Wisdom Can Be Painful: The Evolutionary Origins of Third Molar Impaction in Humans

One of the major changes in the course of human evolution has been the dramatic increase in brain size.  This expansion modified the shape and dimensions of the braincase, in particular, broadening skull width and widening the distance between the mandibular condylar articulations with the braincase base. This architectural reconfiguration of the skull resulted in the repositioning of the face more underneath the base of the braincase.  One consequence of these changes has been the widening and a relative shortening of the dental arcade.  The shortened dental arcade in living humans, in turn, is no longer large enough to house the complete adult dentition.  As a result, in many adolescent humans, the last tooth to erupt in human dental development, the third molar ('wisdom tooth'), cannot erupt normally into its occlusal location with opposing teeth but ends up either out of occlusion or impacted,  unerupted, in horizontal contact with the second molar in front of it or with soft tissues. This condition can lead to chronic pain and to reduced reproductive fitness. Some thousands of years ago, a probable genetic change, or changes, brought about the suppression, or agenesis, of the calcification of the third molar. Recent research has isolated a number of possible mutations that may have been responsible for the loss of the third molar. Because it reduced the incidence of third molar malocclusions and impactions, there was positive selection for this variation and the frequency of third molar agenesis increased, with 25% of individuals in many human populations lacking at least one third molar. With the development of medical intervention in the developed world, the positive selective value of agenesis has been reduced but it remains a ‘scar of human evolution’ in much of the developing world.