Imago Dei and Natural Selection

John Derbyshire offers some provocative reflections on his own journey away from Christianity.  (HT: Volokh)  Many of his points are great conversation-starters.  Here's one:

I can report that the Creationists are absolutely correct to hate and fear modern biology. Learning this stuff works against your faith. To take a single point at random: The idea that we are made in God’s image implies we are a finished product. We are not, though. It is now indisputable that natural selection has been going on not just through human prehistory, but through recorded history too, and is still going on today, and will go on into the future, presumably to speciation, either natural or artificial. So which human being was made in God’s image: the one of 100,000 years ago? 10,000 years ago? 1,000 years ago? The one of today? The species that will descend from us? All of those future post-human species, or just some of them? And so on. The genomes are all different. They are not the same creature. And if they are all made in God’s image somehow, then presumably so are all the other species, and there’s nothing special about us at all.

Now of course there are ways to finesse that point — intellectuals can cook up an argument for anything, and religious intellectuals, who cut their teeth on justifying some wildly improbable stuff, are especially ingenious — but the cumulative effect of dozens of factlets like this is devastating to the notion that human beings are a special creation. And without that notion, traditional religious belief is holed below the water line. 

I've often wrestled with a similar question: at what point of human evolution did our fallen nature kick in?  Was the first human ancestor who was capable of deliberate decision-making sinful in that decision-making?  Or is sin such a uniquely "human" endeavor that it only arose among creatures who looked pretty much like we do?  If so, is it sensible to think of "the Fall" as occurring at a particular point in time or is it more properly considered as a disposition that unfolded gradually across the evolutionary landscape?  Can anyone recommend any resources on these questions?

Rob