High-resolution measurement of Neandertal tooth enamel thickness by micro-focal computed tomography

THE BIOLOGICAL DISTINCTIVENESS OF Neandertals remains the focus of intense palaeoanthropological research and debate. It has been claimed that Neandertals are characterized by having thin molar tooth enamel in comparison to penecontemporaneous humans from North Africa and the Levant and to recent people. Thin enamel has been related to evidence for an accelerated rate of crown formation in the Neandertals. This, in turn, has been interpreted as signifying a ‘major difference’ from modern humans in the early stages of somatic development and, more particularly, a notably shorter maturational period. This developmental difference has been employed to support a specific distinction for H. neanderthalensis. Other workers, however, have interpreted thin tooth enamel in Neandertals as a manifestation of their chronic ‘suboptimal health’. Like enamel hypoplasia, which has been reported to affect Neandertal teeth in high frequencies, thin enamel has been viewed as a result of ‘metabolic depressions which affect/arrest the development of mineralized tissue.’ If thin enamel is related solely to systemic stress in Neandertal populations, it does not speak to their rate of somatic development and is mute on the issue of their biological distinctiveness. However, a recent analysis of the incidence of linear enamel hypoplasia in a somewhat more representative sample of Neandertals concluded that they were no more stressed than present-day foragers such as the Alaskan Inuit. If enamel thickness is demonstrably thinner in Neandertals than in penecontemporaneous humans, this would be useful in the diagnosis of fossils (for example, the Tabun C2 mandible) whose attribution is otherwise debated. While enamel thickness in modern humans has been well-documented from physically prepared crown sections, all studies of Neandertals to date have been based on lateral flat-plane radiographs. This method, however, has been shown to yield inaccurate measurements of enamel thickness when compared to physical sections. Thus, despite all that has been made of thin tooth enamel in Neandertals, it remains to be demonstrated whether, in fact, they actually differ from modern humans in this regard. Micro-focal computed tomography (μ-CT) is a non-destructive method by which tooth enamel thickness can be measured accurately. In order to quantify enamel thickness in a Neandertal nondestructively, the maxillary right third molar (M) of the Shanidar 3 specimen was scanned using a Scanco μ-CT-20 machine (Scanco Medical, Switzerland). This produced 635 transversely-orientated slices at 36-μm intervals, in which the enamel–dentine junction could be seen clearly. From these data, we constructed a high-resolution virtual model of the tooth (Fig. 1a,b). This was virtually sectioned through the middle of the buccal and lingual convexities, approximating the transverse axis through the protocone and paracone (Fig. 1c). The printed image of the μ-CT virtual cross section of the Shanidar 3 M was measured by digitization using a SummaSketch III (CalComp, Inc.) tablet. Five measurements were recorded from this section in order to calculate an index of relative enamel thickness (Fig. 2). For comparison, homologous regions of enamel in equivalent planes of section were measured from photographs of whole crown cross sections for a large sample of human maxillary molars (n = 35), and a small sample of chimpanzee molars (n = 5). It has been shown from physical crown sections that chimpanzees News & Views South African Journal of Science 101, May/June 2005 219

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