There is evidence that the teeth of humans and other higher vertebrates originated from the scales of ancient fish ancestors, which can be confirmed in existing sharks. In fact, shark's teeth are specialized scales. Meibomian and teeth are homologous organs with the same structure: there are also enamel and dentin, and there is a pulp cavity inside. There are nerves and blood vessels in the medullary cavity. The important watershed between mammals and their reptilian ancestors is not viviparous or milk secretion, but the differentiation of teeth and the appearance of bicuspids. The so-called bicuspid teeth mean that an animal has only two sets of teeth in its life, which is what we usually call primary teeth and permanent teeth. Reptiles change their teeth all their lives. Mammalian teeth are divided into incisors, canines, premolars and molars. Because of the differentiation of teeth, the chewing surfaces of upper and lower teeth must be completely closed when chewing. If animals change their teeth all their lives, it is possible to break this closure and affect their chewing function.

Classification of teeth

Mammalian teeth can be written as follows: (upper) incisors, (upper) canines, (upper) premolars, (upper) molars/(lower) incisors, (lower) canines, (lower) premolars and (lower) molars. For example, human teeth can be written as 2, 1, 2. It means that there are 2 incisors on the maxillary side, 1 canine teeth, 2 premolars, 3 posterior molars, and so is the mandible. Tooth type is an important basis for mammalian classification. Because human beings belong to primates, the layout and tooth types of human teeth are the same as those of other primates, that is, 2 1, 2,3/21,2,3. * * * has 32 teeth. However, human teeth are also different from those of other primates. The most striking thing is that human canine teeth are not as long and sharp as those of other primates. This is the most striking difference between hominids and other primates, and it is often used to distinguish hominids from apes in the study of hominids. Although human canine teeth are obviously degraded compared with other primates, human canine teeth are still the strongest compared with other human teeth. The roots of canine teeth are longer and stronger than all other teeth, and their roots are deeply embedded in the mandible. If you don't believe me, you can touch the gums and roots of your canine teeth and compare them with other teeth. There are gender differences in human canine teeth, which is an important reference for bone sex identification. In addition, the contraction of human maxilla and mandible makes many people's wisdom teeth, that is, the last molar, not erupt. Many people only have 28 teeth in their lifetime. The growth of teeth has a certain relationship with bones, but the growth of teeth and bones is not completely parallel because of different embryo sources. At birth, the deciduous teeth have been ossified, and the tooth spores of the deciduous teeth are hidden in the jaw and covered by gums; The ossification of permanent teeth began in neonatal period, and the ossification of the third permanent molar began at 18 ~ 24 months old. People have two sets of teeth: deciduous teeth (20 * * *) and permanent teeth (28 ~ 32). Primary teeth begin to erupt at 4 ~ 10 months after birth, and those who do not erupt after 12 months delay their eruption. The eruption sequence of deciduous teeth is generally mandible before maxilla, from front to back, deciduous teeth erupt at about 2.5 years old. The eruption time of deciduous teeth varies greatly among individuals, which is related to genetic, endocrine and food traits.