'The Light-Switch Hypothesis' proposed by Parker (2003) is reviewed on the basis of recent relevant literature in order to test the hypothesis. This review revealed the following: 1) Diversification of bilaterian animals occurred during the Late Ediacaran Period, based on paleontological and molecular clock evidences. 2) Developmental genetic studies of eyes suggest that the eyes of bilaterian animals were formed from those of the Urbilateria, which hypothetically had both rhabdom and cilium photoreceptors during the Ediacaran period. During evolution, vertebrates utilized cilium photoreceptors, while invertebrates selected rhabdom photoreceptors for the development of eyes. On the basis of the detailed research of the ommatidium surface of the low-light adapted compound eye of the fruit fly, the phenotype of corneal nipple protuberances has changed in a extremely short time period from the view point of the geologic time scale. 3) The oldest fossilized eyes discovered are those of trilobite and bradoriid arthropods from 521 Ma. Increases in body size, and the corresponding increase of energy required, during 630Ma-521Ma may have been triggered by the evolution of the eye.'
