Those who believe the Bible to be the true word of God often get uneasy when this story comes up, and for good reason. As we recount the story as it is written in the Bible, ask yourself whether it seems plausible or whether the Divine Default becomes the only possible answer. Additionally, I would ask you to please bear in mind the concept I put forth earlier regarding the burden of proof and apply it to this story.
According to Genesis, God created Adam, the first man, from dust and His divine breath.
Our global population is now 1.
Eve, the first woman, is created by God from one of Adam's ribs.
Our global population is now 2.
Adam and Eve have two sons, Cain and Abel.
Our global population is now 4.
Abel is a shepherd and Cain is a farmer. God accepted Abel's sacrifice of "the firstborn of his flock and from their fats", but for whatever reason, God didn't think Cain's offering of the "fruit of the ground" was good enough. This made Cain angry...angry enough that God asked him "Why are you angry?" Genesis never tells us how Cain responded to God, so we can assume he either explained why he felt angry or he simply ignored God in the same way that a pouty child sometimes angrily ignores his/her parent. Regardless of how Cain left things with God, we soon find Cain and his brother Abel in the field. Cain attacks and kills his brother.
Our global population is now 3.
God asks Cain "Where is your brother Abel?" (Genesis 4:9). Allow me to digress and ask a question that to me seems like common sense. If God is all-knowing, why is it that He has to ask Cain where his brother is? At this point in our history there are only 3 people on our entire planet. If God is all-knowing, surely a global population of 3 isn't too much for an omniscient being to keep track of, is it?
Here's where things get interesting. The conversation between Cain and God comprises Genesis 4:9 - 4:15 with Cain leaving God's presence and moving to the land of Nod which was located east of Eden in 4:16. In the very next sentence, Genesis 4:17, we learn that Cain makes love to his wife who becomes pregnant and gives birth to a son named Enoch.
What? WAIT A MINUTE! How could this be? If the Bible is the word of God and it is an accurate representation of the creation of man, where did this mysterious wife come from? Even an elementary understanding of math tells us that the entire global population was only 3 - Adam, Eve, and Cain. So who is this woman and how did she get here? We can look as hard as we like, but the Bible never mentions another race of people or even what her name was.
To be fair, I'll present the most-often cited reason for what clearly appears to be a major oversight on behalf of the author. In order to make this work, Adam and Eve "must have" had other children who moved away from them and for whom the Bible never speaks of. Biblical literalists claim that God's law against brother-sister marriage wasn't yet applicable, and that there would be no genetic deformities or issues with this close inbreeding because Adam and Eve "must have" had "perfect" DNA. The Bible of course makes no such reference, but according to this theory, Cain "must have" married either his sister or his niece. Again, in the interest of being fair, it is worth noting that no credible geneticist alive today would agree with that conclusion, nor do Biblical pseudoscientists like Ken Ham at Answers in Genesis present any critical evidence to support this theory. We are forced to make assumptions that are never mentioned in the Bible just to support an obvious oversight. Without making these wild assumptions, the Bible would clearly be inaccurate, and if the Bible is inaccurate...
Adam's next son, Seth, was born when Adam reached the age of 130 (Genesis 5:3). First, we need to believe that a human being could live to that age during a point in time when not a single person on Earth knew what a germ was or how to prevent/treat illness and disease. God gave man dominion over fish, birds, livestock, and every creeping thing, however God never gave man dominion over germs. It would be millennia before we had an adequate handle on these little guys (and still no dominion). Germs have always had a say in how healthy a human could be, however it is historically apparent that old age was somewhat of a banality back then as Sumerian and Babylonian kings were said to have lived for tens of thousands of years. According to the Bible, Adam lived to be 930 years old. The vast majority of the non-royal humans were lucky to see their twenties or thirties, so the idea that humans could live for hundreds of years requires a rational person to suspend a tremendous amount of disbelief. I digress one last time. The believer's theory suggests that Cain was in Nod for many years while a great deal of inbreeding occurred giving us a global population of more than 3. Therefore there "must have" been plenty of people on the planet for Cain to have chosen a nameless wife.
To my Christian readers, I have to ask whether or not you are you comfortable with this explanation? Mind you that there is nothing in the Bible or any outside reference that explicitly supports a theory like this. If the Bible is the infallible word of God, why do literalists have to make conjectures and assumptions to try and explain away the contradiction? The Bible literally goes from Cain leaving God's presence right to us meeting his wife. Read Genesis for yourself. The math is fairly straightforward on this one.
Most believers will find any way that they can to try and continue to support the original conclusion, even if it means trying to extrapolate information that isn't directly in the Bible. In our example of Cain and his mysterious wife, literalists like Ken Ham conjure up a bloodline that is never mentioned anywhere in the Bible or in any other texts, because the alternative is unthinkable to them. Now if only they could conjure up a name for her...