Many researchers have found the evolution of the eye attractive to study because the eye distinctively exemplifies an analogous organ found in many animal forms. Simple light detection is found in bacteria, single-celled organisms, plants and animals. Complex, image-forming eyes have evolved independently several times.

The earliest known fossil of complex eyes date from the Ediacaran, with the appearance of the stem mollusk Clementechiton sonorensis. Diverse eyes are known from the Burgess shale of the Middle Cambrian, and from the slightly older Emu Bay Shale.

Eyes vary in their visual acuity, the range of wavelengths they can detect, their sensitivity in low light, their ability to detect motion or to resolve objects, and whether they can discriminate colours.

 

Note:   The above text is excerpted from the Wikipedia article Evolution of the eye, which has been released under the GNU Free Documentation License.