A tooth is a calcified structure that forms in the mouth and is used to chew or otherwise break down food. The teeth may also be used for self-defense. Teeth are rooted in the jaw and the roots and jaw are sheathed by the gums. Contrary to common misconception, teeth are made not of bone but of the same cellular material that produces skin, hair, and nails. In fact, the genetic processes that have created and evolved teeth are similar to the processes that generate scales in fish; it is believed that teeth are actually an evolutionary modification of scales. Many vertebrates have teeth, though the orodental structures vary considerably from species to species. For example, in some species, the teeth are deeply rooted in the jaws, while in others, the teeth may be attached to the jaw by external ligaments, as is the case with sharks, who generate a new set of teeth every two weeks.
Although other animal classes may also have teeth, teeth are a primary feature of mammals and their shape and positioning has evolved based primarily on the diet of the respective mammal. Most mammals develop two sets of teeth; this is exemplified in humans, whose primary teeth erupt at around 6 months of age and whose permanent teeth usually erupt in late childhood or early adolescence, after the primary teeth fall out. Some mammals, including kangaroos, elephants, and manatees, are exceptions; these species form multiple sets of teeth throughout their lifespans, repeatedly replacing one tooth or set of teeth with another. Humans, and many other primates, generally have 20 primary teeth and 32 permanent teeth. The teeth are divided equally between the top jaw, or maxilla, and the bottom jaw, or mandible.
In humans, the teeth function to cut and crush food to prepare it for digestion. Each type of tooth serves a different purpose in this overall function of mastication. Incisors are designed to cut food; canines tear food, and molars and premolars crush food. Animals that eat primarily vegetable matter, herbivores, have more molars than carnivores, or animals who eat primarily animal matter; carnivores have more developed canines. This is because vegetable matter needs to be ground considerably to allow it to be digested, whereas animal matter needs to be torn. Human teeth are made of dentin, cementum, enamel, and pulp, and they are rooted in the alveolar bones, or the bones of the jaw. In all species, teeth are formed from embryonic cells. Certain species are more prone to tooth decay than others, due both to diet and to the pH levels of the species’ saliva.
While most species’ teeth are inside their oral cavities, the tusks of elephants are actually external incisors, which are helpful for self-defense and for acquiring food. Walrus tusks are also external teeth, though these teeth are canines and serve a different purpose than the tusks of elephants. In many species, the permanent teeth erupt fully formed and fixed in size, while in some species, like rabbits, the teeth continue to grow throughout their lifespans. Because of their fiber-rich diet, rabbits’ teeth wear away naturally as they eat, so their constant growth allows them to maintain an ideal length and function. Teeth are resilient and fascinating indicators of evolution and diet. Often preserved in fossil form, teeth can provide invaluable information for archaeologists, paleontologists, and other scientific researchers.