3.4 Stereoscopic vision
DA explains that ancestral monkeys had to 'decide whether a branch was within reach of a stretch or a jump - or beyond either' [p.248]. Individuals that were able to judge distances between themselves and branches more accurately had a competitive edge over other individuals, which led to the development in primates of stereoscopic vision. Stereoscopic vision evolved convergently in carnivores that judge distances to capture fast-moving prey.
The field of view of a single eye is termed the monocular field, which in most species is almost semicircular, i.e. the areas coloured pink and blue in Figure 3a. Many mammals, such as lagomorphs and ungulates, have an eye on each side of their head, i.e. laterally pointing eyes. In such an arrangement, the monocular field of view of the left eye has very little overlap with the monocular field of the right eye (Figure 3a), which maximises the field of view around an animal's head, enabling it to see predators approaching from almost any direction. Note that rabbits have unusually large monocular fields, giving them a circular field of view.
In contrast, primates have forward-pointing eyes, so the monocular field of the left eye overlaps considerably with that of the right eye (Figure 3b). This overlap reduces the total field of view so that all mammals with forward-pointing eyes have a blind zone - an area that their eyes cannot see; for example, you can't see behind you unless you move your head. But this region of overlapping fields is very important in primate vision. Objects within the area of overlap, such as object 1 in the figure, are viewed by both eyes, whereas objects outside this area are seen by either the left or the right eye but not both; for example, object 2 is seen only by the left eye. When the brain simultaneously receives information from both eyes about an object, complex neural processing occurs that builds up a 3-D image of the object. This is stereoscopic vision, which means literally solid vision, i.e. vision in depth, and hence enormously improves the ability to judge the distance between the viewer and the object. Try covering one eye and looking at a view or at a selection of objects on a table. The scene appears very 'flat'. Now look at the same scene with both eyes and note the illusion of depth.
The larger the field of stereoscopic vision, the greater the accuracy in judging distance. Primates, therefore, are able to judge distances with greater accuracy than herbivores. However, it is interesting to note that the large monocular fields in rabbits gives a small area of stereoscopic vision above and behind an individual's head, as well as in front, as you can see in Figure 3a.
All these developments in visual acuity increased the amount of processing that needed to be carried out by the brain and this is thought to have been the main driver of the large brain size that is found in living primates today (see Section 5).
As stereoscopic colour vision developed, so the sense of smell became less important. In the process, the nasal apparatus became less complex and most experts claim that this led to the shortening of the muzzle of anthropoids compared to other mammals. These changes to the muzzle allowed the muscles of the face to extend, giving mobility to the upper lip, enabling anthropoids today to make a wide variety of facial gestures. Facial and vocal communication are very important in anthropoid societies.