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The evolution of the eye has been a subject of significant study, as a distinctive example of a homologous organ present in a wide variety of taxa. Certain components of the eye, such as the visual pigments, appear to have a common ancestry that is, they evolved once, before the animals radiated. However, complex, image-forming eyes evolved some 50 to 100 times using many of the same proteins and genetic toolkits in their construction. Complex eyes appear to have first evolved within a few million years, in the rapid burst of evolution known as the Cambrian explosion. There is no evidence of eyes before the Cambrian, but a wide range of diversity is evident in the Middle Cambrian Burgess shale. Eyes show a wide range of adaptations to meet the requirements of the organisms which bear them. Eyes may vary in their acuity, the range of wavelengths they can detect, their sensitivity in low light levels, their ability to detect motion or resolve objects, and whether they can discriminate colours