Prognathism

Humans have orthognathic faces, that is, faces that lie almost entirely beneath the anterior cranial fossa, whereas other apes (and primates in general) have prognathic faces that project forward of the anterior cranial fossa. Prognathism has been variously defined by different researchers, and is taken by some to refer only to projection of the lower facial skeleton (in which case projection of the upper portion of the face is referred to as “facial projection” rather than prognathism), and by others to refer to overall facial projection. By either definition, humans have less prognathic faces than chimpanzees, gorillas and orangutans. Prognathism is usually measured with the craniofacial angle (also known as the sphenomaxillary angle), which is the angle formed in the sagittal plane between the most anterior points on the maxilla, sphenoid and foramen magnum (because these landmarks can be difficult to observe in intact crania, this angle is generally quantified as the angle between the Frankfort horizontal and a line passing through the osteometric landmarks sellion and prosthion). This angle is obtuse in the great apes and acute in modern humans. The evolution of an orthognathic face in humans is likely related to a reduction in the size of teeth and chewing muscles, and perhaps to changes in masticatory load arm/lever arm relationships reflecting a reduced need for production of powerful bite forces in the anterior dentition.  Both of these changes may reflect dietary shifts in human evolution away from tough skinned fruits and fibrous plant matter and towards greater consumption of meat and cooked foods.