The Human Mandible and the Origins of Speech

Among the unique traits of human mandibles is the finding of relatively greater utilization of cortical bone with respect to other hominoids. The functional significance of this trait is not plausibly linked to masticatory demands given the diminution of the masticatory musculature in human evolution and the behavioral universal of extraoral food preparation in recent humans. Similarly, the presence of more mandibular bone is not a correlated effect of systemic skeletal robusticity, since gracilization of the skeleton is a feature diagnostic of modern humans. The mandibular symphysis in modern humans is manifested as the chin, and it is here where cortical bone hypertrophy is most pronounced. The potential covariation between the expression of the chin and bone hypertrophy is explored in an attempt to clarify their respective biomechanical roles. Current developments in skeletal biomechanics implicate low magnitude, high frequency strains in bone hypertrophy. The physiology of speech production likely produces strains in mandibular bone of greater frequency and lesser magnitude than those associated with mastication. Consequently, language acquisition plausibly accounts for cortical hypertrophy in modern human mandibles. Its role in the evolution and development of the chin is less clear. 1. Introduction No consensus exists that there is a diagnostic anatomical indicator for articulate speech in human evolution. This absence of success cannot be attributed to a lack of effort in identifying suitable candidates [1]. Whether unique aspects of human hyoid, basicranial, or hypoglossal canal morphology indicate the capacity for spoken language is contested [2–6]. Additionally, the appearance of the human chin has been suggested to indicate the advent of articulate speech [7–9], but no empirical data provide unambiguous linkage of the physiology of speech with variation in mandibular symphyseal morphology [10–13]. The human mandible is morphologically distinct from other primates both in terms of its proportions and specific anatomical features [11]. Anterior corpus morphology in humans is truly unique among anthropoids, in that the lingual transverse tori and genial pit are absent, and the labial surface is characterized by a basal swelling (the “chin”) with several definable features idiosyncratic to human jaws (e.g., trigonum mentale, and incurvatio mandibulae anterior). Even though the chin is recognized as diagnostic of our species, its evolutionary and functional significance remain incompletely understood [14, 15]. Despite the ease with which it can be