Malocclusion and civilization.

Abstract Many civilized human groups have developed more severe malocclusions than they had experienced under primitive conditions of life. To the evolutionist, this change would seem rapid as compared with known rates of osseous and dental change in other mammals. He might ask whether natural selection favors malocclusion in civilized man and whether the present polymorphism of human occlusion is now reaching a stable genetic equilibrium. He might even wonder whether human hybridization under civilized systems of mating might have adverse effects on occlusion. Apart from such possible genetic trends, it is useful to define the environmental conditions under which the human face has evolved, especially by comparing facial growth in primitive and civilized children. Part of the story, at least, seems to involve differences in the use of the jaws and in attrition of the teeth. Where the aborigines of Australia live today under primitive conditions, their diet is tough and abrasive. In the aboriginal children the upper sutures of the face probably contribute less to the downward growth of the face than in civilized Caucasian children, and the eruptive distances traversed by their teeth are shorter, especially in the incisor region. The median palatine suture, however, seems to be more active, so that the maxillary dental arch is typically broad and low. These features also characterized the white population of Europe in the late Pleistocene period. The aboriginal child shows two cycles of occlusal change—one for the deciduous teeth and another for the permanent dentition. In infancy, overbite of the incisors and neutroclusion of the deciduous molars are transitory. By the age of 5 years, some children have experienced so much mesial movement and attrition that the incisors bite edge-to-edge. Now the permanent first molars and incisors erupt, and the overbite is temporarily regained. By maturity, however, the aborigine may already be well on his way toward a second edge-to-edge occlusion of the incisors. Attrition in the dental arches is usually so marked that crowding of the teeth is only a transient phase when new anterior teeth are erupting. In civilized children, resorption of the anterior aspect of the mandibular ramus seems to be discontinuous and coincides with the calcification of the crown of the most posterior molar tooth. Perhaps this dental stage is traversed in a more mesial position in the primitive mandible and with less resorption. Accordingly, the primitive mandible at maturity has a broader ramus than is seen in most civilized persons. These conditions may also prevent late eruption and impaction of the third molar. Evidence on how much the dentition of civilized man has become shrunken in size and on whether agenesis of the third molar is increasing is somewhat conflicting. It appears that orthodontic patients in particular show reduced facial musculature and skeletal development, but no meaningful separation of genetic entities among the types of malocclusion is yet possible. Malocclusions in civilized peoples should be thought of as complications over and above the basic polymorphism of occlusion in primitive man. The relationship of malocclusion to other skeletal changes in civilized populations is obscure. Since occlusal deformities are among the most common malformations of civilized man, the whole battery of techniques now used in research on other human deformities, especially prior to birth, should now be extended toward resolving the mysteries of malocclusion.