If Adam and Eve were the first humans and Cain and Abel were their 1st offspring, how did the population grow? What about incest?
Maintaining a literal view of the Bible and the book of Genesis, the only way the earth could be populated from Adam and Eve was for their children to marry and procreate. This would be the second generation of humanity.
Adam lived to the ripe old age of 930 and fathered Seth, his third son, at 130 (Genesis 5:3-5). The Bible says he had other sons and daughters. These sons and daughters married and procreated to continue populating the earth at God’s command (Genesis 1:28).
This is a long time to have children. We have no idea how many children Adam and Eve had. But the Bible also doesn’t tell us when they had their first two children. We can infer from the 800 years Adam lived after he had Seth that he had many children.
There are also other theories about how the population of the Earth expanded. Everything from believing in mythical creatures from the Gap Theory between Genesis 1:1 and Genesis 1:2 to an evil female creature that had children with angels. None of these mythological theories can be found anywhere in the Bible.
The next question I am usually asked when I give this answer is, “What about incest?” Incest would only take place in the second generation. Everyone after that would be marrying cousins and it would continue to disperse through the generations.
The issue of incest would pop up once again after the flood. Noah and his wife, his three sons and their wives, are the only ones to repopulate the entire earth. The second generation would be marrying siblings. The gene pool would disperse with following generations.
Incest is not addressed in the Bible until Moses’ raw for Israel. Leviticus 20:10-20 lays out some of the laws against sexual impurity with close relatives. Before Moses’ directly points out incest, it’s possible that it went unpunished. There are a few examples that show a little of how it was viewed in culture before Moses’ law.
Abraham believed the Egyptians would kill him and take his wife Sarah so he made an agreement with her that she would say she was his sister (Genesis 20:5). His son Isaac, would ask the same thing of Rebecca, his wife (Genesis 26:7).
It is clear from the stories for Abraham and Isaac that God is unhappy with both decisions to say their wives are their sisters. But they commit two sins in not telling the whole truth about their relationships with their wives and then the suggestion of incest.