Japanese have an easy time digesting sushi and other seaweed-wrapped delicacies thanks in part to an intestinal bacterium that hijacked genes from a marine germ, say scientists.
The surprising discovery, published in journal Nature, sheds further light on the trillions of friendly germs that inhabit our gut.
By breaking down starchy plant fibres, these bacteria get their share of nutrition - as do we.
Researchers, led by Dr Mirjam Czjzek of the Station Biologique de Roscoff in France, studied a marine bacterium, Zobellia galactanivorans and found it had an interesting new class of enzyme that busts open a starch molecule called porphyran, found in red seaweed of the Porphyra species.
Scanning publicly-available databases of DNA codes, the team were astonished to find genes coding for the same enzyme in an intestinal bacterium, Bacteroides plebeius, that so far has only ever been noted in the faeces of Japanese.
The scientists believe that B. plebius picked up the genes from a fellow bacterium that lived on seaweed.
In a classic example of Darwinian selection, strains of B. plebius that had the imported genes had a better chance of survival than others, as they could feast on a major component of the Japanese diet.
Gnawing at nori
The Japanese eat around 14 grams of seaweed each day, and Porphyra, known as nori and used in sushi, is the most important food alga of all.
Recorded evidence of the central role of seaweed in the Japanese diet goes back to the eighth century, when tax archives showed that people could use the commodity as a means of settling their bill with the government.
The human gut is alive with bacteria that supply the body with energy by breaking down plant starch through carbohydrate active enzymes, or CAZymes, that the body cannot produce by itself.
"Traditionally, nori is not roasted, and thus contact with associated marine microbes is promoted through Japanese sushi," says the paper.
"Consequently, the consumption of food with associated environmental bacteria is the most likely mechanism that prompted this CAZyme update into the human gut microbe."