This question is a classic. In 1925, William Jennings Bryan was asked this very question in the famous “Scopes Monkey Trial” in Dayton, Tennessee. He didn’t have an answer then, and unfortunately many believers do not have an answer now.
The question goes like this. Adam and Eve had Cain and Abel, Cain kills Abel, and Cain went off and had a family with his wife. But where did Cain get his wife (Genesis 4:1–17)? Well, the Bible is clear that Adam was the first man (1 Corinthians 15:45), Eve is the “mother of all living” (Genesis 3:20), and all humans are of “one blood” (Acts 17:26). The fact that all people are descendants of Adam and Eve means there is only one biological race, and it’s also the reason we’re all sinners in need of a Savior.
So, what’s the answer? It’s closer than one might think. Just flip the page of the Bible to Genesis 5:4 where it is recorded, “After he begot Seth, the days of Adam were eight hundred years; and he had sons and daughters.” Jewish tradition suggests that Adam and Eve had thirty-three sons and twenty-three daughters. That means, originally, brothers married sisters.
Wait, can you marry your relation? Yes, no, probably, only after counseling? The answer is actually yes; if you don’t marry your relation, then you’re not marrying a human, and you’ve really got problems! Abraham was married to his half-sister, and it was not an issue. It wasn’t until the time of Moses, 2,500 years after creation, in the book of Leviticus, when God prohibited marrying a close relative (Leviticus 18:6).
But why was marrying a close relative originally okay and later on forbidden by God? One of the reasons people don’t marry close relatives today is because humans have thousands of years of accumulated mutations (damaged genetic information) within their genome/DNA. If someone marries a close relative, it is probable both of them will have common mutations, which, if passed on by both parties, will likely end up as a birth defect. So, a person marries someone further away in relation to get good genes to mask bad genes and help prevent the expressing of birth defects.
But, initially, this was not a problem. Adam and Eve were genetically perfect, preprogrammed straight from the hand of God. Even when they sinned and mutations came on the scene as a result of the curse, those mutations would have been few in number for quite a while. So, originally, it would have been no problem to marry a close relative until those mutations accumulated to a detrimental point, and that’s when God stepped in.