In the depths of the ocean, where there’s no sunlight and the pressure could crush a submarine, a handful of animals have quietly rewritten their DNA to withstand these conditions. According to a new study, they’ve all done it the exact same way.

A new study in Cell reveals that species living miles beneath the ocean’s surface have independently evolved the same genetic mutation. These species, which diverged on the tree of life millions of years ago, nonetheless share a change to a single gene — a rare and striking case of convergent evolution.

A Mutation Shared by Strangers

To uncover the secrets of life in the hadal zone — the ocean’s deepest region, from 6,000 to over 11,000 meters — researchers sent crewed submersibles and robotic vehicles into the Pacific and Indian Oceans. They collected 11 deep-sea species: snailfish, lizardfish, cusk-eels and more, living as deep as 7,700 meters (25,300 feet).

When scientists examined the fishes’ DNA, they were stunned by one thing. Although the lineages descended into the deep sea at different times — from the Cretaceous period to “only” a few million years ago — they had all independently acquired the same genetic mutation in a gene called Rtf1. To put it simply: this mutation helps the fish keep their genetic machinery humming smoothly, even under conditions that would normally cause serious errors in how their cells function.

Rtf1 controls how DNA is read and expressed. In fish living below 3,000 meters, the same mutation emerged not once, but at least nine times. It is, as evolutionary biologists call it, convergent evolution — nature arriving at the same answer through different routes.

Some fish groups, like the Neoteleostei, have flourished in this unforgiving environment. Seven of the eight deep-sea lineages below 3,000 meters belong to this group, suggesting an evolutionary advantage. By contrast, large clades like Otomorpha, which includes about 10,000 known species, are conspicuously absent from the hadal depths. Latecomers such as the hadal snailfish, now found in nearly every deep trench on Earth, only diverged from its shallow-water relatives about 20 million years ago.

An Alien World in Our Backyard

There are likely many more oddities that we don’t know yet, as the hadal zone is Earth’s least explored ecosystem. Named after Hades, the Greek god of the underworld, this realm sees pressures over a thousand times greater than at the surface. Temperatures hover near freezing. There is no light. And yet, life thrives even here.

Last year, scientists filmed a snailfish gliding at a staggering depth of 8,336 meters — a new world record. Like others of its kind, the translucent creature survives without a swim bladder, the organ that helps surface fish stay buoyant. At these depths, gas-filled cavities would implode. Instead, hadal fish rely on soft, gelatinous bodies and specialized molecules called piezolytes, to keep their cells from being crushed.

“It’s like putting the stakes up in a tent,” deep-sea biologist Tim Shank of the Woods Hole Oceanographic Institution told BBC, describing how these molecules support fragile proteins under immense pressure.

Some species, like the Mexican cavefish, have evolved large red blood cells rich in hemoglobin to maximize oxygen intake in a low-oxygen environment. Others, like the Dumbo octopus abandoned ink sacs altogether, finding little use for them where predators are few.

In the Mariana Trench, researchers have found jellyfish with dual tentacle systems, shrimp-like supergiant amphipods with aluminum-infused exoskeletons, and violet sea cucumbers that float their entire lives, vacuuming nutrients from the water. Even single-celled giants called xenophyophores — each the size of a mango — drift through the darkness.

“It’s a kind of alien life,” said Victoria Heath, who chronicled these strange creatures for Geographical magazine. “But it’s here, on Earth, just out of reach.”

Lost genes are another defining trait for deep-sea fish. Some species, like the Ipnops, exhibit eye degeneration during development, forgoing sight in favor of other senses; after all, sight doesn’t matter too much when light is a scarce commodity anyway. Others retain expanded vision genes, suggesting they may migrate to higher regions where sunlight still penetrates.

The same diversity appears in olfaction: while some lineages have fewer smell-related genes, others—like deep-sea eels—have evolved rich arrays of olfactory receptors, likely aiding them in detecting scarce prey in the dark.

One group, the hadal snailfish, has even reshaped its own skin. The researchers found high levels of collagen and keratin-related proteins in its tissues, hinting at a gelatinous layer that may help the fish hover effortlessly in place, conserving precious energy in an environment where food is rare.

Even Down There, We’re Affecting Ecosystem

Yet not even the trench is safe from the human touch.

The study also found something else inside the trench wildlife: polychlorinated biphenyls (PCBs), toxic chemicals banned decades ago but still circulating the planet. These industrial pollutants, once used in electrical equipment, were found in the livers of hadal fish and in sediment cores taken from over 10,000 meters down.

These chemicals, long banned in many countries, now form part of the biological makeup of some of Earth’s most isolated organisms. It shows the global extent of human impact and could have significant consequences for deep-sea creatures. These pollutants may interfere with the very genetic adaptations that help deep-sea life survive.

The shared mutation in Rtf1 is more than just a genetic quirk. It’s a story of survival written into the genome of creatures that thrive without daylight and how sometimes, the same thing can be evolved more than once. At the same time, these studies highlight our unintended reach. Even here—seven miles beneath the waves—human pollution has an impact.