Unique human orbital morphology compared with that of apes

Humans’ and apes’ convergent (front-facing) orbits allow a large overlap of monocular visual fields but are considered to limit the lateral visual field extent. However, humans can greatly expand their lateral visual fields using eye motion. This study aimed to assess whether the human orbital morphology was unique compared with that of apes in avoiding lateral visual field obstruction. The orbits of 100 human skulls and 120 ape skulls (30 gibbons; 30 orangutans; 30 gorillas; 30 chimpanzees and bonobos) were analyzed. The orbital width/height ratio was calculated. Two orbital angles representing orbital convergence and rearward position of the orbital margin respectively were recorded using a protractor and laser levels. Humans have the largest orbital width/height ratio (1.19; p < 0.001). Humans and gibbons have orbits which are significantly less convergent than those of chimpanzees / bonobos, gorillas and orangutans (p < 0.001). These elements suggest a morphology favoring lateral vision in humans. More specifically, the human orbit has a uniquely rearward temporal orbital margin (107.1°; p < 0.001), suitable for avoiding visual obstruction and promoting lateral visual field expansion through eye motion. Such an orbital morphology may have evolved mainly as an adaptation to open-country habitat and bipedal locomotion.