The structure of the human eye socket (orbit) is unique compared with other apes and may have evolved to expand humans’ peripheral vision, according to a study published this week in Scientific Reports.
Eric Denion and colleagues compared the orbits of 100 human skulls and 120 ape skulls. They found that humans and gibbons had significantly less convergent (front-facing) orbits than chimpanzees, bonobos, gorillas, and orang-utans. They also found that the human orbit is wider than it is high, and that humans have the largest width/height ratio among apes. These features are thought to allow greater lateral vision in humans compared with other apes.
The authors report that the outer edge of the orbits - the lateral orbital margin (LOM) - is set further back in the skull of humans than in non-human apes. This, combined with the further forward position of the human eyeball, reduces visual obstruction and improves the eye's ability to scan the environment. The authors suggest this feature may have evolved as a by-product of selection for other modern human facial features, such as loss of the snout. They also suggest that the evolution of the rearward LOM may have been driven by humans’ migration out of forests, where non-human primates still live and the relatively forward position of the LOM in these apes may provide protection against branch-related eyeball trauma. Finally, they suggest the exposed position of the human eyeball may represent a trade-off between ultraviolent light-related eye conditions, such as pytergia or cataracts, and a large visual field.