Scaling of tooth enamel thickness, and molar crown size reduction in modern humans

Tooth enamel thickness is important in the interpretation of hominoid evolution, but its comparison among teeth of differing size requires the use of a scaling factor to negate the effects of such size differences. There is debate over whether the area of the crown's dentine core or the linear distance between its buccal and lingual cervical margins is the most appropriate scaling factor. These two variables are significantly correlated in modern humans, contradicting arguments that one should be preferred over the other. Nevertheless, they do not necessarily convey the same information because they respond differently to variation in cervical extension, and because upper and lower molars differ in their buccolingual diameters. The dentine core and enamel cap exhibit a positive allometric relationship in deciduous molars, which have significantly thinner enamel than permanent crowns, but they are negatively allometric among permanent molars. Here the dentine core decreases in size, while the enamel cap increases in relative thickness distally along the molar row. Thus, the well-documented overall size reduction from M1 to M3 in modern humans is attained primarily through a differential loss of the tooth's dentine component. The relatively thicker enamel of human third molars can be explained as a simple consequence of tooth size reduction rather than as a functional adaptation.