LOL, I would also believe if you didn’t believe in the bible chances are pretty good we are still the sons and daughters of incest.
Jewish tradition says that Cain did indeed marry his own sister, allowance thereof given to his generation as a special “kindness” by G-d to allow for propogation of the species.
On a semi-related note, all of humanity was killed off during the flood with the exception of Noah, his wife, their three sons, and their wives. I figure we don’t have to go back to A&E to find a genetic choke point; the flood story implies we all have five ancestors: Noah, Mrs. Noah, and their three daughters-in-law. Unless these people were “perfect” in a similar vein as postulated for A&E, we’ve got some major inbreeding problems.
Does the bible explicitly say that?
At first, anyway, until mutations and such begin to stretch the gene pool out again. Anyone who is particulary damaged by being a product of incest probably isn’t going to live very long anyway.
If you look at the Creation story as allegorical, it’s not that difficult. “Adam” was the human race. I have given some thought to the idea that the creation of “Eve” was symbolic of God granting freewill. In other words, “Adam” was merely a species of animal, albeit an intelligent animal, who worshipped God. God found this “worship by design” unsatisfying, and so imbued Adam with freewill, “Eve”, so that his worship would be a conscious decision.
So if we look at “Adam” being the human race in general, we can suppose an original community of people collectively known as “Adam”. Once Adam grew large enough in population to put a strain on the local resources, groups split off to find new homes. “Cain” was one group, “Abel” was another, each perhaps named for the leader of the departing faction.
Then at some point, the community called “Cain” decided to make war upon the community called “Abel”, and was victorious.
This idea also goes some distance in explaining the long lifespans. It makes a bit more sense to assume that Seth, for example, was a community/culture that survived for 912 years, rather than an individual who lived for 912 years.
So, to answer the OP, the continuance of generations can be looked at like this:
One group “married” another by joining together, or melding, into one larger group. The produced children by growing large enough that subgroups broke off to create new communities/cultures elsewhere.
The same principle can be applied to “Noah” and his “sons”, thought I haven’t thought that one through yet 