Evolution of the eye

Many researchers 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....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. Many researchers 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. Complex eyes first appeared during the Cambrian explosion. Prior to the Cambrian, no evidence of eyes has survived, but diverse eyes are known from the Burgess shale of the Middle Cambrian, and from the slightly older Emu Bay Shale.Eyes are adapted to the various requirements of their owners. They 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 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: 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 steps. 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 fossils of eyes found to date are from the lower Cambrian period (about 540 million years ago). 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 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 would still evolve from a patch of photoreceptor cells in less than 364,000 years. Whether the eye evolved once or many times depends on the definition of an eye. All eyed animals share much of the genetic machinery for eye development. This suggests that the ancestor of eyed animals had some form of light-sensitive machinery – even if it was not a dedicated optical organ. However, even photoreceptor cells may have evolved more than once from molecularly similar chemoreceptor cells. Probably, photoreceptor cells existed long before the Cambrian explosion. Higher-level similarities – such as the use of the protein crystallin in the independently derived cephalopod and vertebrate lenses – reflect the co-option of a more fundamental protein to a new function within the eye. A shared trait common to all light-sensitive organs are opsins. Opsins belong to a family of photo-sensitive proteins and fall into nine groups, which already existed in the urbilaterian, the last common ancestor of all bilaterally symmetrical animals. Additionally, the genetic toolkit for positioning eyes is shared by all animals: The PAX6 gene controls where eyes develop in animals ranging from octopuses to mice and fruit flies. Such high-level genes are, by implication, much older than many of the structures that they control today; they must originally have served a different purpose, before they were co-opted for eye development.