Evolution vs. Intelligent Design: 6 Bones of Contention

Some of evolution's most vocal critics are proponents of "intelligent design," arguing that many structures in plants and animals bear the unmistakable signature of design by a supernatural intelligence.

Intelligent design proponents say the eyes of vertebrates--including humans and the common snapping turtle seen above--could not have evolved in a stepwise fashion. That's because the eye is made of several interacting parts, and the removal of any one part will cause the entire system to cease functioning. Thus, the argument goes, the eye must have been produced in one fell swoop.

"If you look at these [evolutionary] schemes, they often very abruptly add a lens or a cornea," said Casey Luskin, a spokesperson for the Discovery Institute, a Seattle-based organization that advocates intelligent design. But things don't just appear suddenly in evolution, Luskin said. "You need to evolve things in a step-by-step fashion.