In 2017 alone,

Americans spent $129 billion on dental visits,

and an estimated 90% of adults in the US have fillings.

But many animals don't have this problem

because they can regrow their teeth,

replacing old damaged ones with brand new pearly whites.

Wouldn't it be great if you could do that?

Well, there's actually an important trade-off

for this nuisance.

Only a handful of mammals can regrow teeth multiple times

compared to the 50,000 species of reptiles and fish.

Take geckos for example.

They replace all 100 or so teeth every three to four months,

and since geckos can live for six to 10 years,

they'll grow anywhere from 1,800

to 4,000 teeth in a lifetime.

That's all thanks to a special type of cell in their gums

called stem cells.

Stem cells are handy because they can morph into different

cells when needed,

like tooth stem cells to build new teeth.

Humans have these stem cells when we're younger,

but after our adult teeth grow in,

the stem cells die and disappear.

To understand why, let's take a journey back in time

to about 320 million years ago when mammals

and reptiles split off.

In addition to many obvious differences,

another change that eventually emerged was tooth shape.

Reptiles, for example, are what's called generalists,

meaning they eat the animals they can get their teeth on.

And for that, they needed teeth with the same size and shape

to keep prey from escaping their mouths.

Mammals, on the other hand, developed more specific diets,

like grazers who only eat grass and hunters who rip flesh

from their kills.

As a result, mammals evolved different-shaped teeth

for different purposes.

And it's this difference that could also explain

why mammals can't regrow more teeth.

Now, let's say you could regrow your molars multiple times, for example.

It's important that the top and bottom sets match up.

Otherwise, they can't grind food as efficiently.

That sounds good in principle, but with each new set,

there's a risk that the regrown teeth won't line up.

So the leading theory is that adult humans can't regrow

our teeth because it was better for survival

to only grow one well-aligned adult set.

However, if you still wish you could regrow a tooth,

there may be a way in the future.

Using lasers and drugs, scientists have helped rats and mice

regrow damaged tissue in cavity-ridden teeth

with the idea that if you could regrow tooth tissue,

you could eventually regrow entire teeth,

though no human testing has been done on that yet.

So keep flossing and keep seeing your dentist.

At least for now.