On eyes, creationists, and why intelligent design is just a pimple on creationism's back
The argument is over the quality of the design of the human eye. The human eye, of course, is pretty much the same as the eye of all other vertebrates. This is a good example of "common descent." A vertebrate ancestor had a functional eye, and all the descendants of that ancestor have been modifying that same starting structure. That initial eye evolved by a process that's well understood and backed by theoretical models and field studies. That's not today's problem though.
Today's problem is whether having all the photoreceptors backwards is a good design. In the vertebrate eye, light has to pass through neurons and a layer of photoreceptors in order to be detected. The rods and cones must be more sensitive and are less accurate because of that.
The ICR, and Discovery Institute by citation, argues that this is a necessary trade-off. No other "design" is possible because of the complexities involved.
Unfortunately, that's a lie.
As this handy graphic illustrates, the cephalopod eye and the vertebrate eye are pretty similar, but the squid photoreceptors face toward the light. This is called the eversed (not "verted," a word invented by the creationists).
Because of this arrangement, squid don't have a blind spot in the center of their visual fields and they get clearer vision. They also focus their eyes differently, moving the lens rather than changing its shape.
Fans of accuracy will note that this disproves the DI/ICR spin, and constitutes a two-fold rejection of their half-baked claims. First, it demonstrates that other designs are not only possible, but have advantages over the vertebrate eye, meaning the vertebrate eye makes a poor claim to "good design," especially if you postulate an omnipotent and omniscient designer. And the IDolators do.
Second, this demonstrates how commonly eyes evolved naturally. This page from a textbook illustrates the stages in the evolution of the gastropod eye (snails, which aren't that different from squid). These intermediate stages exist right now. There is no scientific basis for invoking supernatural design. Natural selection shaped those eyes. Similar series can be constructed to illustrate the evolution of the vertebrate eye. Again, no scientific reason exists to invoke magical designers.
Systematists looking at the diverse structures in vertebrates and invertebrates estimate that eyes have evolved as many as 65 times. Not only is it possible for an eye to evolve, it's commonplace.
One of the hallmarks of evolution is the way that contingent historical events leave traces that pass down through the generations. A backwards retina, five fingers, five toes, six legs in insects, four in tetrapods, these are all our inheritance from ancient ancestors. Similarly, arguments about the perfect design of eyes, the attempt to impose theology on science, and an adherence to poor research are ancestral traits that intelligent design creationism shares with the ancestral stock of young earth creationism.