Abstract
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.
Highlights
No consensus exists that there is a diagnostic anatomical indicator for articulate speech in human evolution
If the chin is the product of sexual selection, it has not resulted in obvious differences in biomechanical performance between males and females, despite the fact that postadolescent growth of the mandible is distinct between males and females [78]
High-frequency, low-magnitude loads associated with articulate speech are hypothesized to explain the apparent paradox of hypertrophied mandibular bone in contrast to the reduced bone thickness that typifies the remainder of the modern human skull
Summary
No consensus exists that there is a diagnostic anatomical indicator for articulate speech in human evolution. A consensus of definition is elusive, and no attempt to resolve it will be made here; instead, the “chin” in this paper refers to an anterior basal swelling of the mandibular symphysis. In this sense, it “requires” anterior incurvatios for its expression. It is assumed here that, cortical hypertrophy and the expression of the chin both delineate humans from other extant hominoids, these features can be considered logically separate in terms of their adaptive or evolutionary significance. In the discussion that follows, “symphysis” refers to the midline of the mandible (i.e., the section transected by the midsagittal plane), “corpus” refers to the tooth-bearing portion of the jaw, and “anterior corpus” is the region of the mandible anterior or rostral to the canines
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