Asked by: Chris Forshaw, Swindon
The evolution of the eye (which is actually an extension of the brain) was a gradual process, beginning with evolution of individual photosensitive cells. True eyes probably started with the development of photosensitive pits, such as those found in planaria (flatworms). There is evidence that snails in the Cambrian period, some 570 million years ago, possessed such pit eyes, but there may have been more than one biological ancestor for the subsequent rapid evolution.
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