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.
There are two interesting features to this process. First, what may seem an arbitrary choice at the time can have far-reaching and unpredictable consequences. Arranging cells in a bowl or on a bulge makes little immediate difference either way; however, while one may lead to the evolution of a lens, the second does not. When insects made their eyes convex, they were unknowingly choosing the visual equivalent of Betamax.
The second interesting feature of these theories is that, to accord with theories of natural selection, each separate step in the development of an eye must of itself confer advantage upon the organism which develops it. Evolution does not have a mechanism whereby it can reculer pour mieux sauter. Nor can evolution travel through intermediate disadvantageous stages before it happens on a new, really useful adaptation.
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