Abstract
The organization of the bony face is complex, its morphology being influenced in part by the rest of the cranium. Characterizing the facial morphological variation and craniofacial covariation patterns in extant hominids is fundamental to the understanding of their evolutionary history. Numerous studies on hominid facial shape have proposed hypotheses concerning the relationship between the anterior facial shape, facial block orientation and basicranial flexion. In this study we test these hypotheses in a sample of adult specimens belonging to three extant hominid genera (Homo, Pan and Gorilla). Intraspecific variation and covariation patterns are analyzed using geometric morphometric methods and multivariate statistics, such as partial least squared on three-dimensional landmarks coordinates. Our results indicate significant intraspecific covariation between facial shape, facial block orientation and basicranial flexion. Hominids share similar characteristics in the relationship between anterior facial shape and facial block orientation. Modern humans exhibit a specific pattern in the covariation between anterior facial shape and basicranial flexion. This peculiar feature underscores the role of modern humans' highly-flexed basicranium in the overall integration of the cranium. Furthermore, our results are consistent with the hypothesis of a relationship between the reduction of the value of the cranial base angle and a downward rotation of the facial block in modern humans, and to a lesser extent in chimpanzees.
Highlights
IntroductionAmong the numerous skeletal modifications which have occurred during hominin evolution (the term ‘hominin’ refers to the members of the human clade and the term ‘hominid’ corresponds to the common ancestor of Homo, Pan, Gorilla and Pongo and all of its descendants), the morphology of the face has undergone several important changes (see e.g. [1] or [2] for studies on early fossil hominin)
Among the numerous skeletal modifications which have occurred during hominin evolution, the morphology of the face has undergone several important changes
Since the posterior maxillary (PM) plane is based on the midsagittal projection of two lateral landmarks [8,20] and since the patterns of integration of midsagittal and lateral face may differ as it is the case in the basicranium [13,24], we choose to defined the orientation of the facial block using midsagittal landmarks, i.e. the staphylion, the foramen caecum and the sphenoidale (Fig. 1.a, Table 1), instead of the PM plane
Summary
Among the numerous skeletal modifications which have occurred during hominin evolution (the term ‘hominin’ refers to the members of the human clade and the term ‘hominid’ corresponds to the common ancestor of Homo, Pan, Gorilla and Pongo and all of its descendants), the morphology of the face has undergone several important changes (see e.g. [1] or [2] for studies on early fossil hominin). [1] or [2] for studies on early fossil hominin) These changes are notable for facial projection, i.e. the degree to which the face projects in front of the cranial base [3] and facial prognathism, i.e. the protrusion of the lower face relative to the upper face [4]. These modifications of facial characteristics raise several important questions. Hallgrımsson and Lieberman and colleagues [16,17] and Hallgrımsson and colleagues [18] have used a combination of GM and linear measurements on mouse crania as a proxy to understand the developmental pathways that express the relationships between primate facial shape and basicranium flexion
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