Did Adam and Eve's Progeny Commit Incest?

Maybe it's an allegory and not intended to be taken as literal as we often do.

I've always thought that there were 'others' with whom Cain mated. Within the known history of the world outside of the Bible, I speculate that the first humans were Adam and Eve. That is, they were the first intelligent beings who's children mated with the Neanderthals.

It is scientifically speculated that Homo Sapiens came out of Africa where they encountered the Neanderthals, who were stronger and better hunters (implying warriors) than the Homo Sapiens. The Homo Sapiens retreated back into Africa for years and then came back north, but this time for some unknown reason (I believe that it's the fruit of knowledge of good and evil) this time they had an enlarged brain. The rest as they say is history.

Further reading about this: http://www.pbs.org/wgbh/nova/insidenova/2010/05/did-neanderthals-mate-with-modern-humans.html

This would also explain why Cain was fearful of the others killing him when he was sent away.

Yes, according to those that hold a literal view of Genesis, they did commit incest, but it wasn't a problem at the time.

The reason why incest is medically bad is because it makes genetic birth defects more likely. Most genetic defects are recessive. If two people who both have the gene have children, the childrenm may inherit the gene from both parents, and it will materialize. But if someone with the bad gene marries someone without the gene, then their children have one good gene and one bad gene, and -- assuming it's receissive -- they will not manifest the defect.

Bad genes are mutations, that is, DNA damaged by toxic chemicals, radiation, etc. Adam and Eve were presumably created with no bad genes. It took time for mutations to accumulate. So incest was not a medical problem for the first generations.

Incest also has social implications, the confusion of roles of "sister" and "wife" or "brother" and "husband". That would have been less of an issue before the Flood, when people lived longer. If Eve lived to be circa 900 years old like Adam, and if she was fertile for the same percentage of her life as a modern woman. That is, if a modern woman is fertile from mid teens to mid 40's, circa 30 years or 1/2 to 1/3 of her life, then if Eve lived to be 900, she might have been fertile for 300 years. She could have had children decades or centuries apart, who would not have been raised together.

And by the way, if Eve was fertile for several hundred years, she could have had dozens or even a hundred or more children in her life. And none of them ever called.

There is a third option -

The prohibition on incest didn't come about until the covenant in Leviticus & Deuteronomy, and to accuse Cain, Abel, and Seth of incest is to accuse them ex post facto.

As an aside, a Young Earth Creationist would date the Creation to 4004 BC, and the Exodus (and hence the Covenant) to about 1440BC. As such, you would be accusing them of a crime that wasn't mandated for nearly 2600 years.

If you want to say, "but surely the law goes back further than the covenant," you get into territory of, "So when did it become a law?" Incest is prohibited in most secular cultures today because it weakens the gene pool. Over time, the level of closeness has gotten wider in order to ensure a wider mixing. In modern times, a first or second cousin is off-limits. If I remember correctly in Leviticus, immediate family was prohibited, but I believe that cousins were ok. If you follow the trajectory backwards, you'd probably arrive at a date at which it was ok even for family members to procreate together, societally speaking.

An objection has been raised worth noting:

regardless, biology hasn't changed. legally incest wasn't "wrong" yet but humanity does not tend to survive well on inbreeding.

My response stands, however:

Agreed that humanity doesn't tend to survive well on inbreeding, but it fares even more poorly if the first of the species refrain from procreation altogether