The Bajau: Genetic Proof That Humans Are Built for the Sea

A Bajau child of eight descends to twenty metres without equipment, spears a fish, and surfaces. No training programme. No AIDA certification. No carbon fibre fins. Just a body shaped, over a thousand years of selection pressure, to be at home in the sea — and science has now confirmed exactly how that shaping happened.

The 2018 publication of a landmark study in Cell gave evolutionary biologists and freediving researchers something they had long suspected but never had the tools to confirm: proof that humans can evolve specific genetic adaptations to underwater life, and that those adaptations are not simply the product of a lifetime of training. They're written into the genome. They're heritable. And they reveal something profound about the relationship between the human body and the ocean.

Who Are the Bajau

The Bajau — also written Bajaw, and sometimes called the Sea Gypsies — are a maritime people inhabiting the coastal waters of the Philippines, Malaysia, and Indonesia. They're believed to have been seafaring nomads for at least one thousand years, and possibly much longer. For much of their history, the Bajau didn't live on land at all; they were born on boats, raised on boats, and spent the entirety of their lives on the water.

Their relationship with the sea is not recreational. It's vocational and nutritional. Traditional Bajau communities subsist primarily on fish, shellfish, and sea cucumbers harvested by hand from the ocean floor. This means diving — every day, for hours, to depths that would leave most trained recreational divers gasping. Estimates from field observations suggest that some Bajau fishermen spend up to sixty percent of their working hours underwater, with diving sessions extending through a full working day at depths of twenty to forty metres.

The tools are minimal by any standard. A wooden mask. A wooden spear powered by rubber bands. No wetsuit, no weight belt, no fins. Just breath, buoyancy, and a body adapted to the task.

The Bajau are not unique among Southeast Asian peoples in their diving traditions. The Moken of Thailand and Myanmar, the Orang Laut of Malaysia, and related groups throughout the archipelago maintain similar maritime cultures. But it's the Bajau who became the subject of the research that changed what we know about human evolution and the sea.

The 2018 Cell Study: A Scientific Breakthrough

In 2018, a team led by Melissa Ilardo at the University of Copenhagen published a study in Cell that became immediately landmark in both genetics and human physiology. The paper, titled "Physiological and Genetic Adaptations to Diving in Sea Nomads," set out to answer a question that had been asked informally for decades: do the Bajau show diving-related physiological differences that are genetic rather than purely training-induced?

The answer was unambiguous. And the mechanism was surprising.

Ilardo and her team travelled to Sulawesi, Indonesia, and measured spleen size in both Bajau and their nearest genetic neighbours — the Saluan people, who are farmers living in the same region with similar ancestry but no diving tradition. Using portable ultrasound equipment in field conditions, they measured spleen volume in dozens of participants from both groups.

The Bajau spleens were, on average, 50% larger than those of the Saluan. This was not a subtle difference at the margin of measurement error. It was a dramatic, clear morphological difference between two genetically related populations — one that dives daily, and one that doesn't (PubMed 29677510).

This was the first documented case in the scientific literature of genetic selection for a diving-related physical trait in humans. And it opened a broader question: what is the spleen actually doing, and why would natural selection favour a larger one in divers?

The Spleen, Oxygen, and the PDE10A Gene

The spleen is not, in popular understanding, a glamorous organ. It's often treated as a physiological spare — something that can be removed without catastrophic consequence. But in the context of breath-hold diving, it plays a critical and irreplaceable role.

During a dive, the mammalian spleen contracts. This contraction — driven by the dive reflex — expels stored red blood cells into the bloodstream, temporarily increasing circulating haemoglobin by eight to ten percent. More haemoglobin means more oxygen available to the brain and muscles during the breath-hold. In a diver with a large, blood-rich spleen, this single mechanism can meaningfully extend time at depth, improve cognitive clarity near the limits of a dive, and delay the onset of hypoxic symptoms.

The Bajau spleen is not just larger because Bajau individuals dive more. Critically, Ilardo et al. found that even Bajau individuals who reported little or no diving activity had spleens significantly larger than the Saluan average. The enlargement is not a training effect — it's a baseline genetic trait.

The genetic mechanism involves a variant in the PDE10A gene — a gene that regulates thyroid hormone levels. The Bajau variant of PDE10A is associated with elevated thyroid hormone concentration, which in turn drives a larger baseline spleen. The connection is physiological: thyroid hormone influences spleen development and the organ's capacity to store and release red blood cells.

In simple terms: a Bajau diver begins every dive with a biological advantage. Their spleen, already larger than average at rest, contracts and releases a larger absolute quantity of stored red blood cells into circulation. Each dive is thus executed with a higher initial oxygen load than an equivalent diver with a smaller spleen — a diver who has not been selected for this trait over a millennium of underwater foraging.

The PDE10A finding was the first identification of a gene specifically associated with a diving-related trait in a human population. It's a rare and unusually clear example of recent, population-specific natural selection acting on a physiological trait directly relevant to a livelihood strategy.

