No chin left behind: the morphological integration and variation of the modern human mentum osseum

Abstract

The chin, or mentum osseum, is regarded as one of the most unique traits that differentiate modern humans from our earlier hominin ancestors and has received intense scrutiny by scholars for well over a century. Several hypotheses are currently being investigated by researchers in attempts to elucidate the nature of the origin and function of the chin, but none of these have been satisfactorily upheld. Additionally, there are debates about what defines the chin and whether it is variable amongst extant modern humans. In an attempt to study this problem in a novel way, the current study examines whether the chin is part of a morphologically integrated set of facial and cranial characteristics, as well as whether it is variable in a diverse sample of modern human skeletal remains. The morphological integration of the mandible with the cranium has been scrutinized in recent investigations, and results have indicated that some morphological aspects of the mandible covary with the cranium. However, these studies do not evaluate the mentum osseum itself. The chin may be independent of integration with the rest of the skull, indicating that it is a feature that evolved in response to other pressures, such as sexual selection or biomechanical constraints. Conversely, if the mentum osseum is correlated to other measurements of the skull, the appearance of the chin in modern humans may have been a pleiotropic effect of selective forces acting to reduce facial prognathism. A diverse modern human sample was analyzed in order to test the degree of correlation and variation found in the mentum osseum. Results indicate that the mentum osseum is not statistically correlated with the majority of measurements from the mandible and cranium and may be independent of any morphological integration. Additionally, the results further demonstrate that the mentum osseum is highly variable in modern human populations.