Around 450 million years ago, one of our ancient fishy ancestors still boneless and jawless began building itself some armor.

Eriptychius had a built-in shield that had evolved to do more than one job.

It not only protected its head, but also provided key information about the shallow, murky waters surrounding it.

And believe it or not, this shield was laying the groundwork for our toothaches of today.

If you’ve ever experienced dental pain, you've probably wondered why our teeth would evolve to function like an oversensitive, or sometimes straight-up faulty, alarm system.

To find the answer, we have to go way back to when teeth first evolved, as part of the armored skin of ancient fish.

Life’s tough in the ocean.

It’s a fish-eat-fish world out there.

It’s also an arthropod-eat-fish world, and a mollusc-eat-fish world, and all kinds of other combinations.

And that’s not only true today, but it was hundreds of millions of years ago, too.

The world’s animal diversity seems to have really taken off in the Early Cambrian Period, around 530 million years ago, bringing a bunch of new predators onto the scene.

And this would have put pressure on a lot of different organisms to find ways to avoid falling victim to a deadly bite.

Our vertebrate ancestors began solving this predator problem some 500 to 460 million years ago when they started to armor up, back when all fish were still boneless and jawless.

Armor was a great place to start, because it could both act as a protective coating on otherwise soft bodies and also stiffen those bodies, improving a fish’s ability to swim away from danger.

Eriptychius was one of the first examples that we know of of a fish embracing an outer armor made specifically of dentine.

Now, dentine might sound kind of familiar it’s the hard tissue in our teeth.

Dentine, dentist; you get it.

And when we’re talking body armor, we’re talking about odontodes, also known as dermal teeth.

Which means, yup, skin teeth.

Eriptychius’s armor covered its head, because, if you’re going to start protecting key body parts, well, brains are pretty important.

But this thin body armor stopped there it didn’t extend to anywhere else on its body.

Now we don’t totally know why ancient fish like Eriptychius armored up, but there are a lot of hypotheses, many related to fending off attacks.

But some scientists also wonder if their hardened exterior might have served as a place for their muscles to attach to, for example.

Their environment might have played a major role in the shift, too.

Because, living close to shore, like Eriptychius did, came with sacrifices.

Like, some minerals become less abundant where salt meets freshwater, so maybe these protective elements might have actually been a form of mineral storage.

Plus, as a shoreline dweller instead of an open water swimmer, protecting the noggin from not only predators, but also from surface bumps and scrapes seems like a pretty good strategy for Eriptychius to adopt.

So while we don’t know for sure exactly what triggered it, ancient fish likely armored up for at least one, or maybe several, of these reasons.

But with this hard external layer came another complication to work around.

See, fish have always needed to pick up cues from the water.

And since fish are cold-blooded, they rely on the external environment to regulate their temperature.

Which means constantly monitoring changes that can tell you if you’re in the wrong place and need to head to cooler or warmer waters, for example.

And that’s not all that fish are perceiving from the water.

Chemical signals, electrical currents, and mechanical waves are all important factors to pick up on as well.

So evolving sensors on the outside of their bodies makes a ton of sense.

But, how could fish balance the need for sensors with the need for a layer of protection?

Well, Eriptychius had a built-in workaround.

Their external dermal teeth were full of tiny branching tubes.

Thanks to a bunch of pores that opened to the surrounding waters, these interconnected tubes probably transported important information from the external environment inward.

And underneath this dentine was a soft pulp cavity that connected to its internal nerve systems, which would have passed along that external sensory information.

This means that exposed, porous dentine surface would have allowed for strong and immediate, but brief, sensations to tell them about the world around them.

This would have been key for Eriptychius, given that maintaining a strong sensory connection to their environment would be vital for their survival .

Especially in the shallow, murkier waters they called home, where the specific cues you're looking for might be more subtle than in more open waters they might be literally buried in the muck.

So while this early fish didn’t have a whole lot of body armor overall, the dentine plates on their heads offered some early protection of key soft tissues, while also keeping them tuned in to the outside world.

And another armored jawless fish that lived around the same time, Astraspis, put their own twist on this special outer layer.

Their name appropriately means 'star shield,' referring to the shape of the tubes within their dentine-laden dermal teeth.

But Astraspis was a little different, in that they were working with even more head protection than Eriptychius.

They had an extra coating of hard mineral build up over their dentine, similar to, but probably not quite like, the enamel we have on our teeth.

