CLAIM: Genesis teaches that Cain and Abel were the first two descendents of Adam and Eve. But, in a moment of fear and panic, Cain told God that “whoever finds me will kill me” (Gen. 4:14), and later, Genesis states that “Cain had relations with his wife” (Gen. 4:17). Where did these other people come from?
RESPONSE: Adam lived for an extremely long amount of time: 930 years. During this time, he was told to be “fruitful and multiply” (Gen. 1:28). The Bible also states that some time had passed before Cain killed his brother (Gen. 4:3). Moreover, Genesis 5:4 states that Adam had “other sons and daughters.” While the Bible doesn’t say how many, we can be sure that there must have been many (What else did Adam and Eve have to do besides make babies? It isn’t as though there was much else going on at the time). Therefore, Cain must have gotten his wife from one of his sisters, which was incest.
George Kufeldt notes that rabbis from Talmudic times believed that Cain took one of his sisters for his wife. However, because the Jews looked down upon incest (Lev. 18:9), they didn’t explicitly mention this in the text. I find this hard to believe, however, because the Bible doesn’t seem to hold back on sharing gory details like incest (Gen. 19:30-38; 20:2). Instead, it’s more likely that the information simply wasn’t important to the progress of the story, so it was left out.
One of the major reasons why incest is so immoral is because it leads to higher rates of physical deformities in children. When two parents have similar genes (from being related), they are more likely to share the same genetic mistakes. However, because the first humans had no genetic mistakes, this wasn’t an issue. Later in human history, God strictly prohibited incest for this reason (Lev. 18:9).
Finally, we might point out that the naturalistic account isn’t any more sophisticated than the biblical one. In the naturalistic account, we descended through slow mutations on the genome, until the human race was born. This breeding between our primate ancestors led to mutations which gave rise to Homo sapiens—hardly an endearing view of our ancestors.
 Youngblood, Ronald F. The Genesis Debate: Persistent Questions about Creation and the Flood. Nashville: T. Nelson, 1986. 159.
 Ross writes, “The timing of this command makes perfect sense from a biological perspective. Genetic defects as a result of intrafamily marriage develop slowly. They would present no risk until the first several dozen generations.” Ross, Hugh. The Genesis Question: Scientific Advances and the Accuracy of Genesis. Colorado Springs, CO: NavPress, 1998. 105.