The evolution of the eye has been a subject of significant study, as a distinctive example of a homologous organ present in a wide variety of species. Certain components of the eye, such as the visual pigments, appear to have a common ancestry – that is, they evolved once, before the animals radiated. However, complex, image-forming eyes evolved some 50 to 100 times – using many of the same proteins and genetic toolkits in their construction.
Complex eyes appear to have first evolved within a few million years, in the rapid burst of evolution known as the Cambrian explosion. There is no evidence of eyes before the Cambrian, but a wide range of diversity is evident in the Middle Cambrian Burgess shale, and 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 acuity, the range of wavelengths they can detect, their sensitivity in low light levels, their ability to detect motion or resolve objects, and whether they can discriminate colors.
The complex structure of the eye has been used as evidence to support the theory that they have been designed by the Creator, as it has been said to be unlikely to have evolved via natural selection. In 1802, the philosopher William Paley called the eye 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 was perfectly feasible:
...if numerous gradations from a perfect and complex eye to one very imperfect and simple, each grade being useful to its possessor, can be shown to exist; if further, the eye does vary ever so slightly, and the variations be inherited, which is certainly the case; and if any variation or modification in the organ be ever useful to an 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, can hardly be considered real.
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 extant 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.
The first fossils of eyes that have been found to date are from the lower Cambrian period (about 530 million years ago). This period saw a burst of apparently rapid evolution, dubbed the "Cambrian explosion". One of the many hypotheses for "causes" of this diversification, the "Light Switch" theory of Andrew Parker, holds that the evolution of eyes initiated an arms race that led to a rapid spate of evolution. Earlier than this, organisms may have had use for light sensitivity, but not for fast locomotion and navigation by vision.
Since the fossil record, particularly of the Early Cambrian, is so poor, it is difficult to estimate the rate of eye evolution. Simple modelling, invoking small mutations exposed to natural selection, demonstrates that a primitive optical sense organ based upon efficient photopigments could evolve into a complex human-like eye in approximately 400,000 years.
Whether one considers the eye to have evolved once or multiple times depends somewhat on the definition of an eye. Much of the genetic machinery employed in eye development is common to all eyed organisms, which may suggest that their ancestor utilized some form of light-sensitive machinery – even if it lacked a dedicated optical organ. However, even photoreceptor cells may have evolved more than once from molecularly similar chemoreceptors, and photosensitive cells probably 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 protein from a more fundamental role to a new function within the eye.
Shared traits common to all light-sensitive organs include the family of photo-receptive proteins called opsins. All seven sub-families of opsin were already present in the last common ancestor of animals. In addition, the genetic toolkit for positioning eyes is common to all animals: the PAX6 gene controls where the eye develops in organisms ranging from mice to humans to fruit flies. These high-level genes are, by implication, much older than many of the structures that they are today seen to control. They must originally have served a different purpose, before being co-opted for a new role in eye development.
Sensory organs probably evolved before the brain did. There is no need for an information-processing organ -- there is no need for a brain -- before there is information to process.
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