Why Do We Have 2 Eyes?

Reason For 2 Eyes

The reason for only two eyes (and not one or four) is probably due to the limitations of embryological development and evolution.

The body is generally symmetrical, and has been for almost the entire evolution of multicellular animals. So left-right duplication is the default, making one eye development difficult.

Advantage Of 2 Eyes

In most animals (insects, fish, reptiles, birds, rodents), each eye handles vision on half of the body. Humans and monkeys are unusual in that both eyes face forward, have overlapping visual fields, and can rotate in their socket across a wide field of view. This makes 3D stereo vision possible. Owls also have front-facing eyes with stereo vision to help with hunting.

 But Why Not More Than Two Eyes? 

 Vision is so valuable to humans that an extra pair of eyes would seem useful. Spiders have 8 eyes, and some jellyfish have 24.

The reason for "only two" is probably due to the limits of the evolutionary process. The two-eye body plan goes back 500 million years to fish and spans all vertebrates. The eye is so complex and specialized, that there may just not be an incremental way to get from two functioning eyes to four in a sequence of individual genetic mutations. (In contrast, extra ribs, spinal vertebrae, fingers, and toes occasionally occur.) When extra eyes do occur in mammals, it is because the whole face or head is (partially) duplicated, and survival rate is low.

Adding to the complexity of eyes in humans is the fact that the left side of both retinas goes to the left side of the brain and vice versa. This happens in a neural crossover called the optic chiasm (http://en.wikipedia.org/wiki/Opt...). This way each side of the brain can receive the two overlapping frontal visual regions to achieve 3D stereo depth vision.

Inner Structure of The Eye

As a result, adding extra eyes would require quite a bit of additional complex circuitry, which evolution cannot easily accomplish incrementally.