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.

Purpose Like many mammal predators, Humans have frontal (forward‐facing) orbits. This design allows a large overlap of monocular visual fields with good stereoscopic vision but is considered to harm lateral space perception. In Humans, on average, temporal visual field extends 95° in primary position of gaze but 128° with eye abduction. Which anatomical peculiarity may allow such a visual field expansion? Methods Comparative orbit osteology study in 100 human skulls and 120 Apes’ skulls (30 gibbons; 30 orang‐utans; 30 gorillas; 30 chimpanzees and bonobos). Orbit width and height were recorded. Using a protractor and laser levels two orbit angles were recorded: “convergence angle” (the lower this angle, the more frontal the inner orbital rim orientation) and “opening angle” (the higher this angle, the more backward the temporal orbital rim position). Results The largest orbit width/height orbit ratio is 1.19, in Humans (p < 0.001). Humans have a higher “convergence angle” (98.1°) than all Apes except gibbons (99.2°; p > 0.05). Humans have by far the largest “opening angle” (107.1°; p < 0.001) and the largest difference between “opening angle” and “convergence angle” (9°; p < 0.001). Conclusion The largest orbit width/height ratio found in Humans suggests a design that favours lateral vision. More specifically, human orbital rim is unique in that, while frontal, it has by far the most backward temporal orbital rim. This peculiarity ‐ likely and adaptation to terrestrial life with upright bipedal locomotion ‐ allows both good stereoscopic vision and large temporal visual field extent through eye motion.

Scientific Reports 5: Article number: 11528; published online: 25 June 2015; updated: 20 July 2016 This Article contains typographical errors. In the Methods section under subheading ‘Measurement error evaluation’, “Measurement errors were 1.33° degree for OA, 0.93° for OA; 1.12° for OA-CA; 0.44 mm for orbital width; 0.