unfortunately all only show is a lack of comprehension of the bible (no offense to you)
The when looking at the Hebrew translation of Genesis the word used for day can mean a given point in time.
http://www.accuracyingenesis.com/day.html
Cain and Able were possibly not Adam and Eve's only children, there may have been various others. (I understand that this leaves open only incestrial breeding but that is beside the point of erradiating possible extinction based on the inability to breed)
(assuming your pending arguements on genetic deteriation of inbreeds and the idea that incest is a sin... will deal with later)
First, the idea of "in his image" does not have to be a physical sense. Second, the idea of prehistoric men is still speculative. Although this is not an area of scientific knowledge that I am particularly coherent in, from what I understand the fossils are inconclusive at best.
Most scientist's problem is this: They see science as something that can answer more than the question "what is?" They preordain a certain idea of reality, completly uncontingent of science (ontological questions are not completed by what is observed). They then base the other non scientific questions, how and why, off their scientific findings in a way that solely supports thier worldview. In effect they say: We know this is, I think god does not exist, therefore a plausible explanation of why it is based on my worldview is this.
i am in no way saying that anyone is right or wrong, but merely that science cannot answer why, nor should it, as its job is of observation of what is.
Why the human race exists cannot be explained by science. It can give possible reasons how it exists but that is not the quesion. The idea of god as a reality cannot even begin to be refuted by science nor can the idea of ethics or purpose without first identifying a biased worldview to base the scientific facts on.
I know this seems like a pointless rant...sorry, I got carried away a little. I am just annoyed by the consistant ignorance of people (of idiotic anti-religion atheists and their ignorant counterparts as the majority of professing christians, both of whom are equally unintelligent)
perhaps I'll spend some more time writing this out tomorrow after I have been flamed to death!