Unique morphology of the human orbit among the Hominoidea

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Abstract

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.

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