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From the Department of Ophthalmology and Visual Science, Albert Einstein College of Medicine, Bronx, New York; and New York Eye and Ear Infirmary of Mt. Sinai, New York, New York.
The author has no financial or proprietary interest in the materials presented herein.
Charles Darwin considered the eye such a complicated organ of extreme perfection that he understood his theory of evolution would be challenged to explain its origins. In The Origin of Species by Means of Natural Selection , he writes, "To suppose that the eye with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I confess, absurd in the highest degree." However, Darwin's own beliefs were the opposite. He later stated, "The difficulty of believing that a perfect and complex eye could be formed by natural selection, though insuperable by our imagination, should not be considered subversive of the theory."
It all started with isolated photosensitive cells and evolved into an incredible variety of seeing machines we call "eyes." In the first stage of visual evolution, isolated photosensitive spots (found on many marine larvae and earthworms) let organisms tell whether it was day or night, thereby giving them a competitive advantage by letting them know when feeding is best or predators are most prevalent. These spots allowed for sensing depth, both in the water and on land. They also made circadian rhythms possible.
The second phase of this evolution included the addition of a small pigment shield in front of the photosensitive cell that conferred the ability to detect the direction of light. An organism could then move purposely toward or away from light using its one-pixel directionality. A sea urchin's body functions...