As much as people like to bash the validity of the bible I think it is shockingly accurate with many things. For example, we take it for granted now since the science of the universe and Earth are common knowledge, but for Genesis to accurately describe the order of creation before Galileo, Darwin, or access to the fossil record is pretty amazing.
As far as the story of Adam and Eve, while much of these types of stories are symbolic, with any new heterosexual species there did have to evolve an initial male and female of that species. Incestual interbreeding would not have been as detrimental because you didn't have an abundance of recessive genes like today.
The Noah story is pretty far fetched though, highly symbolic most likely.