Yeah... I used to wonder about the Cain and Abel thing as well.
Christians can 'explain' everything away though. I asked a friend how old he thought the world was. He said he wasn't sure. He believes God created the universe and the world but he wasn't sure if the universe was billions of years old or if it was 6000 years old. He had a whole bunch of theories based on time compression - which is actually good. If you have a theory that time was compressed in the past, it's possible to make testable predictions. Too bad they've never come up with a cohesive theory.