Tertiary dentine frequencies in extant great apes and fossil hominins

Tertiary dentine forms when an odontoblast is directly affected by stimuli, commonly through occlusal wear. In this study the presence of tertiary dentine is recorded in three South African fossil hominin species ( Australopithecus africanus , Homo naledi and Paranthropus robustus ), and two extant great ape species ( Gorilla gorilla gorilla and Pan troglodytes ). Frequencies of tertiary dentine were calculated for each species based on macroscopic observations of teeth with dentine exposed through occlusal wear. Overall, the three hominin species have similar tertiary dentine frequencies ranging from 12% to 16.13%. In contrast, over 90% of gorilla teeth with dentine visible show tertiary dentine. Chimpanzees fall between these extremes with 47.21% of teeth affected. Species variances are not related to differences in occlusal wear. Instead, some species appear predisposed to produce tertiary dentine earlier and/or faster than other species. Therefore, tertiary dentine formation has the potential to provide useful information on fossil specimens. For example, the uniformly low rate of tertiary dentine formation in hominins may be due to thick enamel having a similar role in preventing loss of function of teeth, i.e., extending the life of a tooth. In contrast tertiary dentine is clearly an important mechanism for normal dental function in gorillas, and may have evolved to maintain sheering surfaces for masticating tough vegetation.