The Yin-Yang of Ultraviolet Radiation

From SPACE.com:

Now that we have passed through summer, replete with warnings about the health hazards of exposure to ultraviolet radiation, the subdued light of autumn provides the ambiance in which to take a more balanced look at what UV radiation has meant to life on Earth. We need UV radiation to synthesize vitamin D, which is critical for calcium absorption. UV radiation is used by some organisms as an environmental cue, and it aids in some repair mechanisms for DNA damage. Further, UV-catalyzed reactions in the atmosphere and on the early Earth were critical to providing the conditions for life to arise. But the fact remains: UV radiation itself is hazardous to carbon-based life such as ours. Why should this be so, and how has life overcome this obstacle to thrive on Earth?

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