One of the most dramatic scenes in the play Inherit the Wind occurs when the fundamentalist preacher acting as prosecuting attorney is confronted with a challenge to his beliefs. Certain that the Bible is true and complete in every way, he would admit no possibility that Darwin’s theory of evolution presented a more reasonable account of the origins of humanity. But when faced with the probing questions of a clever attorney he faced a problem in logic that his faith could not overcome.
According to the scriptural text, God created Adam and out of Adam, Eve. The couple had two children: Cain and Abel. But Cain kills Abel, leaving only the three aforementioned human beings. As punishment for his crime, Cain is banished and compelled to wander the earth (Genesis 4:12). It is in exile that he takes a wife and fathers a family (Genesis 4:17ff). The piercing question, of course, is from where did Mrs. Cain come? She is not mentioned at all in the earlier narrative. If the Bible is true and complete in every way, there is no explanation for her existence! The fundamentalist preacher was confounded. And even though it was not enough to win the case, the important point came through. Reading the Bible literally can lead to logical difficulties.
Jewish commentators never had problems with the origin of Mrs. Cain, essentially because Jews have never read the Bible literally. Consequently, there was room for allowing for the appearance of Mrs. Cain. For example, the Babylonian Talmud (Sanhedrin 58b) and the Apocryphal book of Jubilees (4:9) both maintain that Cain and Abel were both born as fraternal twins: each having a sister. (In the generations before Sinai brothers and sisters could marry.) That this fact is excluded from the narrative does not make any less plausible. The narrative was interested only in telling the story of the moral development of humanity. Hence, only those characters whose actions of achievements were essential for telling that story are mentioned by name (See Maimonides, Guide of the Perplexed, Book III, Chapter 50.) Because Mrs. Cain was not a key character in the unfolding drama of humanity, she is mentioned only by allusion.
What the commentaries are really saying is that the Bible is not a phone directory (remember those?) listing every person who lived in ancient times. The Bible is a selective account of a few people whose lives and adventures had something to teach us about ourselves. There were many other people around back then, we may assume. Consider the narrative in Genesis 5:4. After the birth of Seth, Adam fathered many sons and daughters. They are never mentioned.
Judaism teaches us to read the Bible seriously, but not literally.