My theory is Adam and Eve had evolved beyond the existing humans, based on a more advanced mind/brain. There were also other humans at that time, based on DNA. but they were prehumans in comparison. Adam and Eve as the first humans is about the mind/brain and not the DNA.
I infer this from the story and Cain and Abel. Cain kills Abel and is sent away for his crime. Cain complains whomever shall come upon me shall kill me.
The question I asked was, why was he afraid of these whomevers, if only Adam and Eve were left? They would have a better chance to kill him, if he stayed instead of left. Yet he was not afraid to stay. He was afraid to leave. This means the whomever were different than Adam and Eve. They were the prehumans, who had human DNA, but a retro mind/brain.
The story of Adam and Eve times out with the initial formation of civlization. This required a more advanced mind/brain compared to the advanced animal behavior of the prehumans.
An analogy is you explore the Amazon and come upon an isolated tribe. They may not be able to advance beyond tribal, on their own, even though they have human DNA. DNA is not the pivotal point, rather their mind is stuck in prehistoric times closer to natural instinct. Based on mind, you are so much more advanced, you may appear like a God. You can still breed with them, since they have human DNA, but the new history of that quickly advancing tribe will be centered on your quantum jumping them into the future.
If we change the scenario slightly, there is another person helping you advance the tribe. The two of you have a fight and you kill him. The primitives understand fighting and death, in their own primitive way, and some may not like you after that. You are now afraid to walk alone at night now since these savage whomevers might get even.
God gives Cain a sign for protection so the whomevers will let his crime against Abel slide. Cain is banished but is able to live among the pre-humans and can breed with them since both he and they have human DNA. The females are wild like prehuman animals, so he is encourgaged and can use his magic prestige to gain selective advantage. His children are half breeds who are still instinctive but with some modern learning power.