Where did Cain get his wife?
This is certainly one of the most ancient of all questions raised by Bible critics, and we can be sure that the superficial contradiction it implies did not escape notice by the original writers of the Bible. Cain was apparently the first son of Adam and Eve (Genesis 4:l) and Abel the second (Genesis 4:2). After Cain had murdered his brother Abel (Genesis 4:8), God punished him by sending him away from his home and from God’s presence forever. But then we are told that Cain was fearful of vengeance by others who might slay him (Genesis 4:14), that he knew his wife (Genesis 4:17), and even that he built a city. The descendants of Cain and the antediluvian civilization which they developed are described in Genesis 4:17-24. Skeptics have, of course, "wondered" where all those other people came from if no one except Adam, Eve, and Cain were living at this time. The idea that there might have been in the vicinity a "pre-Adamic" race of men is clearly precluded by the one equivocal Bible teaching that Adam was the "first man" (1 Cor. 15:45, etc.) and that Eve was "the mother of all living" (Genesis 3:20).
In the beginning, according to Scripture, man was created "very good" and would have lived forever had he not sinned. But, 'by one man sin entered into the world, and death by sin' (Romans 5:12). Even after the reign of decay and death entered the world at the time of God’s great curse on man’s dominion (Genesis 3:17), most men did live for hundreds of years and undoubtedly had large families. Adam and Eve are said to have had both "sons and daughters" (Genesis 5:4) during the 930 years of Adam’s lifetime, and the same is true of each of the other antediluvian patriarchs lists in the genealogies of Genesis 5. The average life-span of these patriarchs (excluding Enoch, who was taken out of the world before he died) is 912 years. Since the Bible does not indicate at what period of his life he murdered his brother, took his wife, or built his city, there is obviously no contradiction in the record. Consequently, neither the original writer of Genesis 4 nor any later editors ever felt there was a problem that needed explanation. Now of course, at least one son and one daughter of Adam and Eve had to marry each other in the first generation after the beginning in order for the race to get started at all.
However, there were no mutant genes in the genetic systems of Adam and Eve, as these had come directly from the creative hand of God Himself. Thus no genetic harm could have resulted had Cain or some other son of Adam married his sister. In fact, it would undoubtedly have taken many generations before enough genetic mutations (which are random, and therefore harmful, changes in the highly ordered structure of the germ cell, brought about by penetration of the cell by shortwave-length radiation or some other destructive agent) could have accumulated in the human race to make such marriages of close relatives genetically harmful.