Sagittal Crest of the Skull

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Human Uniqueness Compared to "Great Apes": 
Likely Difference
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The sagittal crest is a prominent ridge of bone that projects superiorly (upwards) from the cranial vault along its midline, most commonly seen in adult male gorillas and orangutans. Sagittal crests are rare in adult male chimpanzees and female gorillas, and are unknown in female chimpanzees, female orangutans, and humans and bonobos of both sexes (and are also absent in juveniles of all species). The crest provides a surface for the attachment of the large chewing muscle, temporalis. In humans, who have large brains (and hence large cranial vaults) relative to their body size, the temporal muscles occupy a position on the lateral walls of the cranial vault, and extend only about halfway up the vault surface. In apes the superior extent of the temporalis muscles is positioned higher on the cranial vault (because brain size is smaller, and the muscles of mastication are larger than those found in humans), such that the right- and left-side superior temporal lines approach one another at midline. In male gorillas and orangutans (and some species of fossil hominin), in which very large chewing muscles are anchored to a relatively small cranial vault, the right and left superior temporal lines not only converge at the midline of the top of the cranial vault (along the sagittal suture), but also require the development of a sagittal crest to provide sufficient attachment area for the temporalis muscles.

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: 

Bilsborough & Rae, 2007. Hominoid Cranial Diversity and Adaptation.  In Henke & Tattersall (eds.), Handbook of Paleoanthropology. Berlin: Springer. pp. 1031-1105.

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