Even the gentlest sip of a cold drink can send a jolt through our teeth. That familiar sting, long thought to be a side effect of enamel wear, might instead hint at something far more ancient — the remnant of a time when teeth weren’t for chewing at all.

A surprising new study rewrites the story of our teeth’s evolutionary past. Led by paleontologist Yara Haridy at the University of Chicago, the research uncovers striking evidence that the first “teeth” didn’t emerge in the mouths of early fish. Instead, these projections sprouted on their skin — and they weren’t for eating. They were for sensing.

The Ancestor of the Smile

Scientists believed that structures called odontodes — the small, tooth-like projections found in ancient vertebrates — were early versions of teeth used for protection or perhaps mineral storage. Some of the earliest known fossils appeared to support this idea. One fossil in particular, Anatolepis heintzi (roughly 500 million years old), had long been hailed as the first vertebrate with dentin, the sensitive tissue at the core of teeth.

But when Haridy examined Anatolepis — widely considered a jawless “first fish”— using high-resolution synchrotron scans, she saw something unexpected. The microscopic tubules once thought to be dentin looked eerily similar to sensilla, the sensory structures found in modern arthropods like spiders and crabs.

These weren’t teeth at all. They were part of a sensory system. That revelation not only dethroned Anatolepis from its vertebrate status — it turned it into an arthropod, part of an ancient group called aglaspidid.

In other words, one of the most celebrated ‘first fish’ wasn’t a fish at all.

The First Teeth Weren’t Meant for Chewing

What started as an effort to trace the first vertebrates evolved into something much broader: a discovery of convergence. Despite being separated by hundreds of millions of years of evolution, early vertebrates and arthropods both developed similar tools to feel their environments.

Senior author Neil Shubin, also at the University of Chicago, explained the stakes of this shared evolutionary journey. “These primitive animals evolved in a pretty intense predatory environment,” he said. “Being able to sense the properties of the water around them would have been very important.”

The team scanned dozens of ancient and modern creatures and focused on their dentin. Some of the structures on catfish and sharks, it turned out, weren’t so different from the hair-like sensilla on a crab’s claw.

They used immunofluorescence to analyze external dentin structures in living sharks and catfish. In all species, they found nerves wrapped around or entering the dentin, particularly at the base of external odontodes. The nerves form part of what the team calls “sensory armor”. In this defense system, protective skin structures doubled as environmental sensors.

“We performed experiments on modern fish that confirmed the presence of nerves in the outside teeth of catfish, sharks and skates,” Haridy told AFP.

The researchers conclude that “tooth tissues of odontodes outside the mouth can be sensitive — and perhaps the very first odontodes were as well.”

From Anatolepis to Eriptychius

With Anatolepis out of the picture, the study moves the origin of true vertebrate dentin forward by 40 million years, from the late Cambrian to the Middle Ordovician. That’s when a jawless fish called Eriptychius swam through prehistoric seas. Its skin bristled with external odontodes made entirely of dentin, a tissue that, even in modern creatures like humans, sends information to nerves when exposed.

In the 460-million-year-old Eriptychius, the researchers found large, open pulp cavities and branching dentin tubules — an arrangement almost identical to modern sensory teeth. These tubules, lacking any capping enamel or enameloid, would have been exposed to the outside world. That, they suggest, made them excellent sensors.

Some modern animals seem to retain this ancestral feature. Blind cavefish use sensitive odontodes to navigate pitch-black environments. Narwhals, whose iconic tusks are packed with nerves, likely sense their surroundings through their teeth.

The Origins of Dentin — and the Toothache

“Dentine evolved as a sensory tissue in the exoskeleton of early vertebrates,” the authors wrote. And that function remains with us. The intense sensitivity we feel when biting into something cold or sweet? It’s a vestige of an ancient warning system, honed not in mouths, but in armored skin.

Eventually, those pointy sensory structures drifted toward the mouth. As fish evolved jaws, the placement of odontodes shifted from the body surface to the oral cavity. “Little by little some fish with jaws had pointy odontodes at the edge of the mouth and then eventually some were directly in the mouth,” Haridy said.

According to the researchers, invertebrates like Anatolepis and early vertebrates like Eriptychius independently evolved hard, sensory nodules on their skin.  “These two very different animals needed to sense their way through the muck of ancient seas,” said Haridy.

By the time true teeth emerged inside the jaws, their job had changed — from sensing the world to biting into it. But they kept their wiring. Modern teeth still bristle with nerves, and our odontoblasts — the cells that help form dentin — retain their ancient role as sensory sentinels.

That’s why even today, a toothache is a reminder of an ancient, skin-bound signal.

The findings appeared in the journal Nature.