Rosa Rubicondior: Unintelligent Design - Why Your Toothache Has Ancient Origins

CT scan of the front of a skate, showing the hard, tooth-like denticles on its skin (shown in orange).

Yara Haridy

Toothache from eating something cold? Blame these ancient fish | Biological Sciences Division | The University of Chicago

Have you ever taken a mouthful of ice cream or cold water, only to be rewarded with a sudden, stabbing pain in your teeth? It vanishes in a few seconds, but for that brief moment, it’s excruciating.

You might wonder what kind of intelligent designer would produce such a feature—one that serves no apparent purpose other than to make you suffer. The answer, of course, is that it wasn’t designed at all.

Like so much else about the human body, the sensitivity of our teeth is the product of evolution—a long, meandering process shaped not by foresight or intent, but by whatever natural selection happened to favour at the time. That over-sensitive layer of dentine beneath the enamel traces its origins back to ancient jawless fish, whose bony body armour included sensory structures capable of detecting changes in their environment. These structures were crucial for survival and heavily favoured by selection.

As evolution repurposed this structure over millions of years—eventually becoming part of our teeth—there was no strong selection pressure to reduce its sensitivity. In the aquatic world of those primitive fish, a keen sensory system might have meant the difference between life and death. In modern humans, however, it serves no meaningful function. A momentary sting when we drink something cold doesn’t affect our survival or reproductive success, so we’re left with a redundant sensory feature that occasionally causes pain.

If ever there were a perfect illustration of how evolution works—and why no intelligence is required—this is it. Unless, of course, you believe the designer in question is a malevolent deity with a particular interest in toothaches.

The discovery of this direct connection with our remote ancestral past was made by researchers at the University of Chicago who have just published their findings, open access in Science.

What information do you have on the evolution of odontodes? The Evolution of Odontodes: From Armour to Teeth

What are Odontodes?
Odontodes are small, tooth-like structures that first appeared on the skin of early vertebrates. They are made of dentine, often covered with a cap of enamel or enamel-like tissue, and underlain by bone. Although they resemble teeth, odontodes originally evolved as part of external body armour rather than in the mouth.

Fossil Evidence: The Case of Anatolepis
One of the earliest known examples of such structures comes from Anatolepis, an extinct, jawless fish from the Late Cambrian to Early Ordovician (\~500 million years ago). Its fossilised scales are covered in minute odontodes, representing one of the earliest examples of mineralised tissue in vertebrates. These structures likely served a sensory or protective function, possibly helping the animal detect changes in water pressure or deter predators.

How Did Teeth Evolve from Skin Structures?
The prevailing hypothesis is that teeth evolved from dermal odontodes that migrated into the oral cavity. This is supported by embryological and genetic studies in living vertebrates, which show that the same genetic pathways involved in tooth formation are active in the development of skin denticles (such as in sharks). This “outside-in” theory suggests that evolution repurposed existing body structures rather than inventing new ones from scratch.

Why Are Odontodes Important?
The evolution of odontodes marked a turning point in vertebrate history. These mineralised tissues provided early vertebrates with better protection, sensory capabilities, and eventually more effective feeding strategies. Over time, odontodes diversified into the teeth, scales, and dermal armour seen across many groups of fishes and their descendants.

A Legacy Still Felt Today
Although modern mammals no longer have body armour covered in odontodes, our teeth are direct descendants of these ancient structures. The same sensitive dentine that once detected changes in the environment now causes that sharp pain when you bite into ice cream—a vivid reminder of our deep evolutionary past.

The research team explain their research and its significance in a University of Chicago news item by Matt Wood:

Toothache from eating something cold? Blame these ancient fish
New research on fossils shows that teeth first evolved as sensory tissue in the armored exoskeletons of ancient fish.
Anyone who has ever squirmed through a dental cleaning can tell you how sensitive teeth can be. This sensitivity gives important feedback about temperature, pressure—and yes, pain—as we bite and chew our food. However, the sensitive parts inside the hard enamel first evolved for something quite different.

New research from the University of Chicago shows that dentine, the inner layer of teeth that transmits sensory information to nerves inside the pulp, first evolved as sensory tissue in the armored exoskeletons of ancient fish.

Paleontologists have long believed that teeth evolved from the bumpy structures on this armor, but their purpose wasn’t clear. The new study, published this week in Nature, confirms that these structures in an early vertebrate fish from the Ordovician period about 465 million years ago contained dentine, and likely helped the creature sense conditions in the water around it.

