On the timeline, seems plausible that God may have created any number of people aside from Adam and Eve. Bottom line is we just don't get that much information and there's probably a reason for that, but another thing I noticed is that they had all kinds of things going on, they were mining and forging bronze and iron, making music, building tents, etc. Also when Cain and God talk Cain they refer to other people in the world who would kill him. It's also interesting that Cain is the only one mentioned who wasn't making large enough sacrifices. Seems that it was wild times, and there must have been other selfish people in abundance.

In the NLT in the last verse it says that "they first began to worship Lord by name". I often wonder how the Jews so quickly became isolated as the only people who believed in yahweh. So I guess it makes some sense if they already had cities and Cain early in the story was already afraid of getting murdered if he left his farm by strangers. Adam and eve must have passed on some tradition to their kids... but not enough to inspire worship I guess. Pretty sad. And think of the generational consequences just 100 years later let alone 6000. God left it up to man to a great degree to teach others about him. Good reminder today of that responsibility.