<P> The earliest predecessors of the eye were photoreceptor proteins that sense light , found even in unicellular organisms , called `` eyespots '' . Eyespots can only sense ambient brightness : they can distinguish light from dark , sufficient for photoperiodism and daily synchronization of circadian rhythms . They are insufficient for vision , as they can not distinguish shapes or determine the direction light is coming from . Eyespots are found in nearly all major animal groups , and are common among unicellular organisms , including euglena . The euglena 's eyespot , called a stigma , is located at its anterior end . It is a small splotch of red pigment which shades a collection of light sensitive crystals . Together with the leading flagellum , the eyespot allows the organism to move in response to light , often toward the light to assist in photosynthesis , and to predict day and night , the primary function of circadian rhythms . Visual pigments are located in the brains of more complex organisms , and are thought to have a role in synchronising spawning with lunar cycles . By detecting the subtle changes in night - time illumination , organisms could synchronise the release of sperm and eggs to maximise the probability of fertilisation . </P>
what is the first step in the evolution of the eye
photoreceptor proteins that sense light , found even in unicellular organisms , called `` eyespots ''