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 huntergatherer 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.aging | compensatory mechanisms | toothpick | local periodontitis | early Pleistocene Homo A lthough patterns of dental micro-and macrowear and wearrelated pathologies are amply documented in the hominin fossil record (1-5), processes of in vivo dentoalveolar remodeling (3, 6, 7) and their potential influence on dentognathic morphology are only beginning to be studied in fossil hominins (8). In modern human hunter-gatherer populations, remodeling of dentoalveolar hard tissue is triggered mainly by dental wear, aging, pathologies, and trauma. Wear-related remodeling can be understood as a mechanism of in vivo modification that maintains masticatory function (3,7,(9)(10)(11)(12)(13)(14). Three main processes are typically identified (Fig. 1): i) Wear-induced reduction of dental crown height leads to alterations in masticatory biomechanics. This triggers alveolar bone remodeling, yielding dislocation of dental structures and eventual continuous eruption of all teeth. As an effect, occlusal contact between upper and lower teeth is maintained (15-17), and the position and orientation of the occlusal plane relative to the temporomandibular joints (TMJs) is held approximately constant, thus preventing the "wear-out" (18) of the TMJs. ii) The reduction of mesiodistal crown dimensions through interproximal dental wear triggers alveolar bone remodeling in the mesiodistal direction. This leads to mesial drift of the postcanine dentition and shortening of the dental arcade (19).iii) In the anterior dentition, remodeling induced by interproximal wear results in increas...

Comparative analysis of dentognathic pathologies in the Dmanisi mandibles

“…Dental wear was scored using an extended Molnar scale (Molnar 1971;Margvelashvili et al 2013) (see Fig. 2 for dental wear frequency distribution per population).…”

“…The craniomandibular morphology of the Dmanisi paleodeme has been described previously (Gabunia and Vekua 1995;Gabunia et al 2000;Gabunia et al 2001;Gabounia et al 2002;Lordkipanidze et al 2005;Lordkipanidze et al 2006;Rightmire et al 2006;Rightmire and Lordkipanidze 2010;Lordkipanidze et al 2013;Margvelashvili et al 2013). Cranial and mandibular morphological variation within the Dmanisi paleodeme is wide, but well within the range of variation seen in modern humans populations, and in populations of chimpanzee subspecies (Skinner et al 2006;Lordkipanidze et al 2013;Margvelashvili et al 2013;Zollikofer et al 2014). Variation in size, shape and dentognathic features among the Dmanisi mandibles has received special attention and has been studied to address questions of phylogeny, taxonomy, sexual dimorphism, diet, aging, and in-vivo modification of the dentognathic system (Gabounia et al 2002;Skinner et al 2006; 19.01.2016 3 Margvelashvili et al 2013;Martín-Francés et al 2013;Bermudez de Castro et al 2014;Schwartz et al 2014;Zollikofer et al 2014).…”

“…The only tooth present pre mortem (but lost postmortem) was the left canine, as the LC tooth-socket had not undergone noticeable bone remodeling (Lordkipanidze et al 2005;Lordkipanidze et al 2006;Margvelashvili et al 2013). The alveolar process is almost completely atrophied, and the in-vivo loss of the alveolar bone reaches a depth of 16mm (Fig.…”

Comment on “A Complete Skull from Dmanisi, Georgia, and the Evolutionary Biology of Early Homo ”

“…Because root number has taxonomic valence, it is noteworthy that D2600's P 1 bears three robust, long, separate roots, whereas D211 and D2735's P 1 is single-rooted, with grooves above a bifid tip. D2735's M 3 roots are expectedly less splayed than M 1 's (7), whereas D2600's M 3 roots are unusually well separated and D211's M 2 -3 roots are single (6). Because root divergence appears if and when interradicular tongues grow into the cervical loop (7), these differences, which exceed variation in H. sapiens (7), are important.…”

Response to Comment on “A Complete Skull from Dmanisi, Georgia, and the Evolutionary Biology of Early Homo ”

“…(iv) Schwartz et al ignore a recent study quantifying the effects of wear-related dentognathic remodeling on symphyseal shape and orientation and on molar outline shape (12), such that they mistake in vivo modifications for taxonomic differences. Also, the symphyseal alveolar bone of D2600 (12) is far from being "disease-free" and "unaltered" (1).…”