And here is an example of how, at any minute, the evidence used in an educated guess can change.
http://www.theaustralian.news.com.au/story/0,25197,22289797-30417,00.html
While this doesn't have anything too do with The God Dellusion, it proves that what Dawkins thought was correct is now being questioned.
Although only a minor fact it still shows how much we really don't know about our past. Hardly something I would base such a large argument on.