Is the Eye Too Complex to have Evolved Naturally?
It is often argued that the eye is so complex that it could not have evolved naturally. The idea being that an eye with only "half" the parts in place would not work, therefor evolution would have never favored it and the more complex human eye we have would never have evolved.
However, the evolution of the eye has been studied extensively, and its history is more-or-less well established. First, there were light-sensitive cells which merely indicated which way was the Sun. A slight indentation makes a sense of direction possible. Mucus in the pit focuses the light. If the mucus hardens, you have a proper lens, and so on.
In fact, every stage of the eye's development is still around on Earth today. A snail's eye is less than "half" a human eye, yet it serves the snail well enough to help its survival.
Please watch this video which beautifully describes the evolution of the eye for a more visual example.
[ References: Wikipedia - Evolution of the Eye ]