Wholly apart from whether evolution successfully accounts for the diversity (which Intelligent Design theorists vigorously dispute), evolutionists themselves admit that it has nothing to say with respect to the origin of life. Indeed, it does not even attempt to do this; rather, it is concerned only to explain what it sees as the transition from a single form of life to the enormously varied forms that we now find. As such, even if one grants the entire evolutionary story, the existence of life as such is left unexplained. Thus the need exists to augment the theory on the supposition that it is true, and it is not at all clear that that supposition is actually justifiable.
You may also be interested in reading Darwin's Predictions