Why should God be excluded? For what good reason? I submit you reject this answer for purely religious reasons.
You take great stock in rationality, so explain how the designers came into being if not directly created by God. Is there more than one God capable of creating life? I am really curious how ID is different from creationism.
I use rationality because God created people as rational beings, giving us the ability to understand His creation so that we can live effective lives. There is only one God. There are not gods. How do I know? Let's use logic on this one too. There are many great religions, but they contradict each other. Christianity says (the bible says) there is one God and that Jesus is God (Triune God); islam says there is one God but He is not triune and Jesus is not God. Both cannot be true at the same time and in the same sense. The Law of Contradiction does not allow it. The only possible answer could be that both are false or ONE is true, however, both cannot be true. In addition, the rational person looks at the evidence for all the religions - which one has the best evidence. Well, hands down - it is Christianity. It is the only one with any evidence to speak of. That's why you never see skeptics challenging the koran or the Hindus bibles or Buddhism - they argue only against Christianity because it is the one with the real evidence. The bible is a historical narrative - it begins from creation, and goes to the end of creation, Jesus is historical, the Apostles are historical, the old testament is historical.