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Here’s what you’ll learn when you read this story:
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A new study from the University of Chicago suggests our sensitive teeth may be an evolutionary leftover that once helped our fishy ancestors sense their surroundings.
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Researchers studied modern fish with dermal teeth and determined they are innervated, meaning they deliver sensory signals through nerves like our own teeth would.
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The study supports the “outside-in” theory which suggests sensitive “teeth” developed on exoskeletons before they appeared in mouths.
Some would say there’s nothing better than taking a big ol’ bite out of an ice cream cone—but that’s not necessarily a popular opinion. If the thought of sinking your pearly whites into a frozen dessert makes you cringe, then you probably have something in common with our earliest fishy ancestors. According to a new study published in Nature, teeth may have originated as sensory organs that helped ancient fish navigate murky waters.
“When you think about an early animal like this, swimming around with armor on it, it needs to sense the world,” senior author of the new study Neil Shubin said in a press release. “This was a pretty intense predatory environment and being able to sense the properties of the water around them would have been very important. So, here we see that invertebrates with armor like horseshoe crabs need to sense the world too, and it just so happens they hit on the same solution.”
Lead researcher on the study Yara Haridy wasn’t always looking for ancient smiles—she made the discovery while trying to find the oldest vertebrate in the fossil record. Haridy identified armor bumps called odontodes on samples of the Cambrian fossil Anatolepis. It seemed like there were dentine tubules just beneath the odontodes—a hallmark sign of a vertebrate.
Upon further inspection, Haridy realized the tubules were more akin to the sensory organs on crabs’ shells called sensilla, although the pockets did contain dentin (A.K.A. the hard tissue that makes up the teeth in your mouth). The discovery sparked a new hypothesis for Haridy and the rest of the team: teeth may be sensory even when they’re not in the mouth.
“We’ve been wondering ‘why would we chew with these painful things? Why are they so sensitive in the first place?” Haridy said in a video. “And it turns out, maybe this is a leftover from one of our most ancient ancestors.”
Surprisingly, external teeth aren’t uncommon in the animal kingdom today. Sharks, skates, and catfish all have tiny tooth-like structures called denticles lining their skin, making them feel like sandpaper. To better understand whether these external teeth were innervated (connected to nerves), Haridy studied some modern fish with denticles. She determined that the denticles were, in fact, connected to nerves like human teeth would be. Haridy said in the press release that the resemblance between armored fish’s denticles and the sensilla of arthropods (like the ones seen in the Anatolepis samples) was striking.
“We think that the earliest vertebrates, these big, armored fish, had very similar structures, at least morphologically.” she said. “They look the same in ancient and modern arthropods, because they’re all making this mineralized layer that caps their soft tissue and helps them sense the environment,”
According to Shubin, these early sea creatures would have existed in a “pretty intense predatory environment,” so the ability to sense the world swimming around them would have been extremely important. The team’s findings support what is known as the “outside-in” hypothesis, or the theory that sensitive structures developed on exoskeletons first and then sensitive teeth followed. This contradicts the “inside-out” hypothesis that assumes teeth arose first and were later adapted for exoskeletons.
“The more we look at the fossil record, the more we put those fossils in an evolutionary sequence, the more that we see the ‘outside-in’ hypothesis is likely correct,” Shubin said in the video. “Our teeth originally evolved as the armor on the outside of the body of the earliest fish.”
Moral of the story is: next time an annual cleaning leaves you with a toothache, don’t blame the dental hygienist—blame your gilled ancestors!
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