Where did Cain get his wife?

If the human population only started out with two people, Adam and Eve, where did Cain get his wife? The most obvious answer is, Cain married his sister (gasp). Marrying blood relatives is a social taboo today, but there's no reason to believe it was taboo in our ancient antediluvian past.

The Bible only mentions three named children belonging to Adam and Eve: Cain, Able, and Seth, all male children. But Genesis 5:4 says that Adam had sons and daughters. Clearly then, Adam and Eve had other children that were not specifically mentioned.

So, let's take a speculative journey of how many children Adam could have fathered. I used ChatGPT to help me work out the hypotheticals. We have a lot of unknown variables: how long did Eve live? How long were her childbearing years? How long would she have waited to conceive between children? We can only guess here, so we're never going to know the exact numbers.

For the sake of this discussion, let's make some assumptions. Let us assume Eve's lifespan was equally as long as Adam's, who lived to be 930 years. Let's also assume Eve's childbearing years were proportionate to her lifespan. With these assumptions in mind, Eve could have potentially been fertile for 400 years. That could have allowed her to conceive hundreds of children in her lifetime. ChatGPT gives 150-200 as a hypothetical number, though it could have been much greater or lesser.

Here's another question: how high was the population prior to the Genesis flood if we start out with only two people? The Genesis flood was 1,656 years after the creation of Adam (there are other figures given, but this is the traditional number). If everyone prior to the flood had equally long lifespans, and there's no reason to suggest they didn't, the population could have been in the millions. ChatGPT suggests it could have been roughly 86 million people, but again, this is a hypothetical number.

The point being, it would not have been difficult for Cain to find a wife or Seth for that matter. This goes back to the subject of incest. One of the reasons it is taboo, and in many parts illegal, today is because it poses a genetic risk to the offspring. If two blood relatives have the same harmful recessive genes, it will become expressed in their offspring and produce physical and mental impairments. But this may have not been an issue with pre-flood humans.

There is something called genetic entropy. In simple terms, genetic entropy is the genetic mutations that accumulate over many generations. Every time DNA is copied, it produces small errors. Those errors are then passed onto the next generation. Because Adam and Eve were the first, their DNA would have likely been far less corrupted than our own, as were their subsequent descendants. So, genetic entropy wouldn't have been a problem for them like it is for us, at least up until a certain point.

After the flood, they stopped marrying their full-blood relatives. Abraham married his half-sister, while Isaac and Jacob marry their first cousins, allowing for some genetic diversity. During the time of Moses, Levitical laws forbid sexual relations between close relatives, including half-siblings, though marrying cousins was still permitted. So there was a later progression to mitigate the effects of genetic entropy.