Most multicellular animals have photoreceptor cells that respond to light. Animals with simple forms of photosensitivity can move toward or away from light sources. The photoreceptors of other animals are arranged to form eyes that produce an image. The eyes of some of these animals, such as those of annelid worms and arthropods, are adapted to detecting the motion of shapes and simple patterns of light. Other animals, such as vertebrates and cephalopod mollusks like squid and octopus, perceive sharper images.

Although animals have evolved a variety of light-sensing organs, all rely on the same light-sensitive protein, called opsin, to convert light energy into electrical signals in the receptor cell. Opsins are ancient molecules. They evolved early in the tree of animal life as receptor proteins sensitive to many different stimuli, including light. They are G protein-coupled receptors. As we saw in Chapter 9, G protein-coupled receptors activate G proteins, leading to a cellular response. In this case, the cellular response is a change in membrane potential. The observation that different types of animal eye use a common light-sensitive protein suggests a common evolutionary origin for animal eyes about 500 million years ago in the Cambrian Period.