I’ve been leaning toward the idea that “Adam” was the race of humanity in general. Note that Adam is not called Adam until the second chapter of Genesis. In the first chapter, it just says “man”, and seems to describe “man” as the whole race:
1:27 So God created man in his own image, in the image of God created he him; male and female created he them. (emphasis mine)
So this isn’t necessarily the “patriarchy” thing some like to claim.
My theory is that God created mankind as a complete race, as opposed to two specific individuals, “Adam and Eve”. But at the beginning, they were simply “Adam”. They may have existed for quite some time as “Adam”. Adam was a race created for the purpose of worshipping God, and Adam did so. But God found it unsatisfying - His creation worshipped Him because they were designed to do so. They had no choice in the matter.
So “Eve” was not the creation of “woman”, the female sex. Eve was God’s granting of freewill to the human race. Now that man had freewill, he could worship God, or not, by his own choice. When mankind worshipped out of its own free choice, that worship was far more satisfying to God than the previously “programmed” worship.
However, it was also that freewill, or “Eve”, that allowed mankind to make the wrong choice, and fall from grace. So it wasn’t a case of the “woman” or “female” screwing things up - it was a free choice made by the race as a whole.
In the same manner, if “Adam” was the human race in general, and not a specific individual, then it makes sense that “Cain” and “Abel” were not specific, individual children of Adam, but rather they were new communities which grew out of the original Adam culture. Perhaps the original region where Adam was placed eventually became too small to support the growing population, so smaller groups broke off to move into new areas. The group called Cain migrated to one area - the group called Abel went somewhere else. Cain murdering his “brother”, Abel, was likely a case of the Cain community making war on the Abel community.
This also explains the longevity of the early Biblical characters. Doesn’t it make more sense for the Enosh (Gen. 5:9) culture to last 905 years, rather than a single, 905-year-old man named Enosh? And so when the Enosh culture had been around for about 90 years, a subgroup, Kenan, broke off and formed a new community, and so on and so on.