"From nothing, we have almost everything, almost overnight, geologically speaking. Most of the major forms of animals appear suddenly in a geological sense.
In Darwin's theory, if you think of the branching tree, Darwin's branching tree with the common ancestor at the bottom, and the different modern forms of animals up top. You would have one form to begin with, then it would gradually diverge into slightly different forms, and more and more different, until you get the major differences that we see now.
The problem with the Cambrian Explosion is that all these major differences appear together at the same time, with no fossil evidence that they descended from this common ancestor."