This Thai fish walks like a mammal. That may reveal secrets about our evolution.

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This tiny Thai "cave fish" could hold key information about how humans evolved.

This tiny Thai "cave fish" could hold key information about how humans evolved.

Daphne Soares/New Jersey Institute of Technology

It’s eyeless, translucent pink, and dwells in dark caves along the Thai-Myanmar border. And it might just help humanity understand how our aquatic ancestors emerged from the sea.

This species — colloquially called the “waterfall cave climbing fish” — is only an inch long. But attached to its tiny frame are four fins that function remarkably like legs.

How it uses those fins is astounding. While some fish such as mudskippers are said to “walk,” they’re really just dragging their bodies along by using two fins as crutches. Other species do it by sucking on a rocky surface and heaving their bodies forward.

But this one (cryptotora thamicola) straight-up walks, one fin in front of the other. Like a salamander, mouse or many other vertebrates with four legs.

And in doing so, it offers a glimpse into humanity’s past.

“Its pelvic bone almost looks like a mammal, somewhat close to a human being,” says Apinun Suvarnaraksha, a zoologist at Maejo University in Chiang Mai, Thailand. “Through this fish, it’s possible to better understand human evolution.”

Apinun didn’t discover the fish. That happened in the the mid-1980s, he says. But he and a team of scientists at the New Jersey Institute of Technology have revealed that its walking style is startlingly like that of a tetrapod, or four-footed animal.

The fish’s gait is, “from an evolutionary perspective, a huge finding,” according to fellow researchers Brooke Flammang and Daphne Soares. They say it “looks different from anything else we’ve ever seen” and provides insight into the “first limbs that evolved in our earliest ancestors.”

Scientists believe our evolutionary forebears first slinked out of the ocean more than 400 million years ago. It’s possible, Apinun says, that they walked like this fish: jerkily, but with four fins functioning as tiny legs. “It looks almost like a reptile crawling,” he says.

Scientists who puzzle over the “fin-to-limb transition” have mostly dealt with ancient, sterile clues. It’s essentially a cold case, relying on 400 million-year-old footprints left by long-extinct creatures.

This fish, however, offers a living, squiggly clue. Researchers will now scrutinize this modern-day creature to trace our evolution back to those first slithery steps out of the sea.

The tiny fish is only known to exist in a remote cave system along the Thai-Myanmar border. On the Thai side is Mae Hong Son, a mountainous area blessed with hot springs and spicy noodle dishes. On the other, an unruly zone controlled by drug-trafficking militias.

The fate of this species is tenuous, Apinun says. Its exact population is unknown but he suspects it numbers in the mere thousands. He is so fretful over its status that he didn’t remove the fish from its cave. Instead, he gingerly plucked two from a stream, recorded their gait on camera and returned them to the water. To study the species’ bone structure, he used dental equipment to scan a specimen already housed in a government museum.

And though the fish lives in a remote place — a three-hour walk from the mouth of a cave through total darkness — it is nevertheless vulnerable. Mae Hong Son is home to a cottage trekking industry and local guides have been leading tourists inside the cave to spend the night.

“Yes, I’m happy we made our discovery,” Apinun says. “But the government needs to take care of this area. Tour guides need to be more careful. There’s a lot more conservation that needs to be done.”