Photoreception is very old, with various theories of phylogenesis. The common origin of all animal eyes is now widely accepted as fact. This is based upon the shared genetic features of all eyes; that is, all modern eyes, varied as they are, have their origins in a proto-eye believed to have evolved some 540 million years ago, and the is considered a key factor in this. The majority of the advancements in early eyes are believed to have taken only a few million years to develop, since the first predator to gain true imaging would have touched off an "arms race"among all species that did not flee the photopic environment. Prey animals and competing predators alike would be at a distinct disadvantage without such capabilities and would be less likely to survive and reproduce. Hence multiple eye types and subtypes developed in parallel (except those of groups, such as the vertebrates, that were only forced into the photopic environment at a late stage).
Eyes in various animals show adaptation to their requirements. For example, have much greater visual acuity than humans, and some can see light. The different forms of eye in, for example, vertebrates and molluscs are examples odespite their distant common ancestry. Phenotypic convergence of the geometry of cephalopod and most vertebrate eyes creates the impression that the vertebrate eye evolved from an imaging , but this is not the case, as the reversed roles of their respective ciliary and rhabdomeric opsin classes and different lens crystallins show.
The very earliest "eyes", called eyespots, were simple patches of in unicellular animals. In multicellular beings, multicellular eyespots evolved, physically similar to the receptor patches for taste and smell. These eyespots could only sense ambient brightness: they could distinguish light and dark, but not the direction of the light source.
Through gradual change, the eyespots of species living in well-lit environments depressed into a shallow "cup" shape, the ability to slightly discriminate directional brightness was achieved by using the angle at which the light hit certain cells to identify the source. The pit deepened over time, the opening diminished in size, and the number of photoreceptor cells increased, forming an that was capable of dimly distinguishing shapes However, the ancestors of modern , thought to be the were evidently pushed to very deep, dark waters, where they were less vulnerable to sighted predators, and where it is advantageous to have a convex eye-spot, which gathers more light than a flat or concave one. This would have led to a somewhat different evolutionary trajectory for the vertebrate eye than for other animal eyes.
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