[Smokin' Joe]

It is easier to sort out familial obligations when the family tree doesn't look like a briar patch. It also makes it easier to determine who is related to who, at least closely enough that bad genetics aren't amplified--the results of which would be evident even before genetics were understood.

Whether people simply listened to scripture, or whether they had worked out that this was best, having codified it with rules for marriage made sense for a healthier population.

[SMARTY]

Right, and I think too that people understood early on that the possibility of birth defects increased in children born to parents who were TOO closely related.

All sorts of marriage taboos were worked out to avoid this. Also, I think that the 'raids' (to steal women) and fighting between tribes was a way to expand the gene pool... even though primitive people didn't comprehend DNA.