(SOUNDBITE OF DRILL WHIRRING)
ARI SHAPIRO, HOST:
The sound of a dentist's drill - did it make your teeth quiver? Well, it turns out the sensitivity of our teeth which causes them to ache can be traced back to the exoskeletons of ancient armored fish. Here's NPR's Ari Daniel.
ARI DANIEL, BYLINE: Yara Haridy is an evolutionary biologist at the University of Chicago, and she wanted to know, how did our skeletons first come to be?
YARA HARIDY: Before there was bone, there was no bone. What happened in between?
DANIEL: Haridy decided to look for clues in what was considered the earliest vertebrate on record, from roughly 500 million years ago, something called Anatolepis.
HARIDY: It's unfortunately probably the world's least exciting-looking fossil.
DANIEL: It's just a smattering of these tiny bumpy flakes...
HARIDY: Thought to be the scales of this ancient fish-like thing.
DANIEL: Forming a kind of armor - the Anatolepis fragments hadn't been imaged in detail before, so Haridy and her team booked time at a particle accelerator to use its powerful X-rays to get high-res 3D scans.
HARIDY: You get to, like, stay overnight in a really crazy building, but it looks like it's out of "The Terminator."
DANIEL: When Haridy analyzed her scans back in the lab, she noticed something bizarre.
HARIDY: It was just too much anatomy and too much complexity.
DANIEL: The reason - Anatolepis was no vertebrate. It was an invertebrate, a kind of arthropod.
HARIDY: Think of it like an ancestor of spiders and scorpions.
DANIEL: 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 built her whole study around this thing being the earliest vertebrae.
HARIDY: Well, it signified that my project was almost broken. I was so crushed.
DANIEL: But Haridy's adviser helped her take the project in a different direction. These tiny flakes of invertebrate armor looked a lot like the teeth of vertebrates. Why was that? Maybe it had to do with sensitivity.
HARIDY: You know, our teeth are sensitive.
DANIEL: And the structure she'd found in the armor of Anatolepis, the now arthropod, was for sensing.
HARIDY: All this extra anatomy - it's to sense pressure or to sense chemistry in the water.
DANIEL: So Haridy turned to ancient vertebrate fish, whose bumpy exoskeletons scientists believe evolved into our teeth, and when she scanned its fossilized bumps, the anatomy was very similar to Anatolepis.
HARIDY: These early vertebrates - they lived in mucky seas. They probably needed every inch of sensation they could get. What we couldn't ask from the fossils is, hey, ancient fish, were those teeth sensitive?
DANIEL: Haridy then considered the embryos of modern-day sharks, skates and catfish, and when she looked at their tooth-like scales, she found nerves. Combined, these results suggest 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 ability to sense.
HARIDY: And that's the story of evolution.
DANIEL: The research is published in the journal Nature. Joseph Keating is a paleobiologist at the University of Bristol who wasn't involved in this study. He says unraveling skeletal evolution may help us understand the abnormalities that sometimes arise in our own skeletons, and it explains why we get toothaches.
JOSEPH KEATING: It's because once upon a time, your great-great-great-many-times-over-grandparent was a jawless fish sensing its environment through tooth-like structures on its body, the evolutionary predecessors of the teeth in your mouth.
DANIEL: So the next time you smile at yourself in the mirror, think about the ancestral fish grinning back at you with feeling. Ari Daniel, NPR News.
(SOUNDBITE OF KAYTRANADA SONG, "LOVER/FRIEND") Transcript provided by NPR, Copyright NPR.
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