How did we end up with just two sets of teeth, our baby ones and our adult ones?

One of the proudest moments of your childhood is the day you lose your first tooth. For what seemed like forever, your front tooth was all wiggly and loose until one day…POP! out it comes. Maybe it was a bit unsettling at first, but as more and more of your teeth started to fall out (and maybe you grabbed some sweet cash from the tooth fairy in the process), you realized that this must be a normal thing. After all, your friends at school had the same thing happen to them. This happens to every kid. Well, almost every kid. We’ll get to that part.

Big smiles

Some very proud young ladies who have lost their front teeth! Photos courtesy of Jordan Mallon, Fiona Rawle, and Joe Aberant. Thanks for the smiles!

It becomes clear pretty quickly why your baby teeth fell out. You start to notice that newer, much larger teeth slowly fill in the gaps in your smile. But these new teeth stick around for the rest of your life, or, if they do fall out or have to be pulled out later in life, that’s it. No more new teeth. Why is that? Why do we have two sets of teeth and why can’t we make more of them once our adult teeth come in?

The short answer to this complicated question may surprise you. You’re weird. I don’t mean you personally (I’m sure you’re a great person), I mean you as a mammal are weird. You’re not weird because you replace your baby teeth with bigger adult ones, but because you only do this once in your life.

All mammals, including marsupials and monotremes (represented today by the echidna and the platypus) all share the ability to only replace their teeth a set number of times. Some teeth, like our molars, don’t get replaced at all. By comparison, if you were to look at a lizard, crocodile, or even a dinosaur, you’d find that they constantly replaced their teeth with new ones throughout their lives.

Tyrannosaurid tooth replacement

This cross-section through the jaw of a tyrannosaur (a meat-eating dinosaur…but you knew that already didn’t you… ;)) shows how often dinosaurs replaced their teeth. There are two developing replacement teeth  (2 and 3) that would have eventually replaced the first one. You would never see this in a jaw of a mammal.

How did we end up being so different? To answer this question, we have to go on a brief tour of some ancient animals and look at the teeth of our more distant, reptile-like ancestors. Yes, your distant ancestors, stretching back over 300 million years ago, looked more like reptiles than they did a furry little mammal. We will start with one of my favourite members of our evolutionary line: Dimetrodon. Dimetrodon was a sail-backed carnivore with sharp, serrated teeth.

1024px-Dimetrodon_grandis

An artist’s reconstruction of the sail-backed carnivore, Dimetrodon. A very distant relative of yours! By DiBgd at English Wikipedia, CC BY 2.5, https://commons.wikimedia.org/w/index.php?curid=2443149

Like lizards, crocs, or even dinosaurs, Dimetrodon replaced its teeth constantly. We know this, because fossils of these animals tend to preserve lots of teeth that are in the process of being replaced.

Dimetrodon tooth replacement

Dimetrodon replaced its teeth constantly. This cross-section through a Dimetrodon jaw shows one tooth that was in the process of being replaced when the animal died.

There are plenty of other extinct species from our evolutionary lineage that preserve this condition. It turns out that for most of our evolutionary history, our reptile-like ancestors constantly replaced their teeth. Then at some point along our evolutionary story, this pattern changed.

Over millions of years, the rate of tooth replacement in mammals has slowed and it continues to slow down. We now only replace our front teeth (incisors, canines, and premolars) and then add a few teeth at the back as adults (our molars). Even in modern humans, there are occasional blips in this “normal” pattern and some funny things can happen. For example, some people simply don’t develop wisdom teeth (your back molars). It’s not that they don’t come out into the mouth; they just don’t form at all. This is called third molar agenesis (meaning “no formation”). Even stranger, some people don’t even get to replace some of their baby teeth. Here is an example:

Pan.jpg

Panoramic X-rays of a person who didn’t develop wisdom teeth or replace some of their baby teeth (top), versus someone with the full number of adult teeth (bottom). Black arrows: empty spaces where wisdom teeth SHOULD BE in the top picture, position of wisdom teeth in the bottom picture; white arrows: baby teeth in the top photo, adult teeth in the bottom photo. Both of these people are adults and are about the same age.

Researchers have actually figured out HOW mammals stop replacing their teeth, thanks to several studies of modern mammals and reptiles. The organ that makes teeth is called the dental lamina. It’s a sheet (that’s where word “lamina” comes from) or string of cells that originate from our gums and it folds into our jaws to produce the outline of a developing tooth. From there it recruits surrounding cells to start forming the various parts of a tooth. The dental lamina in something like a lizard or a crocodile is a permanent structure that stretches across the jaw, constantly plunking down new teeth.

Caiman sclerops functional and replacement tooth wholeview slide

A thin section through a tooth in the jaws of a modern caiman (a relative of crocodiles). Near the bottom of the image, you can see a new tooth forming from the tooth-producing organ, the dental lamina.

In mammals, the dental lamina slowly begins to disintegrate after it forms the first sets of teeth. After the dental lamina disappears, no more teeth can be produced.

The jury is still out on WHY mammals are so unique. It would sure be great to just replace a tooth with a new one any time it breaks or develops a cavity. Some researchers think that the reason mammals have limited tooth replacement has more to do with the way our bodies grow than anything else. Teeth are odd structures, because they can’t grow in size once they break the gums and enter the mouth. In order to keep up with the growing jaws, reptiles replace smaller, older teeth with larger ones. Sometimes the differences in size can be dramatic, especially early on when the animal is growing really fast:

Vac-High PC-Std. 10kV x40 (8mm)

A Scanning Electron Microscope (SEM) image of a smaller tooth being replaced by a much larger one in the jaws of a baby alligator.

Unlike most reptiles, mammals tend to grow fast early on and then plateau when they reach adulthood. It’s thought that maybe this is the reason why we have evolved to stop replacing our teeth after our adult ones come in, given that they’re the right size for our jaws for the rest of our adult lives. It’s an interesting idea, but we’re still waiting for more evidence, especially from the fossil record, to answer this question more thoroughly.

If you enjoyed this blog entry and have a tooth-related question for me to tackle in a future entry, post a comment, send me an email at arl@ualberta.ca, or tweet me (@AaronLeBlanc6).

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