What This Means: Humans Evolved for the Water

The Bajau genome finding sits within a broader scientific discussion about the relationship between human evolution and aquatic environments. For decades, the hypothesis that early hominids spent significant time in or near the water — the so-called "aquatic ape theory" in its various forms — has generated both interest and controversy. The genetic evidence from the Bajau doesn't resolve that broader debate, but it makes one thing clear: when human populations are placed under sustained selection pressure related to aquatic activity, evolution responds.

The coastal African populations from whom all modern humans descend were, according to the current archaeological consensus, intensively exploiting marine food sources for at least 165,000 years before the out-of-Africa dispersals. Shell middens — ancient rubbish heaps of shellfish remains — at sites like Pinnacle Point and Blombos Cave in South Africa demonstrate a long human relationship with the intertidal zone. If those populations also dove for shellfish — as their descendants in similar environments do today — then some of the genetic architecture for aquatic adaptation may be far older than the Bajau-specific PDE10A variant alone.

The implication for all modern humans is this: the diving reflex, the spleen contraction response, the cardiovascular efficiency of breath-hold — these are not vestigial anomalies. They're maintained, functional adaptations that most people simply never activate. The Bajau have pushed those adaptations further through selection. But the underlying machinery is universal.

The Ama Divers of Korea and Japan

The Bajau are the most genetically studied diving population, but they're not the only one with a deep tradition of aquatic subsistence. The Ama — a word meaning "sea women" in Japanese — are a group of traditional female breath-hold divers with a documented history spanning at least 1,500 years in coastal Korea and Japan.

The Ama dive without modern equipment (though contemporary Ama typically wear wetsuits and use modern masks), to depths of twenty to thirty metres, for five to eight hours per day during the summer diving season. They harvest abalone, sea urchin, seaweed, and oysters. At their peak in the mid-twentieth century, tens of thousands of Ama worked Korea's southern coastlines. Today the tradition is declining as the older generation retires, but it persists and is recognised as a UNESCO cultural heritage practice.

Research by Schagatay and colleagues (2005) examined spleen characteristics in Ama divers and found the same pattern seen in the Bajau: the Ama showed significantly enlarged spleens compared to non-diving age-matched controls, along with enhanced splenic contractility — a greater proportional release of red blood cells per dive than non-divers (PubMed 16004372). The convergence of this finding across two genetically distinct populations — Southeast Asian Bajau and East Asian Ama — is not coincidental. It reflects the same selection pressure acting on similar physiology through similar mechanisms.

The Ama findings also matter for a reason that's directly relevant to recreational freedivers: the Ama splenic adaptation is not purely genetic. Training contributes. Young Ama women who begin diving in their teens show progressive increases in spleen size over the following years of practice. The genetic baseline matters, but the training effect is real and measurable — and available to anyone who commits to consistent breath-hold practice.

Can Training Give You a Bajau-Level Spleen?

This is the question most recreational freedivers ask when they learn about the Bajau study, and the answer is nuanced but encouraging.

You can't acquire the PDE10A variant through training. The genetic basis of the Bajau's baseline spleen size advantage is real and non-trainable. But the spleen is a dynamic organ, and it responds to training stimulus in ways that are now well documented.

Research in trained breath-hold athletes consistently shows spleen volumes significantly above those of untrained controls — on the order of twenty to forty percent larger after sustained training periods. This is not at the scale of the Bajau's genetic fifty percent advantage, but it's a meaningful and functional adaptation. It means that a recreational freediver who trains consistently over months develops a measurably larger oxygen reservoir than an otherwise equivalent untrained person.

The mechanism works the same way whether the enlargement is genetic or training-induced. A larger spleen, regardless of its origin, contracts during a dive and releases more red blood cells. The oxygen advantage scales with spleen size. The math is direct: approximately fifty percent larger spleen produces roughly ten percent more circulating haemoglobin per dive, which at typical recreational freediving depths translates to fifteen to thirty additional seconds of comfortable bottom time before hypoxic discomfort sets in.

Follow-up research by Stromberg and colleagues (2019) in the Journal of Physiology confirmed the genetic basis of the Bajau advantage specifically, while also establishing that training-induced adaptations in non-Bajau populations are real and accumulate progressively with training volume. The Bajau start with an advantage, but everyone who trains is moving in the same direction.

The Oxygen Advantage: A Numbers Breakdown

To make the Bajau physiological advantage concrete, consider the following estimate grounded in published physiology.

A Bajau diver with a spleen fifty percent larger than the population average might have a baseline spleen volume of approximately 500–600 mL, compared to the average non-diving adult's 200–250 mL. Upon dive-reflex activation, this spleen contracts by approximately forty percent of its volume, releasing 200–240 mL of packed red blood cells into the bloodstream.

A non-diving adult releasing from a 200 mL spleen contributes approximately 80 mL of stored cells per dive. The difference — 120 to 160 mL of additional red blood cells circulating per dive — represents a haemoglobin advantage of roughly eight to twelve percent above the non-diver's in-dive haemoglobin concentration.

