Eyes appear in the fossil record about 540 million years ago, at the dawn of the Cambrian period. On some television shows about paleontology, the evolution of the eye is cited as the cause of the Cambrian explosion, the rapid diversification of animal life during the Cambrian period, but this is just speculation. Numerous organisms with eyes appear in the fossil beds of the Cambrian, including trilobites, conodonts, and the large shrimp-like predator Anomalocaris.

At one point, it was thought that eyes probably evolved from light-sensitive cells in at least five different evolutionary lineages, but more recent findings have cast this into doubt, strongly implying that all eyed organisms come from a common ancestor. Of course, some organisms, such as certain types of cave fauna, are descended from eyed organisms but have lost their eyes or possess only vestigal eyes. All eyed animals use the same type of proteins, called opsins, for vision. There is still the possibility that these proteins evolved independently in different lineages.

Eyes are sometimes cited by creationists as an example of an organ that is too complex to have evolved incrementally, and must have been created by a designer. However, paleontologists have since demonstrated how the eye could have evolved incrementally from humble origins. Simple eye-like organs can be found in organisms such as planarians and numerous single-celled organisms including euglena, which has a light-sensitive patch.

The steps of the evolution of the eye were roughly as follows:

1) A region of photosensitive cells.
2) Photosensitive cells in a depressed area, allowing directional sensitivity.
3) A spherical water-filled chamber with a hole, also known as a pinhole eye, which is found in the nautilus.
4) A transparent humor fills the chamber, which is then sealed from the outside.
5) Distinct development of a lens and cornea.
6) Development of an iris and more sophisticated lens and cornea.

The eye is one of the oldest evolutionary developments, and emerged prior to the evolution of many modern phyla. The eye evolved not very long after the first shells, another very simple adaptation.