That zing in your teeth from a cold treat? Blame this ancient armored fish

Yara Haridy, an evolutionary biologist at the University of Chicago, likes to stun people by telling them that our skeletons evolved from a jawless fish.

"Much of what we have today has been around for upwards of 500 million years," she says of the bony structures in our bodies. "And getting people to understand that, I think, gives them a better appreciation for their Earth, for the environment, and sometimes for one another."

Now, Haridy has a new surprise to share. In work that she and her colleagues have published in the journal Nature, they find that the sensitivity of our teeth (which causes them to ache and zing when confronted with an icy treat or a dentist's drill) can be traced back to the exoskeletons of those ancient, armored fish.

An ancient case of mistaken identity

The reason that you or your dog or the bird outside aren't puddles of flesh and muscle is because all our soft stuff hangs together on internal scaffolding — a skeleton. That skeleton is what makes us vertebrates.

And Haridy wanted to know: How did our skeletons first come to be?

"Why are we not goopy creatures?" she asks. "Before there was bone, there was no bone. What happened in between? Making a new skeleton is a brand new thing that hadn't existed in evolutionary history, so I want to know how those cells evolved this ability and what tissue they made first."

Haridy decided to look for clues in what was considered the earliest vertebrate on record from roughly 500 million years ago, something called Anatolepis.

"It's unfortunately probably the world's least-exciting-looking fossil," she says. That's because it's just a smattering of these tiny bumpy flakes, "thought to be the scales of this ancient fish-like thing," forming a kind of armor.

Because the Anatolepis fragments hadn't been imaged in detail before, Haridy and a small team booked time at the Argonne National Laboratory's particle accelerator to use its powerful X-rays to get hi-res 3D scans.

"You get to stay overnight in a really crazy building full of vacuums," recalls Haridy. "Everything's chrome — it looks like it's out of The Terminator."

For those 24 hours, she ate a lot of pizza — not deep dish, though.

"I just can't — it's so much dough and cheese," she says. "I'm not a deep dish person. Oh, that's going to get me canceled in Chicago!"

Back in the lab, when Haridy analyzed her scans, she noticed something bizarre. "I just expected to see a really simple structure," she says. "But instead, it was just too much anatomy and too much complexity."

The reason, it turned out, was that Anatolepis was no vertebrate. It was an invertebrate — a kind of arthropod. "Think of it like an ancestor of spiders and scorpions," she says.

This discovery pushed the emergence of vertebrates some 20 to 30 million years later than once thought.

But the finding was hard for Haridy to stomach. She'd built her whole study around Anatolepis being the earliest vertebrate — when it actually wasn't a vertebrate at all.

"It signified that my project was almost broken," she says. "I was so crushed."

Letting the teeth do the talking

Haridy's advisor at the University of Chicago, paleontologist Neil Shubin, helped her reframe her thinking and take the project in a different direction by considering an alternative question.

The anatomy of these flakes of invertebrate armor resembled the teeth of vertebrates. Why was that? In other words, says Haridy, "What problem are these two animals solving that gave them such similar structures?"

Shubin wondered whether maybe it had something to do with sensitivity. There's the fact that human teeth are sensitive. Plus, the structure that Haridy had found in Anatolepis (the now-arthropod) was for sensing. "All this extra anatomy," she says, "it's to sense pressure or to sense chemistry in the air or water."

So Haridy turned to ancient vertebrate fish whose bumpy exoskeletons scientists believe evolved into our teeth. "As fish evolved a jaw and started to feed more like predators," she says, "it becomes very advantageous to have pointy things around your mouth." But no one had known whether there was more to the armor than self-defense.

When Haridy scanned the animal's fossilized bumps, the anatomy was very similar to Anatolepis. "These early vertebrates, they lived in mucky, shallow-bottomed seas." she says. "The visibility probably wasn't great. They probably needed every inch of sensation they can get. What we couldn't ask from the fossils is, 'Hey, ancient fish, were those teeth sensitive?'"

Haridy figured that modern-day fish might help her answer the question. So she looked at the tooth-like scales of the embryos of sharks, skates and catfish — and she found nerves.

Combined, these results suggested that the armor of ancient fish likely allowed them to sense the water around them. Hundreds of millions of years later, our teeth — which evolved from that armor — have inherited that same ability to sense things like pain and temperature.

"That means those scales were useful every step of the way," says Haridy. "And they just changed through time. And that's the story of evolution."

Joseph Keating, a paleobiologist at the University of Bristol who wasn't involved in the study, says unraveling skeletal evolution may help us understand the abnormalities that sometimes arise in our own skeletons.

"The developmental mechanisms that control bone growth — especially growth of the skull roof — are ancient and evolved first in jawless [fishlike animals] 480 million years ago," he says. There are a handful of rare diseases "where those ancient developmental mechanisms go awry."

In addition, the results of the new research explain why we get toothaches. "It's written in our evolutionary history," says Keating.

"It's because once upon a time," Keating adds, "your great great great many times over grandparent was a jawless fish swimming in the sea, sensing its environment through tooth-like structures on its body."

These, he says, are "the evolutionary predecessors of the teeth in your mouth."

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