Proving that evolutionary science is a cabinet of curiosities more bizarre and – oftentimes – chilling than any work of fiction, new evidence suggests that the reason humans have sensitive teeth today is to do with the exoskeletons of armoured fish some 465 million years ago.

It’s long-established that teeth sensitivity sends important feedback about temperature, pressure, and pain, as we bite and chew our food. But it turns out that the sensitive parts inside the tooth’s 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 – first evolved as sensory tissue in the exoskeletons of ancient fish.

Published this week in Nature, a new study confirms that the bumpy structures evolved along ancient fish armour – first originating from the Ordovician period around 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 to 540 million years ago) were similar to features in the armour of fossil invertebrates, as well as the sensory organs in the shells of modern arthropods such as crabs and shrimp. These similarities imply, the researchers suggest, that sensory organs in the armour 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 armour 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,” said Neil Shubin, senior author of the new study and the Robert R Bensley Distinguished Service Professor of Organismal Biology and Anatomy at UChicago.

“So here we see that invertebrates with armour, like horseshoe crabs, need to sense the world too, and it just so happens they hit on the same solution.”

The study wasn’t intended to hit upon the origin of teeth but instead answer the paleontological question: what is the earliest vertebrate in the fossil record? 

One of the telltale signs of vertebrate features – at least in later fish – is the presence of dentine inside the bumps on external armour – bumps called odontodes. That telltale sign appeared during a scan of a sample from a Cambrian fossil called Anatolepis which, if it was discovered truly was a vertebrate, would have extended the fossil record back by tens of millions of years.

To confirm this, researchers began analysing images of other specimens; a library of shells and skeletons which including samples from other ancient fossils to modern crabs, snails, beetles, barnacles, sharks, and skates. 

Once they compared the possible vertebrate Anatolepis to a known arthropod fossil from the Milwaukee Public Museum, they realised that what had looked like dentine-lined tubules of a vertebrate were more like the sensory organs on the shells of crabs. 

This meant that Anatolepis (which was claimed to be a vertebrate back in 1996) is, in fact, an ancient invertebrate arthropod instead. 

“This showed us that ‘teeth’ can also be sensory even when they’re not in the mouth,” said Yara Haridy, a postdoctoral researcher in Shubin’s lab who led the study. “So there’s sensitive armour in these fish, there’s sensitive armour in these arthropods. This explains the confusion with these early Cambrian animals. People thought that this was the earliest vertebrate, but it was actually an arthropod.”

Sharks, skates, and catfish also all have ‘tooth-like structures’ called denticles that make their skin feel like sandpaper. Studying the tissues of catfish, Haridy observed that the denticles were connected to nerves, just as a tooth would be.

“We think that the earliest vertebrates – these big, armoured fish – had very similar structures, at least morphologically,’ she continued. “They look the same in ancient and modern arthropods, because they’re all making this mineralised layer that caps their soft tissue and helps them sense the environment.”

There are currently two schools of thought about how these structures eventually became teeth. The first is the ‘inside out’ hypothesis which says the teeth arose first before later adapted for exoskeletons. The second is the ‘outside-in’ hypothesis, an idea this most recent study supports, which suggests that ‘sensitive structures developed first on exoskeletons’ and at some point animals evolved with the same ‘genetic toolkit’ to make sensitive teeth as well.

The study, ‘The Origin of Vertebrate Teeth and Evolution of Sensory Exoskeletons’ was supported by the National Science Foundation, the US Department of Energy, and the Brison Family Foundation.

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