The Bajau spleen is 50% larger, whether they dive or not

Fifty per cent larger. That is the difference the geneticist Melissa Ilardo found in 2015 between the spleens of the Bajau — a people of fishermen on the island of Sulawesi, in Indonesia, who have lived aboard their boats for generations — and those of the Saluan, their inland neighbours. And here is the detail that turns a measurement into a discovery: within the Bajau themselves, the spleen comes out just as large in someone who dives every day as in someone who has never gone into the water at all.

The spleen is a fist-sized organ tucked under the left ribs which, among other things, works as a storeroom: it holds red blood cells loaded with oxygen. When you hold your breath it contracts and releases that reserve into the bloodstream. In an ordinary person that lifts the haematocrit — the proportion of red cells in the blood — by up to 9%. An emergency oxygen tank that opens by itself.

The Bajau have lived off that for generations. They free-dive with wooden goggles and stone weights, going down tens of metres to spear fish and gather sea cucumbers, and they spend up to 60% of their working day underwater: on a long day, five hours of immersion all told.

The obvious explanation was training. Just as running enlarges the heart, diving would enlarge the spleen. Ilardo, then a doctoral student, travelled three times to Jaya Bakti, a Bajau village in central Sulawesi, with a portable scanner, and measured the spleen of everyone who would let her.

They came out the same.

A large spleen is not built by the job: it comes as standard. The man who minds the shop, the one who works wood and the one who never gets his feet wet have the same outsized spleen as the best spear fisherman in the village. The Saluan live a few kilometres away and share the same market, the same climate and the same food.

When the team went looking for the trace in the DNA, a suspect turned up: a variant of the PDE10A gene, far more common among the Bajau than among their neighbours and associated with spleen size. In mice, that gene is involved in regulating thyroid hormone, which in turn controls how much the organ grows. BDKRB2 also appeared, linked to peripheral vasoconstriction: the reflex that, when you go under, shuts off the blood supply to arms and legs so as to keep the blood where it is needed.

The work was published on 19 April 2018 in Cell, one of the heavyweight journals in biology and was the first documented case of a human population genetically adapted to diving. The textbook examples until then were altitude in Tibetans and milk in adulthood across half of Europe. Neither was about holding your breath.

And the Bajau are giving up diving. They have spent a long millennium aboard, but their boats stay tied up at the quay more and more: the region's governments have steadily settled them on land, and in many places compressor fishing — air pumped down through a hose from the surface — has replaced the lung. The organ Ilardo had just measured is running out of work.

In Jaya Bakti, meanwhile, the spleen still comes out 50% larger even in someone who has never dived.