The research also showed that structures considered to be teeth in fossils from the Cambrian period (485-540 million years ago) were similar to features in the armor of fossil invertebrates, as well as the sensory organs in the shells of modern arthropods like crabs and shrimp. These similarities imply that sensory organs in the armor of diverse animals evolved separately in both vertebrates and invertebrates to help them sense the larger world around them.

When you think about an early animal like this, swimming around with armor on it, it needs to sense the world. 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.

Professor Neil Shubin, PhD, senior author
Robert R. Bensley Distinguished Service Professor of Organismal Biology and Anatomy
University of Chicago, Chicago, IL, USA.

Night at the particle accelerator
Yara Haridy, PhD, a postdoctoral researcher in Shubin’s lab who led the study, wasn’t looking for the origins of teeth when she started the project. Instead, she was hoping to answer another longstanding paleontological question: What is the earliest vertebrate in the fossil record? Haridy asked museums around the country for fossil specimens from the Cambrian period (485-540 million years ago) so she could CT scan them, looking for telltale signs of vertebrate features.

UChicago scientists Yara Haridy and Neil Shubin talk about their latest research showing how our sensitive teeth first evolved as sensory tissue in the armored exoskeletons of ancient fish.
Julian Romano

One of those signs, at least in later fish, is the presence of dentine inside the bumps on external armor, called odontodes. Haridy collected hundreds of specimens, some just tiny fragments that could fit on the end of a toothpick. She then took them to Argonne National Laboratory for an all-night scanning session using the Advanced Photon Source, which captured extremely high-resolution CT images of the fossils. “It was a night at the particle accelerator; that was fun,” Haridy said.

As they started seeing the images from the scans, one of the samples from a Cambrian fossil called Anatolepis looked like it showed the hallmarks of a vertebrate fish. It had a series of tubules, or pores underneath the odontodes, filled with material that bore the chemical signatures of dentine. If it truly was a vertebrate, this specimen would have extended the fossil record back by tens of millions of years.

We were high fiving each other, like ‘oh my god, we finally did it. That would have been the very first tooth-like structure in vertebrate tissues from the Cambrian. So, we were pretty excited when we saw the telltale signs of what looked like dentine.

Dr. Yara Haridy, PhD., first author
Department of Organismal Biology and Anatomy
The University of Chicago, Chicago, IL, USA.

Ancient vertebrate fish (top row), ancient arthropods (middle row), and modern-day arthropods (bottom row) all had convergent structures on their exoskeletons that are connected to nerves that allow the animals to sense their environment.

Alex Boersma.

They had to confirm this, of course, so they began analyzing images of the other specimens Haridy scanned. This library of shells and skeletons included everything from other ancient fossils to modern crabs, snails, beetles, barnacles, sharks, and skates, plus miniature suckermouth catfish that Haridy raised herself in an aquarium.

Once they compared the possible vertebrate Anatolepis to a known arthropod fossil from the Milwaukee Public Museum, they realized that what looked like dentine-lined tubules of a vertebrate were more like the sensory organs on the shells of crabs, called sensilla. This means that Anatolepis, which was claimed to be a vertebrate in the pages of Nature in 1996, is an ancient invertebrate arthropod instead. The large tubules in another Ordovician vertebrate called Eriptychius were similar in structure to these sensilla, but did contain dentine.
This shows us that ‘teeth’ can also be sensory even when they're not in the mouth. So, there's sensitive armor in these fish. There's sensitive armor in these arthropods. This explains the confusion with these early Cambrian animals. People thought that this was the earliest vertebrate, but it actually was an arthropod.

Dr. Yara Haridy, PhD.

Tooth-like structures scattered across the fossil record
Sharks, skates, and catfish also have tooth-like structures called denticles that make their skin feel like sandpaper. When Haridy studied the tissues of her catfish, she saw that the denticles were connected to nerves, just like a tooth would be. She said the similarities to teeth, the ancient odontodes of armored fish, and the sensilla of arthropods was striking.
We think that the earliest vertebrates, these big, armored fish, had very similar structures, at least morphologically. 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.

Dr. Yara Haridy, PhD.

