The platypus is an anthology of weirdness. It has a leathery duck-like bill, a flattened tail and webbed feet. The males have a venomous claw on their hind feet, and the females lay eggs. And if you look inside a platypus, you’ll find another weird feature: its gullet connects directly to its intestines. There’s no sac in the middle that secrete powerful acids and digestive enzymes.

In other words, the platypus has no stomach.

The stomach, defined as an acid-producing part of the gut, first evolved around 450 million years ago, and it’s unique to back-boned animals (vertebrates). It allowed our ancestors to digest bigger proteins, since acidic environments deform these large molecules and boost the actions of enzymes that break them apart.

But over the last 200 years, scientists have shown that many vertebrates have lost their stomachs. The platypus doesn’t have one, nor do its closest relatives, the spiny echidnas. Lungfish, a group of slender freshwater fish that can breathe in air, don’t have stomachs; nor do the chimeras, bizarre-looking relatives of sharks and rays.

And the teleosts—the group that includes most living fishes—have taken stomach loss to extremes. Of the almost 30,000 species, it seems that around a quarter have abandoned their stomachs, including groups like wrasse, carp, cowfish, pufferfish, zebrafish and more. (It’s commonly said that pufferfish puff by expanding their stomachs, but while they have a sac in the right place, it’s not a glandular, acid-secreting one, so it doesn’t really count.)

On at least 18 separate occasions, vertebrates have abandoned their stomachs. And we now know that several of these losses were accompanied by disappearing genes.

Xose Puente from the University of Oviedo first discovered that the platypus has lost its main stomach genes, back in 2008. Now, Filipe Castro and Jonathan Wilson from the University of Porto have found the same pattern in other stomach-less vertebrates, like the zebrafish, pufferfish, medaka, platyfish, and Australian ghostshark.

They scoured the full genomes of these species and showed that they’re all missing the genes for the gastric proton pump—the enzyme that acidifies the stomach. They’ve also lost many of the genes for pepsinogens—digestive enzymes that break down proteins. The pufferfish was the sole exception—like the platypus, it has kept a single pepsinogen gene, which it uses for non-digestive purposes. “It’s a clear-cut pattern of gene loss and stomach loss across all of these species,” says Wilson.

View Images A family tree of vertebrates, showing those that do and don’t have stomachs.

It might seem intuitive that animals which lose a certain feature might lose the genes associated with that feature. But that’s not always the case.

Blind cavefish still have the right genes for making eyes, and if you cross-breed populations from different caves, you can actually make sighted individuals. Toothless mammals still have genes for making enamel—they just don’t work anymore. And birds also have tooth-making genes—relics from their dinosaur ancestors. “You can go to the chicken genome and find that most genes involved in the formation of the enamel are still there, just where you would expect to find them,” says Puente. They’ve been inactivated, but not lost. With the right genetic tweak, you can switch on these dormant programmes and produce chickens with teeth.

But in the case of the stomach-less species, “the genes are just gone,” says Puente. “No trace of them can be found.”

This means that the stomach-less species could only regain their lost organ by reinventing it from the ground-up—a feat that Castro and Wilson deem unlikely. This fits with Dollo’s principle, which says that complex traits that have been lost through evolution cannot be regained.

But why lose a stomach at all?

Castro and Wilson suspect that diet is part of the answer. We know that animals evolve very different sets of pepsinogen genes to cope with the proteins in their specific diets. Perhaps the ancestors of stomach-less species shifted to a different diet that made these enzymes worthless. Over time, they built up debilitating mutations, and were eventually lost.

You can see the first hints of this process at work in animals that still have stomachs. Many newborn mammals use a gene called Cym to digest proteins in their milk, but our version of Cym is inactive because our milk is relatively poor in proteins.

Pepsinogens work best in acidic environments, so if they disappear, you don’t need an acidic chamber any more. Gastric pumps need a good deal of energy to keep the stomach acidic, so if they are no longer needed, they would eventually be lost too.

This is all just speculation; here’s another plausible idea. Some animals eat lots of shellfish and corals, whose shells are rich in calcium carbonate—a substance that neutralises the acid in a stomach. Bottom-feeding fish like wrasses get similar mouthfuls when they suck up large quantities of muck. These species are all effectively gorging on antacids.

So, why bother acidifying your stomach if your food immediately undoes all that work? The gastric pumps are superfluous, so they are soon lost. And without an acidic environment, the pepsinogen genes are also useless, so they follow suit. “Diet most likely has a predominant role, but we’re still working out what that role is,” says Wilson. He notes that all the stomach-less species live in the water (or, like the echidna, had aquatic ancestors). “My gut feeling is that it’s something related to that,” he says.

For now, one thing is clear: many animals cope quite well without a stomach. There are many possible workarounds. The intestine has its own protein-busting enzymes. The throats of some fish have an extra set of teeth that help to break down what they swallow. “You can have a shift of function to other areas of the gut,” says Wilson. “Every which way you turn, there are species that do perfectly fine without a stomach. They aren’t aberrations; they’re quite common.”

PS: The lungfish in the image at the top has a little story. Wilson bought him from a pet store in the UK and posted him to his lab in Portugal. He got lost in the post, and it took him a week to arrive. “I was a little worried when I opened up the package,” says Wilson. “But he’s a lungfish, so he was fine.” For no particular reason, Wilson named him Horatio.