The Bible’s answer
Cain, the eldest child of the first human couple, married one of his sisters or another close female relative. This conclusion may be drawn by considering what the Bible says about Cain and his family.
Facts about Cain and his family
All humans descended from Adam and Eve. God “made out of one man [Adam] every nation of men to dwell on the entire surface of the earth.” (Acts 17:26) Adam’s wife, Eve, became “the mother of everyone living.” (Genesis 3:20) Therefore, Cain must have married a fellow descendant of Adam and Eve.
Cain and his brother Abel were the first of a number of children born to Eve. (Genesis 4:1, 2) When Cain was banished for killing his brother, he complained: “Anyone who finds me will certainly kill me.” (Genesis 4:14) Of whom was Cain afraid? The Bible says that Adam “became father to sons and daughters.” (Genesis 5:4) Evidently, these other descendants of Adam and Eve could have posed a threat to Cain.
Early in human history, marrying one’s relative was not uncommon. The faithful man Abraham, for example, married his half sister. (Genesis 20:12) The first prohibitions against such marriage appeared in the Mosaic Law, which was codified centuries after Cain’s day. (Leviticus 18:9, 12, 13) It seems that children born to close relatives back then were not as prone to birth defects as they are today.
The Bible presents the account of Adam, Eve, and their family as accurate history. Detailed genealogies going back to Adam can be found not only in the book of Genesis, written by Moses, but also in the writings of the historians Ezra and Luke. (Genesis 5:3-5; 1 Chronicles 1:1-4; Luke 3:38) Bible writers cite the story of Cain as a historical event.—Hebrews 11:4; 1 John 3:12; Jude 11.