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Eye Evolution Eyes can provide information such as light direction and contrast between light and dark, some eyes can form focused images. Among multi-cellular animals there are four main types of eyes, flat-sheet eyes, cup-shaped eyes, vesicular eyes, and convex eyes. The structure and function of eyes are to be able to detect the direction from which light has entered the photoreceptive organ. Photoreception is the ability to absorb light used as vision in animals. Photoreceptors are cells that respond to light. All cells are photosensitive. Plants absorb light and use it in photosynthesis. Therefore to me this would give the beginning to evidence photoreceptors have evolved. The evolution of animal photoreceptor cells has been a debatable question in science. With Charles Darwin himself, the great English Naturalists, saying: “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 freely confess, absurd in the highest degree.” Gave Creationist reason for their argument of Intelligent Design being taught in schools. In this paper I will present current evidence that photoreceptors have their origin in a proto eye believed to have evolved some 540 million years ago and the process only took a few million years thru small mutations and natural selection. Monophyly of all animal eyes is more widely accepted based on shared anatomical and genetic features of all eyes. At a molecular level the genes that control eye formation are similar. The gene Pax6 that codes for a transcription factor has been isolated from humans, mice, chickens, sea urchins, and prosophila. Loss of function mutations in this gene caused reduced absent eye structure in both vertebrate and invertebrates. The involvement of Pax6, which encoded transcription factors in the genetic control of eye development in organisms ranging from planarians to humans, argues strongly for a monophyletic origin of the eye. All light sensitive organs rely on photoreceptors systems. Photoreceptive organs range in complexity from single light sensitive cells to complex eyes that can form sharp focused images. The structure of photoreceptor cells is divided into two major types among the animals, rhabdomic photoreceptors and ciliary photoreceptors. All vertebrate photoreceptor cells are ciliary. Ciliary photoreceptors have a single cilium protruding from a cell. Ciliary photoreceptors are subdivided into two subclasses rods and cones. Different mammalian have different rods and cones based on different functionality. An example is nocturnal mammals have a higher number of rods for better vision in dim light. Feature Rods Cones Class of Ciliary Ciliary photoreceptor Shape Sensitivity Type of photo Outer segment rod Outer segment shaped cone shaped Sensitive to dim Sensitive to bright light light One type Up to three types pigment A recent discovery I read in Principles of Animal Physiology, Moyes & Schulte 2006, claimed that rhabdomeric photoreceptors in some vertebrate pass through a developmental stage in which they have cilia suggest the possibility that all photoreceptor cells are derived from an ancestral ciliated cell. Also the bilateral ancestor of the Protostomes and Deuterostomes may have already possessed two types of photoreceptors, one of which may have been lost in some evolutionary linage. W.J Gering (2005, Journal of Heredity) traced back the evolution of the eye beyond bilaterians, he found highly developed eyes in some box- jellies as well as in some Hydrozoans. Hydrozoans have the same six genes required for eye regeneration as in planarians, and in box-jellies, Tripedalia. Sight is obviously a huge survival advantage for a mobile animal. Prey animals and competing predators alike would be forced to rapidly match or exceed any such capabilities to survive. Hence multiple eye types and subtypes developed. Eye variations in animals show adaptation to their requirements for survival, form to fit function. All primates and many predators have binocular vision. This is the ability to combine and compare information from each eye to provide a three dimensional view of the world. An example is convergent evolution of binocular vision in owls and other binocular vision species. Owls, which have excellent binocular vision, have eyes on the front of their heads and a large binocular zone, but all of their optic neurons cross at the optic chiasm. Owls use a different mechanism to process the visual signal and provide binocular vision. The two sides of the brain communicate with the other in other parts of the visual pathway, allowing both sides of the brain images from both eyes and providing for the necessary condition for binocular vision. In conclusion, tracing back the evolution of eyes leads scientist to believe that the eye is a homologous organ present in a wide variety of species. The structural sequence of its’ evolution is evident on a molecular level and thru small mutations because of natural selection. Resources: Arendt, Detlev. “ Evolution of Eyes and Photoreceptor Cell Types.” Journal of Developmental Biology, 2003 47:563571. Gerhrig, W.J. “New Perspective on Eye Development and Evolution of Eyes and Photoreceptors.” Journal of Heredity, 2005 96(3): 171-184. Klanderf, Sherwood. Animal Physiology From Genes to Organisms, 2005 Brooks/Cole division Thomson Learning. Moyes, C & Schutte, P. Principles of Animal Physiology, 2006 Pearson Education, San Fran.