An atheist before Darwin could have said, following Hume: "I have no explanation for complex biological design. All I know is that God isn't a good explanation, so we must wait and hope that somebody comes up with a better one." I can't help feeling that such a position, though logically sound, would have left one feeling pretty unsatisfied, and that although atheism might have been logically tenable before Darwin, Darwin made it possible to be an intellectually fulfilled atheist.I have to disagree with this statement. While I do think that Darwin's work is one of the great scientific advances, I don't place nearly the same exclusive importance on it as does Dawkins.
Darwin answered an important scientific question, his predecessors (e.g. Newton and Laplace) answered important scientific questions, as did his successors (e.g. Einstein, Feynman). There are today still many important scientific questions as yet unanswered. Each and every scientific advance, from the trivial to the spectacular, contributes to the intellectual fulfillment of the scientist and metaphysical naturalist.
To every unanswered scientific question, we must adopt the stance Dawkins attributes to the pre-Darwin atheist: "I have no explanation for thus-and-such, but I know that God isn't a good explanation," and remain unsatisfied until that question is answered. And then we will be unsatisfied at the next unanswered question. This continuing dissatisfaction is a feature, not a bug. It is precisely this sort of dissatisfaction that motivates us to actually answer each new question. Fulfillment is always partial and temporary; if we were completely "intellectually fulfilled", at least as scientists, we would stop looking for answers.
It's a much simpler issue, though, to be intellectually fulfilled as an atheist. All we really need to know is that God isn't a good explanation for anything. And it isn't. Supernaturalism is incoherent and contradictory on its face. It posits something that exists "outside" the universe, but the universe includes, by definition, everything that exists. Paranormalism and "intelligent design" as ontological theories (theories about things that might exist) are actually false or unfalsifiable and therefore vacuous and without explanatory power.
Regardless of unanswered scientific questions, there is simply no coherent, consistent, and substantive alternative to atheism. Theists have since the very beginning offered us nothing but hot air and bullshit in defense of their temporal authoritarianism. Before the invention of the scientific method, they can, perhaps, be excused. Since then, however, there is no excuse at all.