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Teeth

I decided to talk about teeth today for purely personal reasons.

(Picture from here.)

Teeth are a structure unique to vertebrates. There are similar feeding structures in some invertebrates. Some molluscs have a tongue-like radula that is studded with chitinous bits that serve a similar function. But these bear no embryonic relationship with vertebrate teeth.

A tooth is a hard structure in the jaw or mouth of vertebrates possessing them. Some have deep roots as in mammals. Some do not. They use dentine as a hard structure that is often capped with enamel. Teeth development is quite different among the different vertebrate classes. Turtles and birds have lost their teeth, replacing them with what is assumed to be a less expensive alternative: beaks. In both turtles and birds, there was an evolutionary stage where both beak and teeth coexisted. Later, tooth growth was inhibited and only the beak remained. Interestingly, the platypus has no adult teeth but has an egg tooth similar to reptiles and birds to aid in hatching.

Once teeth were introduced to vertebrates, they were subject to natural selection and from that an enormous variation reflecting form and function. It is clear that carnivores have sharp, tearing teeth and herbivores have flat grinding teeth, though this is a serious simplification. However, there can be no natural selection without natural variation. One can deduce that since natural variation has been so well rewarded across vertebrates that variation in tooth expression has been preserved. You can’t get sharp tooth carnivores and flat tooth herbivores if there isn’t variation in tooth form for natural selection to act on.

The scales of sharks, called dermal denticles, and their teeth derive from the same embryological mechanism. Both have an interior cavity surrounded by dentine, a blood supply, and a nerve. This is one basis of the outside-in hypothesis of tooth origin.  The alternative hypothesis, the inside-out idea, is shown with extinct placoderms, who have tooth-like structures in their palate.

Mammals have a single jawbone, the mandible. Reptile jaws are made up of several different bones. The bones of the mammalian middle ear derive from those jaw bones in the reptile. Consequently, one would expect significant variation in tooth development.

Lizards have acrodont or pleurodont teeth. (See here.) Acrodont teeth are sharp and pointy. They’re not attached strongly to the bone. Lizards, for example, do not have tooth sockets. (See here.) Acrodont teeth are usually not replaced. Eventually, as the teeth wear, the biting surface becomes the underlying bone.

Pleurodont teeth have long roots. They are attached directly to the cheek side of the jawbone but not the tongue side. They are held in place with ligaments. While they are more strongly attached to the jawbone, they are not that strongly attached. Those big teeth on the Komodo Dragon Lizard? Pleurodont teeth.

There are ancient, extinct reptiles with complex tooth forms. (See Priosphenodon here.) These have similar complexity to mammalian teeth with dentine and enamel caps. These could have been inherited since mammals and reptiles diverged long ago. This is likely a case of convergent evolution in that selective pressures triggered Priosphenodons to come up with similar structures.

Crocodiles, on the other hand, have socketed teeth similar to mammals. As do dinosaurs. This kind of dentition is referred to as thecodont, and used to refer to the stem reptiles from which dinosaurs, pterosaurs, and crocodilians derived.

Amphibians often have teeth though some frogs and other groups have lost them. Those teeth that remain are either in the rear of the mouth or the front and are small. Amphibians have lost and re-evolved teeth several times over the last 200 million years.

Which brings us to mammalian teeth.

As I said before, mammals lost the complex lower jaw of reptiles, using a single bone, the mandible act as a jaw bone. Other bones of the jaw migrated to new locations and purposes or were lost entirely. Unlike lizards, mammalian teeth have deep roots, embedded in the mandible and surrounded by bone on all sides. It takes a significant effort to remove a mammalian tooth from the socket. I have personal experience with this.

Often, mammals are diphyodonts—they produce two sets of teeth in the organism’s lifetime. In humans, these are the baby teeth and the adult teeth. Monophylodonts have a single set of teeth for the organism. Polyphyodonts have more than two. Some are a blend. Elephants, for example, have a baby tooth tusk that is dropped and replaced by a permanent tusk. The tusk represents the incisor of mammal teeth. The elephant has six sets of four molars. As one wears down, it is replaced.

Humans—and, by extension, other primates—are typical mammals with diphyodont teeth. In our history, like many groups, we’ve had vegetarian relatives (e.g., gorillas and orangutans) and omnivores (humans and chimps.) Their teeth reflect that difference.

One would think that primitive human beings would have died out long before now because of the way their teeth decay. Decay requires a tooth surface, the appropriate bacteria, fermentable carbohydrates, and time. The latter two might have been in shorter supply in ancient human beings. I.e., fewer sugars and a shorter lifespan. This suggests farming might have been the problem.

However, a 15,000 year old find in Morocco, long before farming, gives evidence otherwise. This find showed that they were eating a particularly soft species of acorn that becomes sticky when cooked. Sweet material adhering to the teeth—well, you get the idea. Dental hygiene was not well known at the time. Long before, back in the days of Lucy, there was a shift in diet that caused a change in our ancestors’ teeth. (See also here, here, and here.)

I close with a quote from Jonathan Swift: “I’m as old as my tongue and a little older than my teeth.”

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