CT scan image of tooth like dermal denticles on a catshark. These tooth-like structures are connected to the nervous system, suggesting they create sensation.
Yara Haridy

CT scan of the tooth-like-odontode structure from Astrapsis, an ancient jawless vertebrate fish. The tubules (shown in green) are filled with dentine, the same material that makes up the sensitive inner layer of modern teeth. In red is the vascular system which would have housed the nerves in life allowing for sensation to be transmitted.
Yara Haridy

Segmented confocal scan of the tooth-like-odontode structure from suckermouth catfish fish, showing nerves (in green) that allow transmission of sensory information from the tooth like odontode to the nervous system.
Yara Haridy

CT scan of a whole bamboo shark, showing the hard, tooth-like denticles on its skin (shown in red).
Yara Haridy

CT scan of the front of a skate, showing the hard, tooth-like denticles on its skin (shown in orange).
Yara Haridy

There are two schools of thought about how these structures eventually became teeth. One, the “inside-out” hypothesis, says that teeth arose first, and were later adapted for exoskeletons. This paper would support the second, “outside-in” hypothesis, that says sensitive structures developed first on exoskeletons, and at some point, animals utilized the same genetic toolkit to produce sensitive teeth as well.

While they didn’t pin down the earliest vertebrate fish, Shubin said this discovery was more than worth the effort.
For some of these fossils that were putative early vertebrates, we showed that they’re not. But that was a bit of misdirection. We didn’t find the earliest one, but in some ways, we found something way cooler.

Professor Neil Shubin PhD.

Abstract
The earliest record of tooth antecedents and the tissue dentine^1,2, an early-vertebrate novelty, has been controversially represented by fragmentary Cambrian fossils identified as Anatolepis heintzi^3,4,5. Anatolepis exoskeletons have the characteristic tubules of dentine that prompted their interpretation as the first precursors of teeth³, known as odontodes. Debates over whether Anatolepis is a legitimate vertebrate^6,7,8 have arisen because of limitations in imaging and the lack of comparative exoskeletal tissues. Here, to resolve this controversy and understand the origin of dental tissues, we synchrotron-scanned diverse extinct and extant vertebrate and invertebrate exoskeletons. We find that the tubules of Anatolepis have been misidentified as dentine tubules and instead represent aglaspidid arthropod sensory sensilla structures^9,10. Synchrotron scanning reveals that deep ultrastructural similarities between odontodes and sensory structures also extend to definitive vertebrate tissues. External odontodes of the Ordovician vertebrate Eriptychius^11,12,13 feature large dentine tubules1 that are morphologically convergent with invertebrate sensilla. Immunofluorescence analysis shows that the external odontodes of extant chondrichthyans and teleosts retain extensive innervation suggestive of a sensory function akin to teeth^14,15,16. These patterns of convergence and innervation reveal that dentine evolved as a sensory tissue in the exoskeleton of early vertebrates, a function retained in modern vertebrate teeth¹⁶. Middle-Ordovician fossils now represent the oldest known evidence for vertebrate dental tissues.

Haridy, Y., Norris, S.C.P., Fabbri, M. et al.
The origin of vertebrate teeth and evolution of sensory exoskeletons.
Nature (2025). https://doi.org/10.1038/s41586-025-08944-w

Copyright: © 2025 The authors.
Published by Springer Nature Ltd. Open access.
Reprinted under a Creative Commons Attribution 4.0 International license (CC BY 4.0)

The evolution of odontodes utterly dismantles the creationist narrative of sudden, purposeful design. Structures like those found in *Anatolepis* are not teeth, yet they are built from the very same tissues and governed by the same genetic pathways. They appear not in the mouth, but embedded in the dermal armour of primitive, jawless fishes. What rational designer would start by putting tooth-like organs in the skin, only to later repurpose them for chewing? Evolution, however, thrives on such jury-rigged solutions—modifying what already exists, regardless of how inefficient or indirect the path may be.

Creationists often demand to see transitional forms, yet when we show them—clearly and in abundance—they ignore or dismiss them. The fossil record doesn’t show teeth appearing from nowhere; it shows a slow, piecemeal process of modification, co-option, and adaptation. From dermal denticles in ancient fish to modern mammalian molars, each step is grounded in biology, genetics, and palaeontology. There is no need for magical intervention, only deep time and natural selection.

And let’s not forget the irony: the same dentine that once served a vital sensory role now causes that sharp stab of pain when you sip something cold. This is not the handiwork of a wise and benevolent designer—it’s the leftover consequence of evolutionary tinkering. If this is the best a supposed intelligent designer can do, then either the designer is woefully incompetent, or evolution is exactly what it appears to be: blind, imperfect, and entirely real.

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