Unique human orbital morphology compared with that of apes

Abstract

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.

Figure 1. Histogram of orbital width/height (W/H) ratio (mean +/− SEM) of the different genera.

***p < 0.001. Abbreviations used: Hylobat. = Hylobatidae; H. Sap. = Homo sapiens.

Figure 2. Histogram of the orbital width/height (W/H) ratio (mean + /– SEM) in different human populations.

***p < 0.001. Abbreviations used: N. Am. = North America; R. Cg. = Republic of the Congo; Austr. = Australia.

Figure 3. Histogram of orbital angle values (mean + /– SEM) of the different genera.

Columns filled with different grey-levels and limited by a thin line denote the convergence angle (CA). White columns limited by a thick line denote the opening angle (OA). The difference between OA and CA is represented by an arrow. The arrow points up in all genera except Pongo in whom CA < OA. ***p < 0.001. Abbreviations used: Hylobat. = Hylobatidae; H. Sap. = Homo sapiens.

Figure 4. The human orbital margin falls short of being inscribed in a plane.

Front and side views of human and ape skulls. A 3-cm wide black landmark affixed to each skull gives the scale. Specimen a/a’ = Homo sapiens; specimen b/b’ = Pan troglodytes; specimen c/c’ = Hylobates muelleri; specimen d/d’ = Gorilla gorilla; specimen e/e’ = Pongo pygmaeus. A black cardboard lid (representing the convergence angle) has been placed in the right orbital plane of each skull. In humans, the lateral orbital margin is far more rearward than the lid (a’) because of the high discrepancy between convergence and opening angles. In non-human apes, this discrepancy is lower and the cap is much closer to the lateral orbital margin (b’,c’,d’,e’). The specimens are from the Natural History National Museum (Paris, France) anthropological collections (a/a’) and Comparative Anatomy Laboratory (b/b’ to e/e’).

Figure 5. Histogram of orbital angle values (mean + /– SEM) in different modern human populations.

The columns filled with dark-grey and limited by a thin line correspond to the convergence angle (CA). White columns limited by a thick line correspond to the opening angle (OA). The difference between OA and CA (OA-CA) is represented by an arrow. Statistically significant differences (p < 0.05) for OA, CA and OA-CA among the human populations were noted using red upper case letters, blue lower case letters and green Greek letters respectively. For the OA, there are significant differences between group A (North America and Republic of the Congo) and group C (Europe); group A and group B (China); group AC (Australia) and group B. For the CA, there are significant differences between group a (North America, Europe and Republic of the Congo) and group b (China and Australia). For the OA-CA, there are significant differences between group α (North America, China and Republic of the Congo) and group β (Europe); group α and group γ (Australia); group β and γ. Abbreviations used: N. Am. = North America; R. Cg. = Republic of the Congo; Austr. = Australia.

Figure 6. Overview of the convergence angle and opening angle.

The sagittal plane and the neuro-ocular plane (NOP: yellow) are represented by a cross-laser. The protractor is positioned in the NOP. The most anterior point of the orbital margin is denoted by a. The point where the NOP crosses the inner orbital margin is denoted by a’. In this skull, points a and a’ coincide. The highest and lowest orbital points are denoted by s and i respectively. The orbital plane (OP) crosses points i, s and a. Its position is shown using a red line for laser crossing points i, s and a. The para-sagittal plane (PSP) is parallel to the sagittal plane and crosses points a and a’. The CA is the angle between OP and PSP, measured using the protractor. The OA is the maximum angle allowing point a’ to be illuminated by the line laser placed postero-laterally in the NOP. The difference between the CA and the OA is denoted by “OA-CA”.