In binocular vision, an organism has two eyes which are aligned so that their field of vision overlaps in part. This causes a slight parallax difference between the field of vision of each eye in the area of overlap. The small binocular disparity is automatically interpreted by the brain as depth perception, and is capable of making very precise estimates of distance. As a result, evolution has favored the development of binocular vision in humans.
Binocular vision in mammals
Binocular vision was probably a mammalian attribute as far back as mammals have existed. Functional binocular anatomy has been demonstrated in every early mammal fossil found so far.
This is directly related to the conditions in which mammals evolved. Unlike other taxonomic classes, the earliest mammals were all surface dwellers. The reliable access to light on the surface of the land would have made eyesight an extremely important sense, which would not have been the case for sea creatures outside the limited photic zone.
On average, Earth’s atmosphere has much less refraction distortion than water. Eyesight in early terrestrial mammals was correspondingly more reliable and relied upon.
Early mammals were also probably nocturnal. Binocular vision has the effect of doubling the amount of perceived light in the overlapping field of vision, which improves vision in low-light conditions.
Finally, air resistance is much lower than water resistance, so predators on land can cover distance faster than predators in the water. As a result, accurate assessment of that distance was crucial to survival for both predator and prey. This created strong evolutionary pressures to develop and retain binocular vision for as long as those conditions remained.
Evolution of binocular vision in mammals
After mammals began taking over the ecological niches left behind by the extinct dinosaurs, their eyesight also evolved to match the needs of those niches. While arboreal primates continued to depend heavily on binocular vision, mammals in some other ecological niches evolved away from binocular vision in favor of a wider field of vision.
However, mammals which evolved away from strong dependence on eyesight in favor of other senses did not lose their binocular anatomy. Even blind mammals still retain their vestigial eyes in a binocular arrangement.
Why binocular vision is not completely dominant in humans
Humans do not rely on binocular vision alone for depth perception. Alternate depth cues, such as foreground overlap, texture gradients and relative size, are at least as important as binocular vision. These backup factors allow distances to be estimated accurately even after the loss of vision in one eye. Flat replication of these alternate depth cues also allows spatial depth to be simulated in visual replication and art, with the effectiveness of the depth illusion depending on the degree of internal consistency among those depth cues.
The evolutionary importance of these secondary factors is greatest in environments with many fixed visual reference points, such as most terrestrial surface environments. In environments without fixed visual reference points, such as sky or open sea, these secondary factors have much less importance.
Thus, evolutionary pressure toward binocular vision is strongest in raptors, which depend almost entirely on binocular vision with extreme visual acuity for their survival. While losing an eye would be death for a wild raptor, there is not much danger of that in a raptor’s typical environment.
In contrast, there is much less evolutionary drive toward binocular vision among mammals, most of which have a much greater chance than raptors of accidentally losing sight in one eye during their lifetime. While binocular vision is especially useful for hunting mammals and arboreal mammals, it is just as important to be able to survive with only one eye.
As a result, nearly all people have a dominant eye, which can take over completely in some conditions. As much as 20% of the human population does not have true binocular vision, or experiences headaches or eyestrain in trying to achieve binocular vision.
Binocular vision, alternate depth cues, and modern technology
This evolutionary balance between binocular vision and alternate depth cues can sometimes backfire. Because of the recent increase in the popularity of 3D movies, there is also a corresponding rise in headaches and nausea associated with watching a 3D movie among roughly 10 percent of the population. Another 10 percent of the population can’t resolve the double image of a 3D movie at all.
The problems are even greater at modern transportation speeds. The combination of featureless terrain and ground lighting illusions has been a factor in several airplane crashes. Sector whiteout, another combination of featureless terrain and ground lighting issues, has been a factor in every airplane crash which has ever occurred in Antarctica.
In evolutionary terms, the depth cue challenges posed by modern technology probably don’t have a major influence on reproductive survival. They certainly haven’t been around long enough for them to have evolutionary effects on the human race. In a few generations, new developments in technology may completely change the relevance of binocular vision in modern society.