The evolution of the eye has been a subject of significant study, as a distinctive example of a homologous organ present in a wide variety of species.
The development of the eye is considered by most experts to be monophyletic; 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.
The majority of the process is believed to have taken only a few million years, as the first predator to gain true imaging would have touched off an "arms race". Prey animals and competing predators alike would be forced to rapidly match or exceed any such capabilities to survive.
Hence multiple eye types and subtypes developed in parallel. Eyes in various animals show adaption to their requirements.
For example, birds of prey have much greater visual acuity than humans and some, like diurnal birds of prey, can see ultraviolet light.
The different forms of eye in, for example, vertebrates and mollusks are often cited as examples of parallel evolution.
As far as the vertebrate/mollusk eye is concerned, intermediate, functioning stages have existed in nature, which is also an illustration of the many varieties and peculiarities of eye construction.
In the monophyletic model, these variations are less illustrative of non-vertebrate types such as the arthropod (compound) eye, but as those eyes are simpler to begin with, there are fewer intermediate stages to find.
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