Where did the sons's of Eve get their wives?

Our question this week comes in 2 parts:  Were there other human beings outside of the garden of Eden?  How did Eve’s sons find wives?  

For our answer, let’s go directly to the narrative of Genesis 4.  After Cain murdered his brother Abel, he received just punishment from the Lord.  It appears in verse 14 that he is afraid someone will hear of what he did and kill him for it.  One wonders, who exactly he is afraid of?  

In verse 17 we read: Cain knew his wife, and she conceived and bore Enoch.  At which time the account breaks off into a genealogy, listing Cain’s progeny 6 generations deep.  All took wives.  Lamech, took two.  One wonders, where exactly did they all come from?  

There are 2 things to consider about biblical, historical narratives.  First, they are not exhaustive, meaning that as history unfolds on the page, every detail about every event and every person is not included.  It can’t be.  This is why John famously says at the end of his gospel, “…there are also many other things that Jesus did. Were every one of them to be written, I suppose that the world itself could not contain the books that would be written.”  (John 21:25).

Second, the Genesis narrative does not specify periods of time.  We are not told how much time passes between events.  We simply do not know how long it had been between Abel’s death and when Cain faces judgment and so on.  This opens us up to several possibilities in answer to our question.

One possibility is that there were other people on the earth at that time.  Dr. Michael Heiser explains, some see pre-Adamism, humans around before Adam; and others co-Adamism, humans alongside of Adam.  (https://nakedbiblepodcast.com/wp-content/uploads/2017/08/NB-171-Transcript.pdf).

The text suggests the existence of others besides those named in the narrative, but let me give you a word of caution.  We do not need to operate in the land of suggestion.  We need to the let the Bible speak for itself.  God does not need our inferences or presuppositions to explain Himself.  

Views like this one have become popular as a means of blending biblical revelation with humanistic evolution.  The two are incompatible.  And should be dismissed.  

The other option and the most straightforward is that Eve’s sons took wives from among their sisters.  In Genesis 5 we read:  When Adam had lived 130 years, he fathered a son in his own likeness, after his image, and named him Seth. The days of Adam after he fathered Seth were 800 years; and he had other sons and daughters (5:3–4).  It is plausible that over that span of time the population would have been quite large.

Jewish and Christian traditions alike postulate that Cain took his wife from among his sisters.  The non-canonical book of Jubilees records that Cain’s wife was in fact his sister, Awan.  Intermarriage was common in the antediluvian world.  Later through the law of Moses it was denounced (Leviticus 18:9), and then had more to do with the distinction between Israel and the pagan nations around them.  

This may be repulsive to us as moderns.  However, consider that it was God’s will and purpose for the entire earth to be filled with His image bearers from just 2 people, not many people groups(Genesis 1:28).  Therefore, it is the safe conclusion that Eve’s sons took their wives from among their sisters.