the evolution of eyes started with simplest light-sensitive patches in unicellular organisms. these eye-spots do nothing but detect if the surroundings are light or dark. most animals have a biochemical 'clock' inside. these simple eye-spots are used to adjust this daily clock, which is called circadian rhythm. some snails, for example, see no image (picture) at all, but they sense light, which helps them stay out of bright sunlight.