Origins and diversification of the organs of sight through geologic time
Many scientists have found the evolution of the eye attractive to study because the eye distinctively exemplifies an analogousorgan found in many animal forms. Simple light detection is found in bacteria, single-celled organisms, plants and animals. Complex, image-forming eyes have evolved independently several times.
^""An important cue for understanding eye evolution is the distinction between different types of photoreceptor cells. Salvini-Plawen and Mayr ( 1977 ) noted a remarkable diversity of photoreceptor cell morphology across the animal kingdom, and suggested that photoreceptors evolved independently numerous times." – Land, M.F. and Nilsson, D.-E., Animal Eyes(2-nd ed.), Oxford University Press, Oxford (2012), p. 7.