If Adam and Eve only had two sons, where did all the people come from?
Its a common question for both believers and unbelievers. Where did all the people come from? Where did Cain find his wife? How could he build a city when there should have been only four people?
Lets begin by saying that the Bible doesnt specifically tell us. There are many details that the Bible leaves out, but that doesnt make the story less true. After studying the absence of details and drawing conclusions from reason, the Bible often appears more reasonable than it would if the details were included.
The Bible does tell us that Adam and Eve had many more sons and daughters. In Genesis 5:4, we find, After Seth was born, Adam lived 800 years and had other sons and daughters (NIV). While this does not tell us about the chronology of the events, it tells us that there were more than just four people on earth during the early years of mankind.
Now that we see that there was more than just Cain and Seth, consider mans extremely long lifespan in the days between Adam and Noah. The average man listed in the Bible during this time lived about 900 years. To help you understand this, Adam was still alive when Noahs father was born. There were 8 generations living during Adams lifetime through Seths lineage.
Using this reasoning, the population when Cain took a wife was probably large enough to have several eligible wives for Cain. So Adam and Eve’s children most likely married among each other to start the growth of the population.
Some people bring up the problem of incest when talking about this topic. It was common through Abrahams days to find sibling marriages. This was how God filled the earth. Considering the purity of the gene pool through the first dozen or so generations, it is not likely that this would cause the difficulties it would cause today.