When Cain is sent as a wanderer in the land, he meets a wife and builds a city. Clearly there are other people in the world by this time. But how? Time frame.Not all of Adam and Eve’s descendants are named.
- We don’t know how old Cain was when he married and met others
- Adam was also likely 130 years old by the time he fathered Seth, giving plenty of time to produce other children.
In fact let’s look at this verse:
“Today you are driving me from the land, and I will be hidden from your presence; I will be a restless wanderer on the earth, and whoever finds me will kill me.“ (Genesis 4:14)
By the very mention of “whoever finds me” implies that there were many others on the earth before the time frame of when he murdered his brother.
All the details we do not know.
But there have been several research studies to indicate that humanity comes from a single couple. Time magazine many years ago did a special piece on this. And even recently the Swiss confirmed this: https://nypost.com/2018/11/24/turns-out-all-of-humanity-is-related-to-a-single-couple/
The Swiss research shows that the couple came after a catastrophic even which would fit the descendants of Noah and his wife as they were all who were left after the flood.
Regardless, Adam and Eve produced many, many children, only a few of which are named early in the Biblical narrative.