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Eyes are organs that detect light. The simplest "eyes", in even unicellular organisms, do nothing but detect whether the surroundings are light or dark, which is sufficient for the entrainment of circadian rhythms and may allow the organism to seek out or avoid light. More complex optical systems with resolving power have come in ten fundamentally different "designs", and 96% of animal species possess a complex optical system.[1]

The first proto-eyes evolved among animals 540 million years ago, about the time of the so-called Cambrian explosion.[citation needed] The last common ancestor of animals possessed the biochemical toolkit necessary for vision, and more advanced eyes have evolved on 96% of animal species in 6 of the thirty-something[note 1] main phyla.[1] In most vertebrates and some mollusks, the eye works by allowing light to enter it and project onto a light-sensitive panel of cells, known as the retina, at the rear of the eye. The cone cells (for color) and the rod cells (for low-light contrasts) in the retina detect and convert light into neural signals. The visual signals are then transmitted to the brain via the optic nerve. Such eyes are typically roughly spherical, filled with a transparent gel-like substance called the vitreous humour, with a focusing lens and often an iris; the relaxing or tightening of the muscles around the iris change the size of the pupil, thereby regulating the amount of light that enters the eye,[2] and reducing aberrations when there is enough light.[3]

The eyes of cephalopods, fish, amphibians and snakes usually have fixed lens shapes, and focusing vision is achieved by telescoping the lens -- similar to how a camera focuses.[4]