The Wiki Man: Evolution and the airline seat

How can something as complicated as a human eye possibly arise through a process of natural selection — through trial and error? Most people will have asked themselves this question at some point in their lives, but without bothering to find out the answer. A pity, since the stage-by-stage explanation of how the eye might have evolved is fascinating.

The story begins when organisms develop cells that are sensitive to sunlight. In time, these may develop to a level of sensitivity where they can detect movement. The next stage is for these cells to form themselves into a convex or concave shape to add an extra degree of directional information (with sensors arrayed in a bowl or on a mound, you can better detect in which direction any movement is happening). From the concave version you can progress to a retractable protective covering, to a pinhole-camera opening, to a transparent protective covering and from there to a lens.