Mammalian teeth have evolved over many millions of years in response to the particular requirements of warm-blooded animals to catch and kill or graze and browse their food. Mammals like the hyena are able to crush bones with their teeth to get at the nutritious marrow. The elephant crushes shoots and leaves. While humans are not capable of such feats, we do have the essential equipment for cracking nuts and crunching raw vegetables. The design of mammalian teeth gives them strength to resist fracture and the sharp edges to slice, grind, and grate through fibrous foods. The strength of teeth comes from the contribution of two quite different materials. One, enamel is very hard but brittle. The other, dentin is softer but very tough. The combination provides what we recognize now as a composite material such as fiberglass. The sharpness of teeth comes from the fractured edges of enamel. For these edges to be exposed, some of the enamel coating on the tooth has to become worn away. In fact, wear is a prerequisite for optimal function of mammalian teeth. This chapter sets out the origins of teeth in mammals, which offer useful insights into the function of the jaws and teeth in modern man.
“Man is a fraction of the animal world. Our History is an after-thought, no more tacked on to an infinite calendar. We are not so unique as we would like to believe. And if man in a time of need seeks deeper knowledge concerning himself, then he must explore those animal horizons from which we have made our quick little march.” These words were written by Robert Ardrey in his book African Genesis. In order to understand the origins of teeth, it is worth reviewing their process of evolution.
The origins of teeth can be traced to dermal scales around the mouth which became modified to grasp food. For some animals, it was necessary to reduce the size of a piece of food in order to be able to swallow it. Smaller food particles are also easier to digest and offer rapid access to food energy essential to fuel the active metabolism of small mammals. It was the early mammals who used teeth in this way for the first time (see Appendix A.1 Mastication and Mammals). The work which a mammal’s teeth are required to perform can be compared to tool use in a workshop. There are different processes used in food preparation and the type of teeth required (see Appendix A.2 The Mechanics of Tooth Use).
The mammalian tooth is particularly well designed to be used as a tool. The surface enamel is the hardest substance produced by any living organism. The brittle nature of the enamel makes it vulnerable to cracking, but this tendency is reduced by the core of underlying dentin which provides the compressive strength and resilience that a solid enamel tooth would lack. The collagen fibers of the dentin run at right angles to the enamel prisms which further discourages the propagation of an enamel crack throughout the tooth. So, the two materials complement each other to form a composite material which is both very hard and resilient (see Appendix H.1 Cracks, Composites, and Teeth).
Two major requirements of a chewing tool are firstly, that each tooth has sharp cutting or grinding edges and secondly, that it allows the escape of processed food away from the cutting edges to avoid clogging. The mammalian enamel is characterized by long slender prisms which run from the amelodentinal junction to the surface. This is in contrast to the reptilian enameloid which is an amorphous crystalline structure; enameloid has, as a result, a higher compressive strength than enamel. The advantage of the mammalian enamel as a tooth tool is that when it wears, instead of just a few crystals being dislodged, a whole prism fractures away leaving a freshly sharp square edge behind (▶ Fig. 1.1).
Fig. 1.1 A diagrammatic representation of the influence of cusps on tooth wear. A cross section of a premolar tooth before wear reveals the core of dentin within each cusp. When the cusp has become worn and the dentin exposed, the dentin wears faster than the enamel and prevents clogging with food debris by providing an escape way for reduced food particles. The enamel edges are kept sharp by wear, as entire enamel prisms break away from the enamel surface leaving a sharp edge. The cusps provide an alternating surface of sharp enamel, and an escape way for food.