Why Can't I Grow More Teeth?If you've ever spent time combing the beach for shells or sea glass, chances are you may have found a shark tooth out there as well. They're pretty common, given that sharks are constantly growing new teeth to replace their older ones. But have you ever thought about why they can do that and you can't? It turns out the reason that sharks get infinite chompers while we're stuck patching our teeth together with fillings has an evolutionary history that you can really sink your teeth into.Now, to start, not all teeth are created equal. Well, except for when they are all created equal. So let's go back. Some tooth-having animals, like many sharks, are homodonts, which means that every tooth in their mouth is more or less the same shape, generally bigger at the bottom and pointier at the top. But other animals have fancier teeth with a whole bunch of different shapes, and these animals are heterodonts, including me. Humans and our relatives being heterodonts is the reason that anthropologists can find a single tooth and tell exactly where it went in its owner's mouth. Each tooth is so different in shape that it looks totally unique. And having all those different shapes in your mouth is pretty useful, because each one can have a shape that suits a specific purpose, like having molars for crushing and canines for slicing. It's almost like keeping a multipurpose tool in your pocket all the time, but instead of your pocket, it's in your face. But that's not to say that homodonts aren't doing cool stuff with their teeth. Sharks, for example, are constantly regrowing their teeth on a cycle that results in an alternating pattern of old and new teeth along the road.

If you've ever spent time combing the beach for shells or sea glass, chances are you may have found a shark tooth out there as well.

They're pretty common, given that sharks are constantly growing new teeth to replace their older ones.

But have you ever thought about why they can do that and you can't?

It turns out the reason that sharks get infinite chompers while we're stuck patching our teeth together with fillings has an evolutionary history that you can really sink your teeth into.

Now, to start, not all teeth are created equal.

Well, except for when they are all created equal.

So let's go back.

Some tooth-having animals, like many sharks, are homodonts, which means that every tooth in their mouth is more or less the same shape, generally bigger at the bottom and pointier at the top.

But other animals have fancier teeth with a whole bunch of different shapes, and these animals are heterodonts, including me.

Humans and our relatives being heterodonts is the reason that anthropologists can find a single tooth and tell exactly where it went in its owner's mouth.

Each tooth is so different in shape that it looks totally unique.

And having all those different shapes in your mouth is pretty useful, because each one can have a shape that suits a specific purpose, like having molars for crushing and canines for slicing.

It's almost like keeping a multipurpose tool in your pocket all the time, but instead of your pocket, it's in your face.

But that's not to say that homodonts aren't doing cool stuff with their teeth.

Sharks, for example, are constantly regrowing their teeth on a cycle that results in an alternating pattern of old and new teeth along the road.

What that basically means is that if a shark gets decked in the jaw and loses a bunch of teeth all at once, there's a fresh set just under the gumline that's ready to pop up, so the shark never spends too much time without its chompers.

And when one tooth gets broken or damaged, the replacements are only so many weeks away from coming in.

That wave-like pattern also results in the teeth that are mature and sticking out having somewhat staggered heights, giving them a kind of organic sawtooth shape along the whole road.

So there are some perks to being a little matchy-matchy with your teeth.

But if there weren't upsides to having specialized teeth, nobody would be doing it.

And the kinds of stuff heterodonts can do with those specialized tooth shapes can get really specific.

Take carnivores.

They've got a pair of teeth called their carnassial teeth that are basically nature's scissors.

Their last upper premolar and their first lower molar have long, sharp ridges that rub against each other in a way that's perfect for slicing through meat.

And dental specializations aren't just for slicing and dicing.

Certain monkeys, like baboons as well as a few apes, have what's called a CP3 honing complex, which means that their lower canine and premolar are always rubbing against the top canine, keeping it sharp.

And they need these sharpened canines not to chow down, but to throw down.

See, these animals are pretty territorial, and they fight over access to females for mating.

One study even found that the more often the males of a species fight, the bigger their canines are.

So having two giant sharp knives in your mouth is a great way to defend your territory, or your girlfriend.

And there are a few animals that use their teeth to keep themselves looking neat and tidy.

Lemurs, colugos, and hyraxes all have what are called tooth combs, which they mostly use to groom their fur and pick off parasites.

So the basic idea is that, in order for all these highly specialized tooth shapes and functions to work, all the teeth need to be the correct size so they fit together.

As mammals' teeth gained complexity, they lost the ability to regenerate.

Like, imagine if those carnivores lost a piece from their carnassial complex.

They'd be stuck with one half of a pair of scissors, which is not that great.

And nobody likes a broken comb.

If you're waiting for a tooth to grow in, that's what you're stuck with.

Basically, at some point along the evolutionary track of most mammals, it turned out to be so much better to have specialized tooth shapes that the ability to grow more sets of teeth had to be sacrificed.

That said, there are a few mammals who have managed to find workarounds to the whole one-set-of-adult-teeth thing.

And oddly enough, figuring out how to do that has also meant growing your teeth in a different direction, too.

Take elephants.

All living species of elephants have gigantic molars that look more like car batteries than teeth.

They're huge rectangular blocks made up of thin sheets of enamel.

And when the elephants use them to chow down, they really only chew on the front part of the tooth.

As the plates of enamel at the front wear down, they fall off the molar and out of the elephant's mouth.

They have about six or seven molars in each quadrant of their mouth over the course of their lives.

And as the first one gets worn down, the next one in line moves forwards, like a conveyor belt.

This system means that elephants have a fresh grinding surface for their entire lives.

As in, the elephants will starve to death once their last molar wears out all the way.

And silvery mole rats manage to grow in new molars at the back of the mouth as the ones at the front wear out, without an apparent limit on how many they can grow.

Unlike elephants, they don't just form a whole set and wait to bump them into place.

They actually generate a new batch of teeth when they're needed, which is really rare for mammals.

So there we have it.

The reason you have to suffer through root canals is the same reason you can bite a chunk off of an apple with your front teeth and crunch it to bits with your molars.

And while some may say it was worth it evolutionarily, I'd like them to tell that to my dental bills.

Now, if you enjoy listening to smart people discussing the merits of things like regrowing teeth, we have an entire tooth-related comedic deep dive in our spinoff podcast SciShow Tangents.

It's got games and poetry, and most importantly, every episode will contain at least one butt-related fact.

Satisfaction guaranteed, or your money back.

Except it's a free podcast, so...

Again, that's SciShow Tangents.

You can get it wherever fine podcasts are downloaded.