Among primates, humans have the largest width/height ratio of the palpebral fissure and this is believed to promote EMVF extension 16, 17. Therefore, among mammals, humans may be unique in combining overlapping monocular visual fields with large lateral visual fields, through eye abduction 15. visual field with allowed eye motion) in humans 15. visual field with the eyes facing straight ahead) is much less extensive than “eye-motion visual field” (EMVF, i.e. However, we have demonstrated 14, 15 that “base visual field” (BVF, i.e. With some of the most convergent orbits among mammals 4, 7, 9, these anthropoid primates are assumed to have a limited lateral visual field 4, 5, 6, 13. Humans and their closest relatives, the non-human apes 10, 11, 12, form the Hominoidea superfamily 11. Hence Cartmill’s 1 authoritative definition 4, 5, 7, 9 of orbital convergence as “the dihedral angle between the mid-sagittal plane and the plane of the orbital margin”. In primates, including humans, the orbital margin is considered to be flat and therefore inscribed in a plane 1, 4, 7, 8. In mammals, convergent orbits ensure good stereoscopic vision through overlapping monocular visual fields, but limit the extent of lateral visual fields, while divergent (lateral-facing) orbits allow panoramic visual fields with little overlap between monocular fields 1, 2, 3, 4, 5, 6. Similar content being viewed by othersĪ convergent (forward-facing) orbit, a feature of predators, is marked in animals such as owls, hawks, felids and primates 1, 2, 3. Such an orbital morphology may have evolved mainly as an adaptation to open-country habitat and bipedal locomotion. 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. These elements suggest a morphology favoring lateral vision in humans. Humans and gibbons have orbits which are significantly less convergent than those of chimpanzees / bonobos, gorillas and orangutans (p < 0.001). Humans have the largest orbital width/height ratio (1.19 p < 0.001). Two orbital angles representing orbital convergence and rearward position of the orbital margin respectively were recorded using a protractor and laser levels. The orbital width/height ratio was calculated. The orbits of 100 human skulls and 120 ape skulls (30 gibbons 30 orangutans 30 gorillas 30 chimpanzees and bonobos) were analyzed. This study aimed to assess whether the human orbital morphology was unique compared with that of apes in avoiding lateral visual field obstruction. However, humans can greatly expand their lateral visual fields using eye motion. 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.

360 view of gorilla skull