Tooth wear and dentoalveolar remodeling are key factors of morphological variation in the Dmanisi mandibles

Abstract

The Plio-Pleistocene hominin sample from Dmanisi (Georgia), dated to 1.77 million years ago, is unique in offering detailed insights into patterns of morphological variation within a paleodeme of early Homo. Cranial and dentoalveolar morphologies exhibit a high degree of diversity, but the causes of variation are still relatively unexplored. Here we show that wear-related dentoalveolar remodeling is one of the principal mechanisms causing mandibular shape variation in fossil Homo and in modern human hunter-gatherer populations. We identify a consistent pattern of mandibular morphological alteration, suggesting that dental wear and compensatory remodeling mechanisms remained fairly constant throughout the evolution of the genus Homo. With increasing occlusal and interproximal tooth wear, the teeth continue to erupt, the posterior dentition tends to drift in a mesial direction, and the front teeth become more upright. The resulting changes in dentognathic size and shape are substantial and need to be taken into account in comparative taxonomic analyses of isolated hominin mandibles. Our data further show that excessive tooth wear eventually leads to a breakdown of the normal remodeling mechanisms, resulting in dentognathic pathologies, tooth loss, and loss of masticatory function. Complete breakdown of dentognathic homeostasis, however, is unlikely to have limited the life span of early Homo because this effect was likely mediated by the preparation of soft foods.

Keywords: aging; compensatory mechanisms; early Pleistocene Homo; local periodontitis; toothpick.

Fig. 1.

Mechanisms of in vivo dentoalveolar remodeling. Continuous eruption (CE) is tracked by the distance between the mandibular canal and root apices. Mesial drift (MD) is tracked by the length of the posterior dental arcade from M2 to P3. Lingual tipping (LT) is tracked by the angle of inclination of the anterior teeth (incisors and canines) relative to the mandibular canal.

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.

Fig. 3.

D2735 toothpick lesion. (A) Parallel horizontal scratches on the disto-buccal surface of the RM1. (B) CT cross-section at the level of RM2: arrow indicates local marginal periodontitis. En, enamel; Ce, cementum.

Fig. 4.

Wear-related dentoalveolar remodeling in fossil Homo and modern humans. (A) Relative continuous eruption of first molars (mean of L and R). (B) Relative mesial drift (rMD). (C) Lingual tipping of canines. (D) Relative corpus height at M1. Red symbols and regression lines: Dmanisi [stars, from left to right: D2735 (DW2), D211 (DW3), D2600 (DW8), D3900 (DW9)] and KNM-WT15000 (plus sign); black diamonds and black-stippled line, Tighenif; black squares and solid line, Atapuerca SH; gray symbols and lines, H. sapiens Australia (normal triangles and solid line) and Greenland (inverted triangles and dashed line). Regression statistics are summarized in Table S4.