Eyes appeared in the Cambrian period and are found in various organisms, implying a common ancestor. All animals with eyes use the same protein, opsin. The evolution of the eye can be traced incrementally from simple light-sensitive cells. The eye is one of the oldest evolutionary developments and emerged before the evolution of many modern phyla.
Eyes appear in the fossil record about 540 million years ago, at the dawn of the Cambrian period. In some paleontology television shows, 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 only speculation. Numerous organisms with eyes appear in the Cambrian fossil beds, including trilobites, conodonts, and the large shrimp-like predator Anomalocaris.
At one point, eyes were thought likely to have evolved from light-sensing cells in at least five different evolutionary lineages, but more recent discoveries have cast doubt on this, strongly implying that all organisms with eyes come from a common ancestor. . Of course, some organisms, such as some types of cave fauna, are descended from organisms with eyes, but have either lost their eyes or have only vestigial eyes. All animals with eyes use the same type of protein, called opsin, for vision. There is still the possibility that these proteins independently evolved in different lineages.
The eyes are sometimes cited by creationists as an example of an organ that was too complex to have evolved incrementally and must have been created by a designer. However, paleontologists have since shown how the eye may have evolved incrementally from humble beginnings. 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 stages in the evolution of the eye were approximately as follows:
1) A region of photosensitive cells.
2) Photosensitive cells in a depressed area, which allows for directional sensitivity.
3) A water-filled spherical chamber with a hole, also known as a pinhole eye, found in the nautilus.
4) A transparent humor fills the chamber, which is then sealed from the outside.
5) Clear development of a lens and cornea.
6) Development of an iris and a more sophisticated lens and cornea.
The eye is one of the oldest evolutionary developments and emerged before the evolution of many modern phyla. The eye evolved not long after the first shells, another very simple adaptation.
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