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1 - Department of Cultural Heritage, University of Bologna, Ravenna, Italy; 2 - Department of Human Evolution, Max Planck Institute for Evolutionary Anthropology, Leipzig, Germany;

3 - University of Ferrara, Ferrara, Italy; 4 - UCD School of Medicine, Health Sciences Centre, University College Dublin, Ireland.

3D enamel thickness in Neanderthal and modern human permanent incisors

Laura Buti1, Adeline Le Cabec2, Noemi Dipino3 , Jean-Jacques Hublin2, Stefano Benazzi1,2, Robin N.M. Feeney4 Max Planck Institute for

Evolutionary Anthropology

UNIVERSITA DI BOLOGNA

DEPARTMENT OF CULTURAL HERITAGE laura.buti4@unibo.it

www.unibo.it/sitoweb/laura.buti4

This research was funded by the Max Planck Society and the European Research Council (ERC) under the European Union Horizon 2020 Research and Innovation program (Grant agreement No. 724046– SUCCESS); www.erc-success.eu

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Results

Permutation tests for 3D AET values do not differ between NEA and modern humans (UPMH+RMH) in all four incisor positions. By contrast, mean values for 3D RET significantly differ in LI1 between the two groups for worn teeth at wear stages 4 and 5 (both p<0.05) (Fig.2 a,b,c). Moreover, NEA and MH show significantly different 3D RET values in LI2 at wear stages 1-3 and 5 (p=0.0085; p=0.027; p=0.02 respectively) (Fig. 2 d,e). On the other hand, the UI1s do not differ in 3D RET values between the two groups at wear stages 3 and 4 (p=0.13; p=0.34, respectively) (Fig.3 b,c). Due to different sample sizes between wear stages 1-2 and 5, statistical comparisons were not possible (Fig. 3 a,d). Finally, 3D RET values did not differ for UI2 at wear stages 1-2 and 4 (p=0.17; p=0.3, respectively) (Fig. 3 e,f). The limited sample size for wear stages 3 and 5 only allow for a qualitative investigation, which revealed the same non discriminant result.

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References [1]Olejnickzak, A.J. et al., 2008. J. Hum. Evol. 55:12-23 [2]Buti, L., Le Cabec, A. et al., 2017. J. Hum. Evol. 113:162-172 [3]Benazzi, S. et al., 2014. Am. J. Phys. Anthropol. 153(2):305-13 [4]Smith, B.H., 1984. Am. J. Phys. Anthropol. 63:39-56 [5]Clement, A.F. et al., 2012. J. Hum. Evol. 62:367-376 [6]Fornai, C. et al., 2014. J. Hum. Evol. 76:83-9

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Introduction The study of enamel thickness (ET) distribution in living and extinct hominoids for taxonomic, dietary and phylogenetic purposes has so far focused on permanent teeth, particularly postcanines, benefiting from novel 3D methodologies [1]. Growing attention is brought to the anterior dentition [2], thus fostering the application of a recent protocol [3] to investigate the 3D ET in Neanderthal and modern human (MH) unworn to variously worn upper and lower permanent incisors.

This research aims to:

1) provide new data to discriminate between the two groups

2) explore ET distribution in Neanderthal and MH incisors

Fig. 1 3D Enamel volume (green) and enamel dentine junction (EDJ, red) of the Neanderthal lower right second incisor of Combe Grenal I. La=labial; Li=Lingual. White scale bar: 25mm.

Materials and Methods

MicroCT data of 88 upper (UI1=48; Ul2=41) and 120 lower (Ll1=54; U2=64) permanent incisors from Neanderthals (NEA, upper=38; lower=35) (Fig.1), Upper Paleolithic (UPMH, upper=8; lower=13) and recent modern humans (RMH, upper=37; lower=80) at different wear stages (1 to 5 according to [4]) were segmented in Avizo 7 to generate 3D digital models. The cervical line was digitized on each 3D model in Geomagic Design X to separate the crown from the root dentine. Volumes of enamel and of crown dentine and the enameI-dentine junction (EDJ) surface were measured to compute the 3D Average Enamel Thickness (AET) and 3D Relative Enamel Thickness (RET) indices.

