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Up to this point in Genesis we only know about Adam and Eve, and Cain and Abel. But the most obvious answer to this common question must be that Adam and Eve had other children, including daughters. Indeed, Genesis 5:4 plainly says as much, "[Adam] had other sons and daughters."
Cain must have married his sister. But to admit this is to raise a further difficulty: was he thereby guilty of incest?
At least two things can be said in response to this reproach. First, if the human race was propagated from a single pair, as we believe the evidence indicates, such closely related marriages were unavoidable. The demand for some other way of getting the race started is an unfair expectation.
In the second place, the notion of incest must be probed more closely. At first the sin of incest was connected with sexual relationships between parents and children. Only afterward was the notion of incest extended to sibling relationships.
By Moses' time there were laws governing all forms of incest (Lev 18:7-17; 20:11-12, 14, 17, 20-21; Deut 22:30; 27:20, 22, 23). These laws clearly state that sexual relations or marriage is forbidden with a mother, father, stepmother, sister, brother, half brother, half sister, granddaughter, daughter-in-law, son-in-law, aunt, uncle or brother's wife.
The Bible, in the meantime, notes that Abraham married his half sister (Gen 20:12). Therefore, the phenomenon is not unknown in Scripture. Prior to Moses' time, incest in many of the forms later proscribed were not thought to be wrong. Thus, even Moses' own father, Amram, married an aunt, his father's sister, Jochebed (Ex 6:20). In Egypt, the routine marriage of brothers and sisters among the Pharaohs all the way up to the second century made the Mosaic law all the more a radical break with their Egyptian past.
The genetic reasons for forbidding incest were not always an issue. Close inbreeding in ancient times was without serious or any genetic damage. Today, the risk of genetic damage is extremely high. Since the genetic possibilities of Adam and Eve were very good, there were no biological reasons for restricting marriages to the degree that it became necessary to do later.