That uncomfortable zing you feel when biting into ice cream or during a dental cleaning has a surprising evolutionary origin, according to a groundbreaking study published in Nature. Researchers have discovered that the sensitivity of human teeth can be traced back to sensory structures in the armored exoskeletons of ancient fish that lived approximately 465 million years ago.
"A toothache is actually an ancient sensory feature that may have helped our fishy ancestors survive!" said Dr. Yara Haridy, an evolutionary biologist at the University of Chicago and lead author of the study.
From Ancient Armor to Modern Teeth
Haridy and her team initially set out to identify the oldest vertebrate in the fossil record, focusing on dentine—the sensitive inner layer of teeth that transmits sensory information to nerves. Using high-resolution CT scans from the Advanced Photon Source at Argonne National Laboratory, they examined hundreds of fossil specimens, some tiny enough to fit on the tip of a toothpick.
What they discovered upended conventional understanding about the evolution of teeth. The researchers found that tooth-like structures called odontodes, which appeared as bumps on the external armor of ancient fish, contained dentine-filled tubules connected to nerves—suggesting they evolved primarily as sensory organs rather than for protection or feeding.
"When you think about an early animal like this, swimming around with armor on it, it needs to sense the world," explained Dr. Neil Shubin, senior author of the study. "This was a pretty intense predatory environment, and being able to sense the properties of the water around them would have been very important."
Convergent Evolution
The study also revealed striking parallels between these fish odontodes and sensory structures called sensilla found in the exoskeletons of arthropods like crabs and shrimp. This represents a remarkable case of convergent evolution, where vertebrates and invertebrates independently developed similar sensory mechanisms to navigate their environments.
"These jawless fish and arthropods have an extremely distant shared common ancestor that likely had no hard parts at all," Haridy noted. "Yet amazingly they evolved similar sensory mechanisms integrated into their hard skeleton independently."
The research also corrected a longstanding misidentification in the fossil record. A species called Anatolepis, previously thought to be one of the earliest vertebrate fish, was actually revealed to be an invertebrate arthropod—pushing back the timeline for the emergence of vertebrates by 20-30 million years.
The 'Outside-In' Theory
The findings support the "outside-in" hypothesis of tooth evolution, which proposes that sensory structures first evolved on external exoskeletons before being adapted for use in the mouth. As fish evolved jaws, these sensory odontodes gradually migrated to the edges of the mouth and eventually inside it, where they could be used for biting and chewing while retaining their sensory capabilities.
Evidence for this evolutionary connection continues today in several modern species. Sharks, skates, and some catfish are covered in small tooth-like structures called denticles that make their skin feel like sandpaper. When the researchers examined these denticles, they found they were connected to nerves in the same way teeth are.
"We performed experiments on modern fish that confirmed the presence of nerves in the outside teeth of catfish, sharks and skates," Haridy explained. "This shows that tooth tissues of odontodes outside the mouth can be sensitive—and perhaps the very first odontodes were as well."
The next time you wince from a cold drink or dental cleaning, you can thank your distant fish ancestors, who used those same sensitive tissues to navigate the ancient seas and avoid becoming someone else's dinner.
