There is no evidence that the mechanism of random gene mutation and natural selection is sufficient to explain anything other than speciation. To claim that it explains the kind of diversity found suddenly in the Cambrian explosion is an unfounded extrapolation, and is wholly unscientific. Nobody disputes that evolution occurred on this planet, but by what mechanism is the central point of the debate. The atheist must maintain a piori that the whole process from start to finish is unguided and uniform in its mechanism, when in reality it's much more likely that this is not what actually took place on this planet.