Fig. 2 (left) and Fig. 3 (right) 3D RET box plots (standard deviation interquartile method) for lower and upper incisors at wear stages 1-2, 3, 4 and 5 for Neanderthal and modern humans.

Fig. 4 and Fig. 5 3D enamel thickness distribution maps in a subsample of Neanderthal and modern human lower (Fig. 4) and upper (Fig. 5) incisors (wear stages 1-2) visualized using spectral colors. While the thickest enamel is represented in red, the thinnest enamel appears in violet (see color-scale with the corresponding enamel thickness in mm). La=labial, Li=lingual. White scale bar: 1 cm.

Discussion Our preliminary results suggest that 3D RET may successfully discriminate between NEA and MH lower incisors, including in worn teeth. However, while worn LI1 can be used for discrimination, due to the small sample size of unworn UI1 caution is needed when using this incisor position. The consistent results of LI2 allow us to consider this tooth position the most effective of the four to discriminate between NEA and MH, ranging from unworn to moderately worn. On the other hand, upper incisors do not discriminate between the two groups. In most tooth positions, NEA show statistically greater values for both enamel and dentine volumes, with MH showing greater variability among the single dental components values. We can argue that for some reasons to be determined, even though the amount of enamel and dentine volumes are generally greater in NEA, upper and lower incisors follow different configurations which results in different 3D RET discrimination. This trend partially reflects that of canines [2], with lowers showing slightly more significant discriminant value between the two groups, even though both upper and lower canine positions were found to be discriminant. Moreover, as for molars, Neanderthal tissue proportions evince less variation than the modern human sample. Size reduction usually explains many of the dental differences between Neanderthal and modern humans, but interestingly, as for canines morphological features that usually characterize Neanderthal anterior teeth (canine linguaI tubercle, shovel shaped incisors), they do not appear to be associated with thicker enamel rather to a major dentine presence underlying the EDJ (Fig. 4,5). In a recent study, according to the anterior dental loading theory [5], supported also by their root morphology, it has been suggested that Neanderthals have applied considerably powerful anterior dental loads than modern humans. This further suggest that the increased dental wear on their anterior teeth could only be the result of persistent use. At the same time, the results of this study, however, do not demonstrate that Neanderthals actually possess an increased amount of wear on their anterior teeth, relative to their posterior teeth, compared to samples of both Late Pleistocene hominins and recent huntergatherers. Because these human groups are characterized by strongly different craniofacial morphology when compared with Neanderthal, this last consideration of tooth wear only partially supports the anterior dental loading theory.

Conclusion

An increased sample size may allow for a better understanding of the enamel distribution in incisor teeth of Neanderthal and modern humans (Fig. 4,5), in which to better provide insight to explain the result of 3D RET. It is reasonable to assume that the thickened enamel on the labial aspect of the crown and the reduced enamel on the lingual aspect, similar in NEA and MH, couId be related to the fact that tension forces are more damaging to enamel than that of compression. At the same time, we cannot rule out the hypothesis that the general greater dentine volume in Neanderthals could be an additional reinforcement feature of dental biomechanics, i.e., for withstanding and distributing the occlusal loading, due to the predominant assumption of the use of anterior teeth as tools, witnessed by the presence of a major amount of dentine corresponding to external features such as tubercles in the canines and shoveling in the anterior teeth that could be considered such an advantage to face iterative paramasticatory activities. Moreover, in recent studies Neanderthals have been demonstrated to display wear across the anterior dentition that is not observed in any of the modern human groups. This in combination with the preliminary absence of 3D RET discrimination in upper incisors between the two groups, strongly suggest to deeper explore enamel distribution of the upper tooth classes to understand the role of upper dentition according to Neanderthal’s peculiar facial morphology. Notwithstanding, as worn teeth are more frequently found in the fossil record [6], the discriminant power of 3D RET for lower incisors may be considered as a useful tool for taxonomical purposes.

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