Why do we have
baby teeth?
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Your kid is six. After days of wiggling, a front tooth finally falls out. But around the same time, a big tooth quietly appears at the very back. Lots of parents think it's a baby tooth too. It isn't. It will never be replaced. So why do we grow one set of teeth, only to swap it for another? Let's follow one adult front tooth, from the very beginning.
This tooth starts forming before the baby is even born. It hides deep in the gum, right underneath a baby tooth. Teeth have one odd rule: the hard outer shell is made in one go. The cells that build it are gone once the tooth comes in. So a tooth stays the size it came in, for life. It's like a kid's shoes: feet grow, shoes don't. When they get tight, you buy a new pair.
Then why not grow adult teeth from the start? They wouldn't fit. Adult teeth are big and wide, and a baby's tiny jaw has no room. But kids start chewing food at around one. They can't wait for the jaw to grow. So the body hands out a small set first: twenty baby teeth, small and few, all in by about age three, just enough to fill a little jaw.
The big set waits in line underneath. A few years later, the jaw has grown, and gaps often open up between the baby teeth. Our front tooth is ready now, and starts pushing up. As it rises, it works on the root of the baby tooth above it: dissolving it, bit by bit.
No root, and the baby tooth comes loose. That's why a fallen baby tooth almost never has a long root. Baby teeth aren't just stand-ins: they hold the spot for the tooth below. Lose one too early to decay, and its neighbors drift into the gap, so the adult tooth comes in crooked. Now, back to that big tooth that shows up at six. There's no baby tooth above it. It grows into empty space at the very back. Since nothing fell out first, adults rarely notice it, and cavities in it get ignored. But it never gets replaced. Damage there is for life.
So why can't we be like sharks, and just grow another one? Some sharks lose more than thirty thousand teeth in a lifetime. More than two hundred million years ago, our ancestors were down to a single swap. One guess from scientists: to chew, upper and lower teeth must fit together tightly. Teeth that keep changing would never line up. That's still not settled. So baby teeth are a pair of kids' shoes: worn while the jaw is small, holding the spot, until the jaw grows, and the lifelong pair goes on.
Tonight, ask your kid to open wide, count back from the middle front tooth: is number six there yet? Tell us how old your kid is, and whether it's in.
The big tooth
that shows up at six
Around age six, the front teeth start wiggling and falling out. About the same time, a big tooth quietly appears at the very back of the mouth. Nothing fell out to make room for it, so adults rarely notice it, and often assume it's a baby tooth.
It isn't. It will never be replaced.
To see why, start with the bigger question: why grow one set of teeth, only to swap it for another?
A tooth stays
the size it comes in
A tooth's hard outer shell is made in one go. The cells that build it are gone once the tooth comes in. So teeth can't grow along with you.
Like kids' shoes: feet grow, shoes don't. When they're tight, you get a new pair.
Adult teeth
don't fit a tiny jaw
Why not grow adult teeth from the start? They wouldn't fit: adult teeth are big and wide, and a baby's jaw is tiny. But kids start chewing food at around one. They can't wait for the jaw to grow.
So a small set comes first: twenty baby teeth, all in by about age three, just right for a little jaw.
The adult set, thirty-two teeth in all, waits in line underneath.
The adult tooth pushes up
and dissolves the root
An adult front tooth starts forming before the baby is even born, hiding below the baby tooth. Years later the jaw has grown, and gaps often open between the baby teeth. The adult tooth is ready, and starts pushing up.
As it rises, it dissolves the root of the baby tooth above it, bit by bit.
No root, and the baby tooth comes loose. That's why a lost baby tooth almost never has a long root.
Baby teeth
save the seat
Baby teeth aren't just stand-ins. Lose one too early to decay, and its neighbors drift into the gap. The adult tooth then has no room, and comes in crooked.
They fall out anyway,
but still need brushing.
Take extra care with that big tooth that shows up at six. It doesn't replace anything; it grows into empty space at the back. A cavity there is for life.
Sharks swap thousands,
we swap once
Some sharks lose more than thirty thousand teeth in a lifetime, growing a new one each time. But over two hundred million years ago, our mammal ancestors were down to a single swap.
One guess from scientists: to chew, our upper and lower teeth must fit together tightly, and teeth that keep changing would never line up. That's still not settled.
Why animal teeth differ: see why some animals eat meat and others eat plants. Puppies swap teeth too: see why puppies smell like milk.
saving the seat.
and count back from the middle front tooth.
Is number six there yet?
Quick check
Why do we grow baby teeth first, then adult teeth?
Like kids' shoes: feet grow, shoes don't. A small jaw wears a small set until it's ready for the lifelong one.