Haemoglobin binds approximately 1.34 mL of oxygen per gram. A ten percent increase in circulating haemoglobin therefore translates to roughly ten percent more total oxygen available per dive. At ten metres of depth, where partial pressure of oxygen is approximately doubled relative to the surface, this advantage is somewhat compressed. But at thirty metres — where Bajau fishermen routinely work — the additional oxygen margin becomes the difference between a functional working dive and a marginal survival dive.

The recreational implication is clear. Training-induced spleen adaptation of twenty to forty percent, while smaller than the Bajau genetic baseline, still shifts a diver meaningfully along this spectrum. The oxygen advantage is not binary — not "Bajau genetics or nothing." It's a continuum, and consistent training moves any diver up that continuum regardless of their genetic starting point.

Freediving Heritage in Southeast Asian Waters

The Bajau's story is Southeast Asian at its core — the Philippines, Malaysia, Indonesia, and the broader maritime region where island-hopping and ocean exploitation have shaped human cultures for millennia. Thailand participates in this tradition, though its diving heritage is less formally documented than that of the Bajau or Ama.

Traditional communities of sea gypsies — known in Thailand as Moken or Urak Lawoi — have inhabited the Andaman coast for at least several hundred years, diving for shellfish, sea cucumbers, and fish using breath-hold techniques passed between generations. The Surin Islands, Similan Islands, and the Mergui Archipelago were their territory long before they became dive tourism destinations. Some communities still maintain traditional practices alongside modern fishing, and their children — like Bajau children — grow up in the water with an ease that recreational freedivers spend years trying to cultivate.

Phuket and the surrounding Andaman Sea represent, in this sense, not just a scenic diving destination but a place with living roots in exactly the tradition the Bajau and Ama represent. The underwater world here — the coral systems of Racha Yai, the deep walls of Koh Dok Mai, the channels and currents of the outer islands — is the same environment that sustained maritime peoples for thousands of years. Diving in these waters connects a recreational freediver to something older and more fundamental than a sport.

At ORO Freediving, we operate in this environment daily. The courses we run are modern in their pedagogy and safety standards, but the ocean they take place in carries that longer history. Understanding the Bajau findings makes freediving richer — it transforms breath-hold training from a recreational activity into participation in a physiological heritage that's, in some sense, shared by all humans who have ever lived near the sea.

What Bajau Science Means for Your Training

The practical takeaway from the Bajau research is more motivating than it might initially appear. The headline — "they have a genetic advantage" — could be read as discouraging. The more accurate reading is the opposite.

The Bajau study proves that the human body responds to diving selection pressure by developing specific, measurable physiological advantages. It's not magic. It's biological engineering. And the same engineering process — slower in the absence of the PDE10A variant, but real and progressive — operates in any human who trains consistently in breath-hold disciplines.

Your spleen will grow. Not to Bajau proportions in six months, but measurably, functionally, and in direct proportion to your training volume. Your dive reflex will strengthen. Your resting heart rate will fall. Your RMSSD will rise. Your CO2 tolerance will expand. These are not metaphors for improvement — they're the same categories of measurable, physical adaptation that the Bajau exemplify in their extreme genetic form.

The human body is not indifferent to the sea. It knows what to do when you go there consistently. The Bajau have been going there for a thousand years and their genomes carry the record of what happened. Your training log, if you build one, will carry a smaller but real version of the same story.

Start Your Own Adaptation

The most practical step a person can take after learning about the Bajau is to start building their own breath-hold training history. The adaptations are available. They're not gated behind genetics that you either have or don't.

If you're new to freediving, the Try Freediving session at ORO provides the safest and most effective introduction: proper breathe-up mechanics, the experience of your first comfortable breath-hold, and the foundational safety protocols that make further training both safe and rewarding. It's the session after which most people think, for the first time, "I can actually do this."

For those ready to build the complete physiological foundation — spleen adaptation, CO2 tolerance, vagal tone, depth skills — the Wave 1 course is a structured two-day progression that takes beginners from first static apnea to open-water dives in the Andaman Sea. The ocean around Phuket — warm, clear, and extraordinarily rich in marine life — is one of the best environments in the world to begin this process.

The Bajau have already demonstrated that humans are built for this. The question is only how far you want to take it. Get in touch to find out what's possible for you.

Summary

The 2018 Ilardo et al. study in Cell (PubMed 29677510) established something that had long been suspected but never proven: humans evolve specific genetic adaptations to aquatic lifestyles, and those adaptations are both measurable and functional. The Bajau's fifty percent spleen size advantage, mediated by the PDE10A gene and its effect on thyroid hormone, gives them a per-dive oxygen reserve that translates directly into working depth and time. The Ama divers of Korea and Japan show the same pattern through a different genetic and cultural route, confirming that the adaptation is not unique to a single population but available wherever humans sustain multi-generational diving practice.

For recreational freedivers, the finding is not a ceiling but a compass. Training moves everyone in the direction of the Bajau, building spleen volume, strengthening the dive reflex, improving HRV, and expanding the body's comfort in oxygen-limited environments. The genetic head start matters. But the direction of travel is the same for everyone who enters the water with intention and consistency.

Dive more. The biology follows.