Tooth Wear and Dentoalveolar Remodeling Are Key Factors of Morphological Variation in the Dmanisi Mandibles
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Tooth wear and dentoalveolar remodeling are key factors of morphological variation in the Dmanisi mandibles Ann Margvelashvilia,b,1, Christoph P. E. Zollikofera, David Lordkipanidzeb,1, Timo Peltomäkic, and Marcia S. Ponce de Leóna aAnthropological Institute and Museum, University of Zurich, CH-8057 Zurich, Switzerland; bGeorgian National Museum, 0105 Tbilisi, Georgia; and cOral and Maxillofacial Unit, Department of Ear and Oral Diseases, Tampere University Hospital and Department of Otolaryngology, Tampere University, FI-33521 Tampere, Finland Contributed by David Lordkipanidze, September 2, 2013 (sent for review May 25, 2013) The Plio-Pleistocene hominin sample from Dmanisi (Georgia), dated iii) In the anterior dentition, remodeling induced by interproxi- to 1.77 million years ago, is unique in offering detailed insights into mal wear results in increased lingual tipping; that is, teeth patterns of morphological variation within a paleodeme of early become more upright relative to the alveolar plane (7). Homo. Cranial and dentoalveolar morphologies exhibit a high As an effect of mesial drift, interproximal contacts between degree of diversity, but the causes of variation are still relatively adjacent postcanine teeth are preserved. Similarly, as an effect unexplored. Here we show that wear-related dentoalveolar re- of lingual tipping, interproximal contacts between incisors are modeling is one of the principal mechanisms causing mandibular ’ shape variation in fossil Homo and in modern human hunter– preserved. During an individual s lifetime, the combined, accu- gatherer populations. We identify a consistent pattern of mandib- mulated effects of continuous eruption, mesial drift, and lingual ular morphological alteration, suggesting that dental wear and tipping tend to result in an edge-to-edge bite in the front den- fi compensatory remodeling mechanisms remained fairly constant tition (7) and in signi cant changes in mandibular morphology throughout the evolution of the genus Homo. With increasing oc- (13, 20). clusal and interproximal tooth wear, the teeth continue to erupt, The site of Dmanisi, Georgia, has yielded a crucial sample of ANTHROPOLOGY the posterior dentition tends to drift in a mesial direction, and the early Pleistocene hominin fossils along with a rich vertebrate front teeth become more upright. The resulting changes in dentog- fauna and mode I (Oldowan) lithic implements (21, 22). Site nathic size and shape are substantial and need to be taken into occupation began shortly after 1.85 Ma (million years ago) (23); account in comparative taxonomic analyses of isolated hominin the fossils are dated to 1.77 Ma (21, 23–26). The Dmanisi mandibles. Our data further show that excessive tooth wear even- hominin sample comprises five individuals that document den- tually leads to a breakdown of the normal remodeling mechanisms, tognathic development from adolescence to old age, thus pro- resulting in dentognathic pathologies, tooth loss, and loss of mas- viding a unique opportunity to study wear-related dentognathic ticatory function. Complete breakdown of dentognathic homeosta- variation in a sample that comes from a single point in space and sis, however, is unlikely to have limited the life span of early Homo geological time (26–29). Various hypotheses have been proposed because this effect was likely mediated by the preparation of to explain the high degree of variation seen in the Dmanisi soft foods. mandibles, ranging from intrataxon sexual dimorphism to intertaxon aging | compensatory mechanisms | toothpick | local periodontitis | Significance early Pleistocene Homo The mandibles of the early Pleistocene hominins from Dmanisi, lthough patterns of dental micro- and macrowear and wear- Georgia, exhibit wide morphological variation, but the causes Arelated pathologies are amply documented in the hominin of diversity remain largely unknown. Here we quantify the fossil record (1–5), processes of in vivo dentoalveolar remodeling effects of heavy tooth wear and wear-related bone remodeling (3, 6, 7) and their potential influence on dentognathic mor- on mandibular variation. Using modern hunter–gatherer pop- phology are only beginning to be studied in fossil hominins (8). ulations as a reference, we show that features such as dental In modern human hunter–gatherer populations, remodeling of arcade form, mandibular corpus height, and symphyseal in- dentoalveolar hard tissue is triggered mainly by dental wear, clination change substantially with progressive tooth wear. These data indicate that the Dmanisi mandibles reflect normal aging, pathologies, and trauma. Wear-related remodeling can be within-population