Prognathism

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Human Uniqueness Compared to "Great Apes": 
Absolute Difference
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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.

Timing

Timing of appearance of the difference in the Hominin Lineage as a defined date or a lineage separation event. The point in time associated with lineage separation events may change in the future as the scientific community agrees upon better time estimates. Lineage separation events are defined in 2017 as:

  • the Last Common Ancestor (LCA) of humans and old world monkeys was 25,000 - 30,000 thousand (25 - 30 million) years ago
  • the Last Common Ancestor (LCA) of humans and chimpanzees was 6,000 - 8,000 thousand (6 - 8 million) years ago
  • the emergence of the genus Homo was 2,000 thousand (2 million) years ago
  • the Last Common Ancestor (LCA) of humans and neanderthals was 500 thousand years ago
  • the common ancestor of modern humans was 100 - 300 thousand years ago

Probable Appearance: 
2,000 thousand years ago
Background Information: 

Spoor et al., 2005.  Correlation of cranial and mandibular prognathism in extant and fossil hominids.  Trans Roy Soc S Afr 60: 85-89.
Bromage et al. 2008. Craniofacial architectural constraints and their importance for reconstructing the early Homo skull KNM-ER 1470. J Clin Ped Dent 33: 43-54.

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Referenced By:
Title Certainty
Upper Facial Height Likely

References

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