The first woman
Eve was made from Adam's rib (or side) (Genesis 2:21-24)—this was a unique event. Jesus (Matthew 19:4-6) and Paul (Ephesians 5:31) use this historical and one-time event as the doctrinal foundation for the marriage of one man to one woman.
If Christians cannot defend that all humans (including Cain's wife) can trace their ancestry ultimately to Adam and Eve, then how can they understand and explain the gospel? How can they justify sending missionaries to every tribe and nation? Therefore, one needs to be able to answer the question about Cain's wife, to illustrate that Christians can defend the gospel and all that it teaches.
Cain's brothers and sisters
Cain was the first child of Adam and Eve recorded in Scripture (Genesis 4:1). His brothers, Abel (Genesis 4:2) and Seth (Genesis 4:25), were part of the first generation of children ever born on this earth.
Even though only these three males are mentioned by name, Adam and Eve had other children. In Genesis 5:4 a statement sums up the life of Adam and Eve—“And the days of Adam after he had fathered Seth were eight hundred years. And he fathered sons and daughters.” This does not say when they were born. Many could have been born in the 130 years (Genesis 5:3) before Seth was born.
During their lives, Adam and Eve had a number of male and female children. The Jewish historian Josephus wrote that, “The number of Adam's children, as says the old tradition, was thirty-three sons and twenty-three daughters.”
The Bible does not tell us how many children were born to Adam and Eve. However, considering their long life spans (Adam lived for 930 years—Genesis 5:5), it would seem reasonable to suggest there were many! Remember, They were commanded to “Be fruitful, and multiply” (Genesis 1:28).
If we now work totally from Scripture, without any personal prejudices or other extra-biblical ideas, then back at the beginning, when there was only the first generation, brothers would have had to have married sisters or there would be no more generations!
But what about God's Laws?
Many people immediately reject the conclusion that Adam and Eve's sons and daughters married each other by appealing to the law against brother-sister intermarriage. Some say that you cannot marry your relation. Actually, if you don't marry your relation, you don't marry a human! A wife is related to her husband even before they marry because all people are descendants of Adam and Eve—all are of “one blood.” The law forbidding marriage between close relatives was not given until the time of Moses (Leviticus 18-20). Provided marriage was one man to one woman for life (based on Genesis 1 and 2), there was no disobedience to God's law originally when close relatives (even brothers and sisters) married each other.
Remember that Abraham married his half-sister (Genesis 20:12). God blessed this union to produce the Hebrew people through Isaac and Jacob. It was not until some 400 years later that God gave Moses laws that forbade such marriages.
Biological deformities Biological deformities
Today, brothers and sisters (and half-brothers and half-sisters, etc.) are not permitted by law to marry because their children have an unacceptably high risk of being deformed. The more closely the parents are related, the more likely it is that any offspring will be deformed.
There is a very sound genetic reason for such laws that is easy to understand. Every person has two sets of genes, there being some 130,000 pairs that specify how a person is put together and functions. Each person inherits one gene of each pair from each parent. Unfortunately, genes today contain many mistakes (because of sin and the Curse), and these mistakes show up in a variety of ways. For instance, some people let their hair grow over their ears to hide the fact that one ear is lower than the other—or perhaps someone's nose is not quite in the middle of his or her face, or someone's jaw is a little out of shape—and so on. Let's face it, the main reason we call each other normal is because of our common agreement to do so!
The more distantly related parents are, the more likely it is that they will have different mistakes in their genes. Children, inheriting one set of genes from each parent, are likely to end up with pairs of genes containing a maximum of one bad gene in each pair. The good gene tends to override the bad so that a deformity (a serious one, anyway) does not occur. Instead of having totally deformed ears, for instance, a person may only have crooked ones! (Overall, though, the human race is slowly degenerating as mistakes accumulate, generation after generation.)
However, the more closely related two people are, the more likely it is that they will have similar mistakes in their genes, since these have been inherited from the same parents. Therefore, a brother and a sister are more likely to have similar mistakes in their genes. A child of a union between such siblings could inherit the same bad gene on the same gene pair from both, resulting in two bad copies of the gene and serious defects.
Adam and Eve did not have accumulated genetic mistakes. When the first two people were created, they were physically perfect. Everything God made was “very good” (Genesis 1:31), so their genes were perfect—no mistakes! But, when sin entered the world (because of Adam—Genesis 3:6, Romans 5:12), God cursed the world so that the perfect creation then began to degenerate, that is, suffer death and decay (Romans 8:22). Over thousands of years, this degeneration has produced all sorts of genetic mistakes in living things.
Cain was in the first generation of children ever born. He (as well as his brothers and sisters) would have have received virtually no imperfect genes from Adam or Eve, since the effects of sin and the Curse would have been minimal to start with (it takes time for these copying errors to accumulate). In that situation, brother and sister could have married with God's approval, without any potential to produce deformed offspring.
By the time of Moses (a few thousand years later), degenerative mistakes would have built up in the human race to such an extent that it was necessary for God to forbid brother-sister (and close relative) marriage (Leviticus 18-20). (Also, there were plenty of people on the earth by then, and there was no reason for close relations to marry.)
Where did Cain get his wife?
One of the first puzzles people run into in Genesis is, Where did Cain's wife come from?
"Where did Cain get his wife?" or "Who was Cain's wife?" are commonly asked questions—and reasonable ones! God did not choose to inspire the writers of Genesis to record every detail of the lives of its characters. That is to say, more went on than what we find in the Bible. Indeed, much of Genesis relates only summaries of broad spans of time. The Bible gives us the essential details, and we learn other facts from secular history.
In the case of the family of Adam, he and Eve had more children than Cain and Abel and Seth, although Genesis mentions only these three by name. Genesis 5:4 adds: "After he begot Seth, the days of Adam were eight hundred years; and he had sons and daughters." What can we learn from history on the subject?
Josephus is a widely recognized Jewish historian. The following footnote about Adam's family appears in his Antiquities of the Jews: "The number of Adam's children, as says the old tradition, was thirty-three sons and twenty-three daughters" (Chapter 2, Section 3). We cannot know that Josephus' sources were accurate, but the suggestion that Adam and Eve had many other children besides Cain, Abel and Seth is reasonable.
The logical answer to "who was Cain's wife?" is that Cain married one of his sisters. God's prohibition against marriage between close relatives came many centuries later (Leviticus 18).