Illusions: Seeing in Stereo; July/August 2009; Scientific American Mind; by Vilayanur S. Ramachandran; Diane Rogers-Ramachandran; 3 Page(s)

All primates, including humans, have two eyes facing forward. With this binocular vision, the views through the two eyes are nearly identical. In contrast, many other animal groups, especially herbivores such as ungulates (hooved animals, including cows, sheep and deer) and lagomorphs (rabbits, for example), have eyes pointing sideways (b). This perspective provides largely independent views for each eye and an enormously enlarged field of view overall. Why did primates sacrifice panoramic vision? What benefit did they gain?

We know binocular vision evolved several times independently in vertebrates. For example, among birds, predatory species such as owls and hawks have forward-pointing eyes (c). One theory is that the feature conferred a statistical advantage—two eyes are better than one—for detecting and discriminating objects, such as prey, in low light levels. But whatever the original reason for its emergence, the evolutionary novelty afforded a huge advantage: stereoscopic (literally, solid) vision.