ESV Genesis 1:26a, 27
“Then God said, ‘Let us make man in our image, after our likeness. And let them have dominion . . . So God created man in his own image, in the image of God he created him; male and female he created them.”
Here Walton argues that Genesis 1 is talking about a collective group of humans and not a single human whose name was Adam. The Hebrew word for Adam is the same word for humans. Thus the word “adam" can be used to talk about the human race or a gender or the personal name, Adam. This would mean that God created a group of humans just like he created groups of animals and fish and birds.
This would explain how Cain found of a wife and why Cain was afraid to be cast away from his family and harmed by others. This would also explain how Cain founded a city.
Walton goes on to defend his position through explaining other parts of scripture that seem to be saying otherwise. He argues that the genealogies are tracing their lines back to the “first person of significance in their (the Hebrews) realm of knowledge” (Walton, 188). 1 Corinthians 15’s reference to the “first man” is using this language in relationship to the first man of Adam’s given position; i.e., the first priestly position given. Adam was the first one that failed. Christ was the last one that succeeded. Arguably, Act 17:26, where Paul says “from one man he made all the nations,” is referring to Noah and not Adam. Lastly, Adam naming Eve the “mother of all living things” is a not a biological claim. Likewise the men named afterwards in Genesis 4 were also not biological claims: “father of those who live in tents and raise livestock” and “father of all who play stringed instruments.”
I've always thought that Adam and Eve's long lives provided the time to create monstrously large families living all over the region. However, this assumes that the Cain and Abel story occurred hundreds of years after the fall. And the brother's story seems to suggest that this was their first time doing priestly things similar to what they'd seen their father do. It's almost like a coming of age story. This is me surmising though.
Walton, John H. The Lost World of Adam and Eve. Downers Grove, Illinois: IVP Academic, 2015. Print.