The bible is a history of the Jewish people written quite a few thousand years ago. People were very uneducated then and needed to find ways to explain the things going on around them. They were also very tribal. I suspect that Adam, Eve, Cain and Abel are explainations for the beginings of one particular tribe that grew to be the Jewish people. Most bible stories are parables to teach people how to get along within their tribe and build tribal solidarity. The same us against them mentality that has the world in such turmoil today.