Q: How did Adam and Eve populate the earth without brothers and sisters getting married? This may seem like a silly question, but it has been bugging me.
A: The Catholic Church does not side with either monogenism (an original pair of two human beings) or polygenism (more than one couple of people from the beginning of the human race). The Church does not have a stance on this because each theory does not have enough evidence to prove itself correct. This is a great mystery that will probably never be solved here on earth.
Just to explain this a little we’ll start with monogenism, if the theory of monogenism is true then Adam and Eve really were our first parents and every thing that the Bible says in the start of Genesis is completely accurate and brothers and sisters did marry each other. If the theory of polygenism is true than there were multiple couples from the beginning of our race, and the Bible just didn’t mention it so brothers and sisters didn’t marry each other. The funny thing about this is: in Genesis chapter four is the story of Cain and Abel, in this story Cain gets jealous of his brother Abel and kills him. So God banishes Cain from Eden. Now keep in mind Cain and Abel are descendants of Adam and Eve. Well, the Bible goes on to say that Cain marries a wife and has kids, and then it goes down a list of all Cain’s descendants. If you look closely at chapter four you will see that Cain was banished from Eden to the land of Nod, and married a wife in Nod. If he was banished to Nod and married a wife, where did his wife come from? Was she banished from Eden too? Did the Bible not mention every detail? This is probably the biggest part of the mystery of our origin. These reasons are part of why the Church is undecided on this. Without getting into any more detail, I’ll just say if you look at Genesis 1-6 you’ll see these little details that make our origin a mystery. So there is no positive answer to your question. Even great theologians disagree on this subject.