All animals have biological dysfunctions, genetic junk, signs of evolutionary dead-ends and obscure morphologies (birds that can't fly, male nipples, etc) and countless other little imperfections that belie the any idea that evolution 'knows' what it is doing1. It merely gets by with whatever comes up, and doesn't make future plans. Many features evolve first for one purpose before being modified and finding themselves useful for another. The human eye is a prime example. The history of the eye has been carefully documented by evolutionists; from its basic form in ancient fish, to designs of ever-increasing complexity and functionality. The professor of evolutionary biology Massimo Piugliucci writes that "we now have both a set of computer simulations showing how the complex vertebrate eye can evolve from simple photoreceptors, and a collection of currently living organisms actually displaying many of the predicted forms (all perfectly functional, which answers the classic creationist question of 'what is half an eye good for?')"2
There was no foresight or plan in the development of the human eye, and our vertebrate ancestors evolved an unfortunate feature: an inside-out retina. The nerves that carry signals from the rods and cones in our retina lay on the sensors instead of under them. This is because once something starts evolving, there is no easy way to restart the design. Things move on, and new designs build on old ones.
As the eyes increased in resolution, more and more nerves lay on the inside of the retina; their way to the brain remains a hole in the retina which now features as the blind spot in our vision. A little foresight on the behalf of nature would have led to a much more sensible design! The biologist and philosopher Daniel C. Dennett comments, "no intelligent designer would put such a clumsy arrangement in a camcorder, and this is just one of hundreds of accidents frozen in evolutionary history that confirm the mindlessness of the historical process"3. It is one the many daft features of our bodies which made anthropologist Scott Atran come to call the whole spectacle "unintelligent design" and declare that there is no God running the show - "why did he invert the retina and give humans (but not the octopus) a blind spot?" he asks4.
The eye has developed independently in quite a few species in quite a few different ways. This why there is so much difference between the eyes of species. It is possible to take eye-forms and map them; we find that their forms are similar in species that evolved from common ancestors. This is why all vertebrates have the same style inside-out retina.
“Other animals, such as octopodes and squids, have their eyes wired more rationally. [...] The retina of the eye evolved as a modification of the outer layer of the brain that gradually developed light sensitivity. The eye is neither poorly nor well designed. It is simply not designed. Eyes provide such obvious survival value that they developed at least forty times independently in the course of evolution.”
"God, the Failed Hypothesis: How Science Shows That God Does Not Exist"
Prof. Victor J. Stenger (2007)5
Despite the strength of the evidence, there are still many religionists who believe that the human eye is not evidence for the blind nature of evolution. Instead, they believe it is evidence for intelligent design (creationism) and declare that the human eye displays a feature called "irreducible complexity". This means that it cannot have evolved from its simpler forms because "what good is half an eye". Although this arguments stems from William Paley (early 19th century) many such people remain unaware, or choose to ignore, the multiple simpler forms of the eye which exist in nature. The creationist view is espoused particularly by fundamentalist Christians from the USA.6
To put this discussion of the eye in the larger context, see my page on "Evolution and the Unintelligent Design of Life: Inherited Traits, Genetic Dysfunction and Artificial Life" by Vexen Crabtree (2007).