“So do you think that it is safe to assume that the lens of an eye could have first appeared just as a result of a completely random mutation and in fact was not a lens at all. For example it could be just a kind of a protection screen made of transparent cells which allow the light in but don’t allow in any ‘rubbish’? And after millions of iterations with natural selection applied this protection evolved to become a lens.
Do you think it sounds reasonable?”

Above quote is from the discussion thread A question about evolution

It sounds reasonable. I’d point out that many creatures get by just fine with primitive light sensors that lack lenses. The animal that I have in mind is the garden slug. It possesses simple light sensors at the end of two stalks. More sophisticated light sensors would be of little use to a creature like this unless it also had a brain sophisticated enough to process the visual information, and a means of locomotion quick enough so that it could then act upon the information. Here is a short video I made showing the movements of a garden slug, illustrating  the connection between its problem solving abilities, locomotion, and sensory equipment.