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Throughout the history of life on this planet, there have been millions of organisms which have evolved. Each one has specific traits which makes them different from the other species. Depending on whether you classify a species by their morphological differences, inability to reproduce with each other, or by the habitat organisms live in can affect what people consider a species to be. No matter what way you look at the many species on the planet, there is something that unifies us all. The eye, whether it is vestigial, as in Typhlotriton spelaeus, a species of cave-dwelling salamander, or a necessity for life, is present in one form or another in almost every organism on the planet (Freeman, 31).

But what has caused the formation of an organ such as the eye? How can something that seems so complex be developed through evolution? The answer: One step at a time. Remember, life has been present on this planet for roughly 3.2 billion years. A lot can happen through succession and chance in an amount of time that vast. Especially when there aren't to many ways that an eye can form and be efficient.

So how did it happen? How did eyes such as those of squids, octopuses, fish, and mammals evolve from such a primitive form? Click on the video on the right, to see how Dr. Erik-Nilsson, of Lund University in Sweden, describes the formation of all but the insect eye in a broadcast from Evolution, a PBS series.

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