Phenotypic convergence of the geometry of cephalopod and most vertebrate eyes creates the impression that the vertebrate eye evolved from an imaging cephalopod eye, 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 eye-spots, were simple patches of photoreceptor protein 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 eye-spots 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 effective pinhole camera that was capable of dimly distinguishing shapes. However, the ancestors of modern hagfish, thought to be the protovertebrate, 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.
Eyes are organs of the visual system. They provide living organisms with vision, the ability to receive and process visual detail, as well as enabling several photo response functions that are independent of vision. Eyes detect light and convert it into electro-chemical impulses in neurons (neurones). In higher organisms, the eye is a complex optical system which collects light from the surrounding environment, regulates its intensity through a diaphragm, focuses it through an adjustable assembly of lenses to form an image, converts this image into a set of electrical signals, and transmits these signals to the brain through complex neural pathways that connect the eye via the optic nerve to the visual cortex and other areas of the brain. Eyes with resolving power have come in ten fundamentally different forms, and 96% of animal species possess a complex optical system. Image-resolving eyes are present in molluscs, chordates and arthropods.The most simple eyes, pit eyes, are eye-spots which may be set into a pit to reduce the angle of light that enters and affects the eye-spot, to allow the organism to deduce the angle of incoming light. From more complex eyes, retinal photosensitive ganglion cells send signals along the retinohypothalamic tract to the suprachiasmatic nuclei to effect circadian adjustment and to the pretectal area to control the pupillary light reflex. Overview
Complex eyes distinguish shapes and colours. The visual fields of many organisms, especially predators, involve large areas of binocular vision for depth perception.