Implications of the Relationship Between Basicranial Flexion and Facial Orientation for the Evolution of Hominid Craniofacial Structures
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The basicranium and face have been linked through genetic, developmental, and functional relationships throughout their evolution. As a result, basicranial morphology most likely plays a major role in the evolution of facial structures. We describe the relationships between basicranial flexion and the face in Homo, Pan, and Gorilla to determine the role of cranial base angle reduction in the setup of the short and orthognathic face of Homo. We test the hypotheses that cranial base flexion plays a significant part in variation in facial orientation, length, and projection at the intraspecific level. The sample comprised 125 crania of adult specimens including 66 Homo sapiens, 32 Pan troglodytes, and 27 Gorilla gorilla. We described the cranial base and face using landmarks placed on scans of the surfaces and computed correlations between the cranial base angle and facial orientation, length, and projection. Our results support the hypotheses that cranial base flexion plays a significant part in facial orientation for Homo and Pan and in facial length for Pan. The hypothesis that basicranial flexion is related to a reduction of facial projection is not supported. The findings suggest that basicranial flexion can explain several anatomical specificities of hominins, including the reduction of prognathism and the reduction of the length of the nasopharynx. We found different patterns in the different genera, highlighting the fact that changes in the relationship between craniofacial structures may have occurred during hominid evolution.
KeywordsCranial base Cranium Facial block Facial projection Hominin
We thank the following institutions and people for allowing us access to their specimens or data: Mr. W. Wendelen of the Royal Museum for Central Africa (Tervuren, Belgium), Pr. C. P. E. Zollikofer and Dr. M. Ponce de León of the Anthropologisches Institut und Museum (Zürich, Switzerland), and the Natural History Museum (London). We also thank the following people and facilities for the CT scans data acquisition: the Department of Radiology of UZ Leuven (Leuven, Belgium), the Kantonsspital Winterthur (Winterthur, Switzerland), and the Hammersmith Hospital (London). We thank Pr. D. E. Lieberman and Dr. T. Bienvenu for their comments on previous versions of this work. We also thank S. Ramdarshan for his help improving the English. We thank Dr. J. Setchell and two anonymous reviewers for their valuable comments on earlier drafts of this manuscript. The Agence Nationale de la Recherche (project ANR-09-BLAN-0238) supported this work.
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