Molar crown thickness, volume, and development in South African Middle Stone Age humans

Introduction A current debate in palaeoanthropology centres on the issue of modern human origins, particularly the question of when ‘anatomically modern humans’ (AMH) from the African Middle Stone Age (MSA) became fully modern. Fossil Homo in Africa appears to have undergone a morphological transition from a more primitive form (i.e. Homo heidelbergensis/Homo rhodesiensis) to a more modern form (Homo sapiens) between 150 000 and 200 000 years ago. This evidence is based primarily on external cranial and dental features. Little is known about internal aspects of tooth structure such as tissue thickness/distribution, or patterns of growth, including the speed and duration of development. Studies of incremental dental development in early Homo show a pattern more similar to African apes than modern humans. Aspects of Homo heidelbergensis development also differ when compared to Upper Palaeolithic/Mesolithic populations. Research on brain growth in early Homo also suggests a more rapid period of early development than in modern populations. Given dietary changes and technological innovations during the Late Pleistocene and the Holocene, as well as dental size reduction in modern populations, it is unclear whether dental tissue thickness and development may have undergone corresponding changes, and when the unique, prolonged pattern of growth and development originated. This study represents the first combined investigation of molar enamel thickness, crown tissue volumes, and enamel development in a fossil hominin taxon, aimed at determining whether differences between AMH and living populations can be detected. Micro-computed tomographic (mCT) image data were collected from several molars from the South African MSA localities of Die Kelders and Equus caves; these specimens are from approximately 60 000–80 000 and 33 000–94 000 yr BP, respectively. Aspects of dental development were assessed from high-resolution casts, mCT scans, and confocal microscopy. Data on enamel thickness, crown volumes, and enamel development were compared with a large sample of modern humans from several geographical regions.