Thanks everyone for some really great posts on this thread. Many biologically active molecules absorb light it's to do with the resonance of light with the energy of the electrons involved in the bonds. I know this is a "what" but since simple light absorbing organic compounds could very easily have been created by non-organic means in the Earths early history and have been detected in space. These molecules don't choose to absorb certain frequencies of light it's just intrinsic to the nature of chemical bonds. Now as regards the "astronomical" odds against life or eye's or brains evolving without Devine intervention, we live in an astronomically sized Universe that by definition must contain intelligent life to observe it, and is therefore self-selected. Our job is to explain to ourselves "how" we got here, the "why" of some absolute cause can have no meaning.