This study contains information on the occurrence of agenesis of one to four third molars among the population and ethnic groups of Europe, North America, Africa and Asia (Japan), based on the results of investigations carried out by dozens of authors. Recent discoveries have been supplemented with corresponding data concerning the skeletal remains of the jaws of individuals living from the ice age to the middle ages. The results show unbelievably large diversities as regards the frequency of agenesis of third molars in different populations from practically zero (Tasmania) to nearly 100% (Mexican Indians). This remarkable fact is not, however, the only reason why the problem of third molars remains a major concern for anthropologists, anatomists and stomatologists. Indeed, agenesis of third molars present only one of the most important anomalies which characterizes this tooth. It is necessary to look for the roots of the said variability in the remarkably complicated phylogenetic development undergone by the third molar from the time of our predecessors to the present day. Most of today's authors are of the opinion that although significant involution changes have occurred in third molars, that with the advent of hominization the number of teeth in individuals has stabilized at eight for every quadrant of dentition. For this reason it is not possible to regard agenesis of third molars as the manifestation of the phylogenetic reduction in the number of teeth, but as the developmental anomaly which is the result of a mutation and selection process based on heredity. The results are completed with an analysis of the value of agenesis of one to four third molars according to the individual quadrants and gender. A knowledge of these indicators is necessary in order to study the relationship between agenesis of third molars and hypodontia of the other teeth.