Though by many inerrantists the question of where Cain got his wife would not be considered a problem at all, this question is often used by those who try to demonstrate that the Bible is unreliable in what it claims. How could it claim that Adam and Eve were the first human beings who had two sons, one of whom murdered the other, and yet who produced a large race of people? Clearly, the Bible does teach that Adam and Eve were the first created human beings. The Lord affirmed this in Matthew 19:3-9. The genealogy of Christ is traced back to Adam (Luke 3:38). Jude 14 identifies Enoch as the seventh from Adam. This could hardly mean the seventh from “mankind,” an interpretation that would be necessary if Adam were not an individual as some claim. Clearly, Cain murdered Abel and yet many people were born. Where did Cain get his wife?
We know that Adam and Eve had other sons and daughters in addition to Abel, Cain, and Seth (Gen. 5:4), and if there was only one original family, then the first marriages had to be between brothers and sisters. Such marriages in the beginning were not harmful. Incest is dangerous because inherited mutant genes that produce deformed, sickly, or moronic children are more likely to find expression in children if those genes are carried by both parents. Certainly, Adam and Eve, coming from the creative hand of God, had no such mutant genes. Therefore, marriages between brothers and sisters, or nieces and nephews in the first and second generations following Adam and Eve would not have been dangerous.
Many, many generations later, by the time of Moses, incest was then prohibited in the Mosaic laws undoubtedly for two reasons: first, such mutations that caused deformity had accumulated to the point where such unions were genetically dangerous, and second, it was forbidden because of the licentious practices of the Egyptians and Canaanites and as a general protection against such in society. It should also be noted that in addition to the Bible most other legal codes refuse to sanction marriages of close relatives.
I always find it highly amusing when people use knowledge gained from modern evolutionary theory to justify belief in the literal truth of the Genesis story. Notice also that the writer causally asserts that ‘many, many generations’ from Adam to Moses was enough for mutations to accumulate to make incest undesirable. But according to Bishop Ussher’s chronology that put creation at 4004 BCE, Moses was born around 1600 BCE, about 2400 years from Adam. If we take a generation as 25 years, that only gives us only about 96 generations, hardly enough to accumulate all these degenerative mutations.
But it is even worse than that. According to the Bible, the time taken to go from Adam to Noah’s children (10 generations) was about 1500 years because people supposedly lived for much longer times then and had children when they were well over 100 years of age. This leaves just 900 years to go from Noah to Moses. Assuming that the average generational age suddenly dropped in that period to the present day value of about 25 years, that gives us about 36 more generations, so that the total number of generations from Adam to Moses was about 46, hardly deserving of the description of ‘many, many’ and nowhere enough for all the genetic deficiencies to accumulate that caused Moses to decree laws against incest.
Biblical literalists will have to do better than this to get around their incest problem.