Occipital Bone Morphology

Humans differ from other apes in the overall architecture of the occipital bone (the bone forming the posterior portion of the cranial vault), as evident when the skull is viewed from the back or side. First, apes tend to have prominent temporal and nuchal crests (although these features may be reduced in females of all species, or in bonobo males) that result from large chewing muscles and neck muscles (the latter important in maintaining head posture in a quadrupedal position) attaching to a relatively small neurocranium. In most individuals, the temporal crest meets the nuchal crest, either at two locations on either side of the midline (as in most chimps) or continuously across the back of the cranium (as in gorillas), to form a prominent compound temporal/nuchal crest. Humans have relatively small temporal and nuchal muscles (the former reflecting an overall reduction in the size of the masticatory apparatus [teeth, jaws and chewing muscles], the latter related to upright posture that reduces the need for large neck muscles to counterbalance the head), whose attachments on the cranial vault do not form pronounced crests, and which do not share a compound line of attachment on the occipital. Second, in apes the greatest breadth of the cranial vault is found across the temporal bones near the base of the vault, giving the appearance of an inflated mastoid region. In apes the sides of the cranial vault (the parietotemporal walls) slope upwards and inwards from this wide base towards the top of the cranium. The greater cerebral volume of humans results in vertically set parietotemporal walls, the greatest breadth of the cranial vault is positioned high on the parietals, and the mastoid region appears considerably less prominent. Thus differences between humans and other apes in occipital morphology result from the intersection of three evolutionary trends in the hominin lineage: 1) brain size expansion, 2) reduction of the masticatory apparatus, and 3) upright posture and bipedal locomotion.