ilardo’s team conducted a study published in 2018 in the journal Cell. They examined 59 Bajau using a portable ultrasound and compared the results with 34 residents of a neighboring village, Koyoan, inhabited by the Saluan, a population that does not practice diving.
The findings were striking. Once close relatives were excluded from the analysis to avoid familial bias, the Bajau spleens turned out to be on average 50% larger than those of the Saluan. A substantial difference for an organ not typically linked to physical performance.
The spleen, however, plays a crucial role in apnea. It acts as a reservoir of red blood cells, which it releases into the circulation during immersion. According to Rasmus Nielsen, co-director of the study and a researcher at UC Berkeley, this contraction could mobilize around 10% more red blood cells during a dive, providing a genuine oxygen boost during apnea.
Another troubling finding, and not the least: there was no difference in spleen size between Bajau who regularly dive and those who do not dive. In other words, individual training cannot explain everything. This trait appears to be carved in more deeply, in their genes.
PDE10A, the Gene That Rewrote the Bajau Anatomy
Examining the Bajau’s genetic heritage, Ilardo’s team identified a central actor: the PDE10A gene, active notably in the thyroid. This gene shows strong natural selection among this people, a signal that researchers regard as the hallmark of a real evolutionary adaptation rather than a statistical fluke.
The proposed mechanism rests on mouse studies where PDE10A regulates a thyroid hormone that directly controls spleen size. If this link holds in humans, it would explain why the Bajau possess this abnormal organ from birth, independent of their apnea practice.
The study also detected strong selection on a second gene, BDKRB2, linked to the diving reflex. This reflex encompasses several automatic bodily responses to cold water, such as a slowed heart rate, which helps conserve the available oxygen.
However, not everyone agrees. Anthropologist Cynthia Beall of Case Western Reserve University expressed reservations about interpreting the results. She notes that the Bajau dive more often than their neighbors but not necessarily longer on each dive, which complicates the precise assessment of the evolutionary pressure at work.
To push the inquiry further, Melissa Ilardo and her coauthors published in 2022 a new study in Frontiers in Physiology. This time, they tested a selective PDE10A inhibitor in mice, called MP-10. The result: an increase in spleen size and red blood cell count via a mechanism independent of erythropoietin, the hormone typically responsible for producing these blood cells. A further clue that reinforces the initial hypothesis, though it does not conclusively settle the debate.
What the Bajau story ultimately reveals is humanity’s capacity to adapt to extreme environments in just a few hundred years, a fleeting nod in terms of evolutionary time. Their enlarged spleen is not a mere anatomical accident but the result of a precise selective pressure written into the DNA of this sea-dwelling people. A reminder that our bodies sometimes preserve the silent memory of our oldest ways of life.