Enamel is the hardest tissue found in all vertebrates today.

So having something even close to it would have been great for extra protection.

And not only did Astraspis have this extra layer on its built-in hardhat, but its dermal teeth also changed throughout its lifetime, toughening up underneath that extra hard layer by filling in the sensitive pulp layer below.

Now, like Eriptychius, Astraspis was found in shallow shoreline habitats, so it was probably another bottom-dweller.

But given their bulked-up protection measures, Astraspis may have had an even greater need than Eriptychius to protect themselves.

Not only with that outer layer but with their filled-in pulp, they seemed to be creating a buffer from intense sensations that would stem from a greater dentine exposure.

And while we don’t exactly know why that is, the likes of Astraspis definitely started to change the game in how our early ancestors hardened up, from the outside-in.

Fish continued to evolve and change over millions of years, and their skin teeth were along for the ride.

Take Andreolepis for example, a 420 million-year-old fish that didn’t just have dentine on their head.

Instead, they were fully covered in protective dermal teeth.

And Andreolepis not only had skin teeth, but also teeth teeth, of the mouth variety.

Much like their dermal teeth, these mouth teeth were also made of dentine, and were shed and replaced as they aged.

Andreolepis one-upped Astraspis in the protective layer department, with scales covered in the really tough stuff, enamel.

They still didn’t have any true enamel on their teeth though.

So even when enamel first began to evolve in fish as a protective structure, it still didn’t cover their teeth.

But it wasn’t long before fish finally started to fully embrace the really hard stuff, and enamel became more widespread.

Shortly after Andreolepis, give or take a couple million years, Psarolepis arrived on the scene in the late Silurian Period.

And this bony fish took safety very seriously.

Not only were the scales on its body protected with enamel, but unlike Andreolepis, Psarolepis's enamel made its way up onto the head, reinforcing the dermal plates for seriously strong noggin protection.

In Psarolepis, the thick dermal plates that protected its head had big pores, likely creating a pathway of sorts to maintain some communication lines from the outside in.

And this new enamel protection wasn’t just keeping Psarolepis’s brain safe.

The dermal teeth on their head covered their lips and even went down into the nostrils!

But even with enamel-covered scales lining their mouth, they still had naked dentine on their teeth.

Though their teeth did have some mineral protection, it was nothing like enamel.

This delayed appearance of added dental protection may seem strange, but it actually makes a lot of sense.

Carnivores need a lot less tooth protection than herbivores because they’re just not as hard on their teeth.

It’s a lot of grab, tear, swallow, and not a lot of grind and chew.

But mouth tooth enamel did finally arrive on the scene a little later in the Devonian, just under 400 million years ago.

It coated the chompers of early Sarcopterygians, the group all tetrapods four-limbed vertebrates eventually took off from.

Once enamel joined forces with dentine inside the mouth, they strengthened the teeth for a better bite, making them tougher and less likely to crack.

So while it took some time to do it, dermal teeth flipped the script from protecting when attacked, to gearing up as an attacker.

And from there on out, enamel played a key role in supporting dietary diversity, allowing tetrapods to really branch out into completely new culinary avenues, like plant-eating.

Taking a closer look at our teeth today, we can see the soft inner pulp full of fibers that pass along important sensory and chemical information through our nervous system.

That pulp is overlaid with dentine which, like in our fishy ancestors, remains full of tiny-tubules.

And finally, our dentine is capped off with a generous layer of enamel.

Our thick enamel is what allows our teeth to do so much crunching and grinding.

But those layers of dentine underneath that also protect our teeth will still let us know that the insides are conversing with the outsides under certain conditions.

Whether that’s from damage to the enamel like cracks and cavities, or cold temperatures, or seemingly innocuous food choices, like sugar.

And the level of pain felt is often extremely out of proportion to the damage being done, especially in some cases when no damage is being done at all.

After all, a sip of cold water isn’t exactly giving us frostbite.

But all this makes a lot more sense given the context of our teeth’s beginnings.

Our ancient ancestors were keen on sensing the world around them, but as they shifted habits and habitats, their sensory bits became covered with harder and harder materials.

So even when they started covering their bodies with dentine for protection, they maintained the ability to pick up on all those different cues of temperature, chemical signals, and more.

These protective coverings later made their way in to cover their teeth, too.

But underneath that tough enamel cap still lies dentine just itching to share all kinds of info with its soft pulp interior, making us painfully aware of our fishy origins.