Evolution of the eye

The evolution of the eye has attracted significant study, with the eye distinctively exemplifying an analogous organ present in a wide variety of animal forms. Complex, image-forming eyes have evolved independently some 50 to 100 times.

Complex eyes appear to have first evolved within a few million years, in the rapid burst of evolution known as the Cambrian explosion. No evidence of eyes before the Cambrian has survived, but a wide range of diversity is evident in the Middle Cambrian Burgess shale, and in the slightly older Emu Bay Shale. Eyes show a wide range of adaptations to meet the requirements of the organisms which bear them. Eyes vary in their visual acuity, the range of wavelengths they can detect, their sensitivity in low light, their ability to detect motion or to resolve objects, and whether they can discriminate colours.

In 1802, philosopher William Paley called it a miracle of "design". Charles Darwin himself wrote in his Origin of Species, that the evolution of the eye by natural selection at first glance seemed "absurd in the highest possible degree". However, he went on to explain that despite the difficulty in imagining it, this evolution was perfectly feasible:

...if numerous gradations from a simple and imperfect eye to one complex and perfect can be shown to exist, each grade being useful to its possessor, as is certainly the case; if further, the eye ever varies and the variations be inherited, as is likewise certainly the case and if such variations should be useful to any animal under changing conditions of life, then 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 as subversive of the theory.

He suggested a gradation from "an optic nerve merely coated with pigment, and without any other mechanism" to "a moderately high stage of perfection", giving examples of existing intermediate grades of evolution. Darwin's suggestions were soon shown to be correct, and current research is investigating the genetic mechanisms responsible for eye development and evolution.

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