Evolution of the eye

The evolution of the eye is the origin and development with diversification by natural selection over geological time of organs of photosensitivity and vision in living organisms. Many scientists have found the evolution of the eye attractive to study because the eye distinctively exemplifies an analogous organ found in many animal forms. Simple light detection is found in bacteria, single-celled organisms, plants and animals. Complex, image-forming eyes have evolved independently several times.

Diverse eyes are known from the Burgess Shale of the Middle Cambrian, and from the slightly older Emu Bay Shale. Eyes vary in their visual acuity, the range of wavelengths they can detect, their sensitivity in no 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." In 1859, Charles Darwin himself wrote in his Origin of Species, that the evolution of the eye by natural selection seemed at first glance "absurd in the highest possible degree". However, he went on that despite the difficulty in imagining it, its 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 stepwise evolution from "an optic nerve merely coated with pigment, and without any other mechanism" to "a moderately high stage of perfection", and gave examples of existing intermediate. Current research is investigating the genetic mechanisms underlying eye development and evolution.

Biologist D.E. Nilsson has independently theorized about four general stages in the evolution of a vertebrate eye from a patch of photoreceptors. Nilsson and S. Pelger estimated in a classic paper that only a few hundred thousand generations are needed to evolve a complex eye in vertebrates. Another researcher, G.C. Young, has used the fossil record to infer evolutionary conclusions, based on the structure of eye orbits and openings in fossilized skulls for blood vessels and nerves to go through. All this adds to the growing amount of evidence that supports Darwin's theory.

The first possible fossils of eyes found to date are from the Ediacaran period (about 555 million years ago), while the oldest certain fossilized eye is from a Schmidtiellus reetae fossil from 530 mya, collected in Saviranna in northern Estonia. The structure is similar to the compound eyes of modern-day dragonflies and bees, but with (~100) ommatidia spaced further apart, and without a lens. The lower Cambrian had a burst of apparently rapid evolution, called the "Cambrian explosion". One of the many hypotheses for "causes" of the Cambrian explosion is the "Light Switch" theory of Andrew Parker: it holds that the evolution of advanced eyes started an arms race that accelerated evolution. Before the Cambrian explosion, animals may have sensed light, but did not use it for fast locomotion or navigation by vision.

The rate of eye evolution is difficult to estimate because the fossil record, particularly of the lower Cambrian, is poor. How fast a circular patch of photoreceptor cells can evolve into a fully functional vertebrate eye has been estimated based on rates of mutation, relative advantage to the organism, and natural selection. However, the time needed for each state was consistently overestimated and the generation time was set to one year, which is common in small animals. Even with these pessimistic values, the vertebrate eye could still evolve from a patch of photoreceptor cells in less than 364,000 years.