variation augmented by interindividual dif- understood as a mechanism of in vivo modification that main- – ferences in wear-related bone remodeling. Dmanisi further tains masticatory function (3, 7, 9 14). Three main processes are provides the first clear evidence for toothpick-induced local fi typically identi ed (Fig. 1): periodontitis. This study illustrates how excessive tooth wear i) Wear-induced reduction of dental crown height leads to alter- leads to dentognathic pathology and complete tooth loss, ations in masticatory biomechanics. This triggers alveolar bone which was evidently compensated for by culturally mediated remodeling, yielding dislocation of dental structures and even- food processing. tual continuous eruption of all teeth. As an effect, occlusal contact between upper and lower teeth is maintained (15–17), and the Author contributions: A.M., C.P.E.Z., D.L., T.P., and M.S.P.d.L. designed research; A.M., C.P.E.Z., and M.S.P.d.L. performed research; A.M., C.P.E.Z., and M.S.P.d.L. contributed position and orientation of the occlusal plane relative to the new reagents/analytic tools; A.M., C.P.E.Z., D.L., and M.S.P.d.L. analyzed data; and A.M., temporomandibular joints (TMJs) is held approximately con- C.P.E.Z., and M.S.P.d.L. wrote the paper. stant, thus preventing the “wear-out” (18) of the TMJs. The authors declare no conflict of interest. ii ) The reduction of mesiodistal crown dimensions through inter- 1To whom correspondence may be addressed. E-mail: [email protected] or proximal dental wear triggers alveolar bone remodeling in the [email protected]. mesiodistal direction. This leads to mesial drift of the post- This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. canine dentition and shortening of the dental arcade (19). 1073/pnas.1316052110/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1316052110 PNAS Early Edition | 1of6 Downloaded by guest on September 25, 2021 the middle Pleistocene mandibles from Tighenif (n = 3) and Atapuerca Sima de los Huesos (SH) (n = 4); and mandibles of modern hunter–gatherer populations from Australia (n = 26) and Greenland (n = 15) (Table S1), which all show substantial intragroup variation in dental wear. Measurement protocols are specified in Materials and Methods and in Table S2; abbreviations are listed in Table S3. It is well known that dental wear rates (amount of wear per year of life) vary substantially between Fig. 1. Mechanisms of in vivo dentoalveolar remodeling. Continuous eruption populations as an effect of differences in food properties (e.g., (CE) is tracked by the distance between the mandibular canal and root apices. abrasiveness) and paramasticatory tooth use (32, 33). To permit Mesial drift (MD) is tracked by the length of the posterior dental arcade comparisons between different populations, remodeling rates are from M2 to P3. Lingual tipping (LT) is tracked by the angle of inclination of thus calculated per wear stage. the anterior teeth (incisors and canines) relative to the mandibular canal. Results and Discussion – The Dmanisi mandibles compose a wide spectrum of wear- variation (25, 27 31). Here we focus on in vivo dentognathic re- related dentognathic variation from moderate overall wear in the modeling as a potential mechanism contributing to the remark- subadult D2735 and young adult D211 individuals to wear- able dentognathic variation in Dmanisi and differentiate normal induced pathological changes in the old adult individual D2600 remodeling processes from pathologic alterations. Using direct and edentulousness in the senile individual D3900. observations and data derived from computed tomography (CT) Mandible D2735 is associated with the subadult cranium and scanning electron microscopy (SEM), we quantify dental D2700 (27, 34). The socket of the right third molar (RM3) wear (DW), continuous eruption (CE), mesial drift (MD), and contained a tooth bud, which was found in isolation. The left side lingual tipping (LT) in the following groups: the Dmanisi man- bears no evidence of an M3 socket, possibly indicating LM3 dibles [specimens D2735, D211, D2600, D3900 (Fig. 2 and Fig. agenesis (see refs. 35 and 36 for similar cases). Although the S1) and KNM-WT 15000 (grouped as early Pleistocene Homo)]; second molars (M2) exhibit moderate occlusal wear, the first molars Fig. 2. Dmanisi mandibles. Right lateral and occlusal views are photographs taken from original specimens; left lateral views are CT-based 3D reconstructions highlighting internal structures. The canines and incisors of D2735 were found in isolation and digitally reinserted into their sockets. Arrow indicates toothpick lesion. 2of6 | www.pnas.org/cgi/doi/10.1073/pnas.1316052110 Margvelashvili et al. Downloaded by guest on September 25, 2021 (41–44). The external symphyseal surface below the incisor roots shows evidence of