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TEETH AREN'T BONES
Across the animal kingdom and the fossil record there are teeth of all shapes and sizes. Big flat teeth on plant eaters, sharp pointy ones on meat eaters, you name it. Teeth have been around for hundreds of millions of years, so you might think that we have a pretty good idea of where teeth came from. But among scientists, the origin of teeth is still a hot debate.

Now when we talk about teeth, we're specifically referring to the kind of teeth we humans have. Some animals, like snails, have other mouth parts that they use to munch on food, but they're not technically teeth. True teeth are complex structures made up of a kind of living tissue called dentin and a thin layer of enamel. And while they look white and bony, they're not actually bones. They don't even have to be attached to a jaw. Some vertebrates actually have teeth in their throats. You're welcome for that visual!

OUTSIDE-IN MODEL
So, we know what teeth aren't. They're not mouth bones popping up through our gums. But we don't really know what they are - like what they evolved from. The classic hypothesis since the 1800's has been that teeth evolved from fish scales. And that might sound farfetched,  but teeth actually do look really similar to the scales on sharks and rays known as dermal denticles. Just like teeth, these denticles are isolated structures made of dentin, covered by a thin layer of minerals and attached by a bony base. On top of that, the genes controlling scales and teeth seem related, suggesting that there may be an evolutionary link between the two. So the idea behind the classic hypothesis is that scales originally evolved as armor plating to protect ancient fish against predation. Then, over time, the scales around these fish's mouths began to migrate inward where they became teeth and began helping with eating. This hypothesis is called the Outside-In Model and there is some possible support for it in the fossil record. For example, today many fish have teeth, but the fossils of one early jawless fish show that while it didn't have teeth, it appears to have had large plates on its skin that would have been covered in tooth-like scales. This seems to, at least, support the idea that scales evolved before teeth whether or not they evolved into them. Another possible piece of support for this hypothesis came in 2022, when scientists looking at saw-fish fossils concluded that the spikes on this creature's so-called saw were actually scales that had taken on some tooth-like characteristics.

INSIDE-OUT HYPOTHESIS
That's not to say that this particular adaptation was the evolutionary origin of teeth, but it does show that it's at least possible for scales to evolve into something like teeth. But none of this is definitive evidence that teeth evolved from scales. And in the 1990's, some scientists took a whole new look at the fossil data and how animals develop and grow and they proposed a completely different idea. It was called the Inside-Out Hypothesis and it suggested that teeth evolved in the mouth first then migrated out to become scales. And some fossil evidence seemed to support this idea as well. For example, an extinct group of eel-like animals called conodonts seemed to have tooth-like structures well before scales evolved. In that case, teeth couldn't have evolved from scales if they existed first. But the finding doesn't necessarily rule out the classic hypothesis because these supposed teeth might not actually be related to ours. They may have evolved separately and just look similar. Plus, the conodonts place on of the tree of life is still debated, so even if they did have true teeth, that makes it hard to tell whether their teeth came before scales.

So, it's not clear whether the Inside-Out Hypothesis is the right answer either. For years, scientists have looked for more clues as to which came first between scales and teeth, but unfortunately it's just not obvious from the fossil record. And while scientists were trying to get to the bottom of this debate, yet another possibility came into play.

INSIDE-AND-OUT MODEL
In 2010, some scientists suggested that maybe it doesn't matter whether teeth or scales came first because maybe neither evolved from the other.  Instead, they proposed that both teeth and dermal denticles grew out of some more basic kind of structure that could be found in multiple different types of tissue around the body. According to this hypothesis, known as the Inside-And-Out Model, these basic structures could turn into teeth or scales depending on what instructions their cells received from developmental genes. But this hypothesis hasn't settled the debate either.

CONCLUSION
As of today, the quandary of where teeth came from is still an open question. The original Outside-In Hypothesis has the most support behind it but there are still a lot of details to work out. And no matter what the true origin turns out to be, there's something amazing about the fact that the mouth rocks we use every day can be so mysterious. Outside-In, Inside-Out, or a little bit of both, there's a complicated dance going on and we're still learning the steps.

SPONSOR
Now this video is supported by Cometeer. Many of us can't imagine a better way to start our morning than by covering those mouth rocks in some bean water. And Cometeer is here to make that a reality for you. Cometeer's mission is to make a positive impact on the coffee industry from farm to roaster to your coffee cup. Their supply chain is sustainability focused and every component of the package you receive is recyclable from the capsule itself to the shipping materials. Cometeer is flash frozen to lock in the freshness then, when the coffee gets to you, you can make any coffee style you like whether it's hot, iced, or espresso. Or you can experiment with different styles to create your ideal drink. And when they're done making you barista-quality coffee capsules, Cometeer turns all of their spent grounds into compost for local gardens. For a monthly shipment of next generation coffee sent to your door, you can visit https://cometeer.com/scishowoctober and for $20 off each of your first two orders, you can use the code SciShow. That's $40 off, plus free shipping.

Thank you to Cometeer for supporting this video and thank you for watching.