Your Inner Fish: A Journey Into the 3.5-Billion-Year History of the Human Body by Neil Shubin Page A

Farish’s team uncovered in Arizona, had very precise patterns of biting. Scrapes on the cusps of an upper tooth fit against mirror images of these scrapes on a lower tooth. These patterns of wear are so fine that different species of early mammals can be distinguished by their patterns of tooth wear and occlusion. Farish’s Arizona mammals have a different pattern of cusps and chewing than those of the same age from South America, Europe, or China. If all we had to compare these fossils to were living reptiles, then the origin of mammalian feeding would appear to be a big mystery. As I’ve mentioned, crocodiles and lizards do not have any kind of matching pattern of occlusion. Here is where creatures like tritheledonts come in. When we go back in time, to rocks about 10 million years older, such as those in Nova Scotia, we find tritheledonts with an incipient version of this way of chewing. In tritheledonts, individual cusps do not interlock in a precise way, as they do in mammals; instead, the entire inner surface of the upper tooth shears against the outer surface of the lower tooth, almost like a scissors. Of course, these changes in occlusion did not happen in a vacuum. It should come as no surprise that the earliest creatures to show a mammalian kind of chewing also display mammalian features of the lower jaw, skull, and skeleton.
     

A tritheledont and a piece of its upper jaw discovered in Nova Scotia. Jaw fragment illustrated by Lazlo Meszoley.
     
    Because teeth preserve so well in the fossil record, we have very detailed information about how major patterns of chewing—and the ability to use new diets—arose over time. Much of the story of mammals is the story of new ways of processing food. Soon after we encounter tritheledonts in the fossil record, we start seeing all sorts of new mammal species with new kinds of teeth, as well as new ways of occluding and using them. By about 150 million years ago, in rocks from around the world, we find small rodent-size mammals with a new kind of tooth row, one that paved the way for our own existence. What made these creatures special was the complexity of their mouths: the jaw had different kinds of teeth set in it. The mouth developed a kind of division of labor. Incisors in the front became specialized to cut food, canines further back to puncture it, and molars in the extreme back to shear or mash it. These little mammals, which resemble mice, have a fundamental piece of our history inside of them. If you doubt this, imagine eating an apple lacking your incisor teeth or, better yet, a large carrot with no molars. Our diverse diet, ranging from fruit to meat to Twinkie, is possible only because our distant mammalian ancestors developed a mouth with different kinds of teeth that can occlude precisely. And yes, initial stages of this are seen in tritheledonts and other ancient relatives: the teeth in the front have a different pattern of blades and cusps than those in the back.

    TEETH AND BONES—THE HARD STUFF

     
    It almost goes without saying that what makes teeth special among organs is their hardness. Teeth have to be harder than the bits of food they break down; imagine trying to cut a steak with a sponge. In many ways, teeth are as hard as rocks, and the reason is that they contain a crystal molecule on the inside. That molecule, known as hydroxyapatite, impregnates the molecular and cellular infrastructure of both teeth and bones, making them resistant to bending, compression, and other stresses. Teeth are extra hard because their outer layer, enamel, is far richer in hydroxyapatite than any other structure in the body, including bone. Enamel gives teeth their white sheen. Of course, enamel is only one of the layers that make up our teeth. The inner layers, such as the pulp and dentine, are also filled with hydroxyapatite.
    There are lots of creatures with hard tissues—clams and lobsters, for example. But they do not use hydroxyapatite; lobsters and clams