Cain would have lived much longer than 100 years. More likely closer to the 900 years common back then. Only after Noah was the lifespan shortened.
Not sure why this is important to you but it's obvious that in the beginning it had to be close relatives. It's logical to understand that God would have made possible those reproductive possibilities without negative consequences.
>>With due respect to your faith, I stand by my belief that the OT is mostly fable.<<
I suppose if you ignore all available archaeological and historical proof that's possible. Every historian to date that I am aware of who set out to disprove it ended up believing it.
>>BTW, who wrote Genesis? Serious question.<<
Bottom line is that it is God who is the author as He is all of scripture. Through which human is not totally proven. We do know that cuneiform tablets have been discovered that date all the way back to 3500BC so that would be in Adam's lifetime since he would have died around 3074BC. Most believe that it was Moses and/or Israelite scribes who compiled the information.
Every archaeological and historical proof was written by unaware and superstitious men, especially cuneiform tablets of ancient history. Please give me references of which historians came to believe and why.
The importance to me is the timeline of what was written in Genesis. It doesn't work. But then, some suggest much of the Bible is metaphoric. Okay, but metaphors are subject to interpretation. Therein lies my ultimate quandry, not to mention. There is NO proof that any of Genesis actually occured.
Thank you for addressing the point of siblings having children. I didn't want to add a new debate. So, why did God later decide that incest was bad and caused proven deformaties? Or was that a human decision not to sleep with a close relative?
Plus I did say 100's of years in parentheses for Cain. Thank you for your courteous reply. I'm trying...