The main purpose of tooth extraction is to relieve the presence of crowding and protrusion of teeth. Among all patients with malocclusion, crowded teeth are the most frequent symptom. Why do modern people have more crowded and uneven teeth? This is first of all related to the evolution of human beings. During the evolutionary process from apes to humans, the human chewing organ was degenerating due to the increase of human civilization and the structure of diet from rough to fine. However, the degradation of teeth, bones and muscles is unbalanced, with teeth degrading the slowest, so the smaller jawbone of modern humans has to carry the same number of teeth as our ancestors, and teeth tend to become crowded and uneven. The food of children nowadays is generally high in sugar and too fine, which can easily induce tooth decay and early loss of milk teeth, and if the treatment is not timely, it will cause the obstacle of tooth replacement and produce crowded misalignment. For patients with more severe crowding, orthodontic treatment is usually carried out by reducing the number of teeth, i.e. extracting them. A few less functionally important teeth are extracted, and the gap created by the extraction is used to realign the other crowded teeth with orthodontic appliances. Some patients with prominent lips and exposed incisors (commonly known as “buck teeth”) are often caused by the anterior protrusion of the bicuspid arch. Although most of these patients do not have crowded teeth, it is often necessary to extract the bicuspid teeth on both sides and use the gap between extractions to relax the upper and lower lips, thereby improving the facial shape. Other patients have obvious occlusal disharmony between the upper and lower arches (such as “enamel” or deep overlap), and sometimes it is necessary to selectively extract the upper or lower teeth in order to remove the occlusal obstacle and use the gap to adjust the occlusal relationship.