Most animals — fish, insects, reptiles, birds, rabbits, and horses, for example — exist in non-cluttered environments like fields or plains, and they have eyes located on either side of their head. These sideways-facing eyes allow an animal to see in front of and behind itself, an ability also known as panoramic vision.
Humans and other large mammals — primates and large carnivores like tigers, for example — exist in cluttered environments like forests or jungles, and their eyes have evolved to point in the same direction. While animals with forward-facing eyes lose the ability to see what’s behind them, they gain X-ray vision, according to Mark Changizi, assistant professor of cognitive science at Rensselaer, who says eyes facing the same direction have been selected for maximizing our ability to see in leafy environments like forests.
The phrase "x-ray vision" used here is a bit misleading, since no creature sees literally "through" an object. But to illustrate the point, place your hand with spread fingers in front of (and close to) your face while reading this blog entry. You will be able to see most of the text behind your fingers. Now close one eye and note that the fingers block more of the screen.
An interesting theory, but frankly I would think the distance/depth perception conferred by overlapping fields of vision is a more valuable evolutionary advantage.