Post-orbital constriction

In physical anthropology, post-orbital constriction is the narrowing of the cranium (skull) just behind the eye sockets (the orbits, hence the name) found in most non-human primates and early hominins. This constriction is very noticeable in non-human primates, slightly less so in Australopithecines, even less in Homo erectus and completely disappears in modern Homo sapiens. Post-orbital constriction index in non-human primates and hominin range in category from increased constriction, intermediate, reduced constriction and disappearance. The post-orbital constriction index is defined by either a ratio of minimum frontal breadth (MFB), behind the supraorbital torus, divided by the maximum upper facial breadth (BFM), bifrontomalare temporale, or as the maximum width behind the orbit of the skull. Measurement of cranial capacity in hominis has been long used to examine the evolutionary development of increased brain size, allowing for comparing and contrasting among hominin skulls and between primates and hominins. Similarly, the post-orbital constriction index has become a form to compare and contrast craniums with the possibility of determining the relative age and evolutionary place of a new found hominin. Cranial capacity and post-orbital constriction index can demonstrate a correlation between increased brain size and reduced post-orbital constriction. For example, the average cranial capacity for Australopithecines is 440 cc, and the post-orbital constriction index is 0.66. However, with the evolutionary change in brain size in Australopithecines to the Homo genus, the average cranial capacity for Homo Habilis is 640 cc, and the post-orbital constriction index is 0.72. More specifically, in a departure from Homo erectus, Homo sapiens manifests a absolutely reduced post-orbital constriction (post-orbital constriction disappears) and a post-orbital constriction index of 0.92 due to increase in cranial capacity, about 1,350 cc.