Before Darwin, evolution as a hypothesis (Note: Hypothesis, not theory. Scientifically, something's not allowed to reach the theory stage until it has been tested and at least semi-proven) allowed for the existence of God as the designer and planner of evolutionary pathways. (Think dog-breeding for a possibly more-understandable example.) Darwinian evolution, which is what most people think of today when they hear the term, rejects that option, insisting that everything happened completely randomly, without any form of outside interaction.
And the Cambrian Explosion does fly in the face of Darwinian evolution. If Darwin was correct, and everything does happen in moderate micro-steps, there simply wasn't enough time for what happened during the Cambrian Explosion to occur...yet the fossil record shows that it did.
And @ 'Stein: I apologize about getting you confused with someone else. I'll do my best not to do that again.