Spend enough time interacting with those who believe evolution and the issue of Cain’s wife is bound to come up. As you defend the idea that all people came from the union of Adam and Eve and this question will arise.
If you have ever seen the movies Inherit the Wind or Contact, you will remember that this question and the believers’ lack of an answer is a turning point. Could you answer this question?
Why is this important? Many skeptics have claimed that for Cain to find a wife, there must have been other “races” of people on the earth who were not descendants of Adam and Eve. To many people, this question is a stumbling block to accepting the creation account of Genesis and its record of only one man and woman at the beginning of history. Defenders of the gospel must be able to show that all human beings are descendants of one man and one woman (Adam and Eve) because only descendants of Adam and Eve can be saved. Thus, believers need to be able to account for Cain’s wife and show clearly she was a descendant of Adam and Eve.
Since the Bible describes all human beings as sinners, and
we are all related (“And He has made from one blood every nation of men to dwell on all the face of the earth,”
Acts 17:26), the gospel makes sense only on the basis that all humans alive and all that have ever lived (except for the first woman) are descendants of the first man Adam. If this were not so, then the gospel could not be explained or defended.
Thus, there was only one man at the beginning—made from the dust of the earth (
Genesis 2:7).
This also means that Cain’s wife was a descendant of Adam. She couldn’t have come from another race of people and must be accounted for from Adam’s descendants.
In
Genesis 5:4 we read a statement that sums up the life of Adam and Eve: “After he begot Seth, the days of Adam were eight hundred years; and he had sons and daughters.”
During their lives, Adam and Eve had a number of male and female children. In fact, the Jewish historian Josephus wrote, “The number of Adam’s children, as says the old tradition, was thirty-three sons and twenty-three daughters.”
Scripture doesn’t tell us how many children were born to Adam and Eve, but considering their long life spans (Adam lived for 930 years—
Genesis 5:5), it would seem logical 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 extrabiblical ideas, then back at the beginning, when there was only the first generation, brothers would have had to marry sisters or there wouldn’t have been any more generations!
We’re not told when Cain married or many of the details of other marriages and children, but we can say for certain that Cain’s wife was either his sister or a close relative.
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 marriage. Some say that you can’t marry your relation. Actually, if you don’t marry your relation, you don’t marry a human! A wife is related to her husband before they are married because all people are descendants of Adam and Eve—all are of one blood. This law forbidding close relatives marrying was not given until the time of Moses (
Leviticus 18–20). Provided marriage was one man for one woman for life (based on
Genesis 1–2), there was no disobedience to God’s law originally (before the time of Moses) when close relatives (even brothers and sisters) married each other.
Remember that Abraham was married to his half-sister (
Genesis 20:12). God’s law forbade such marriages, but that was some four hundred years later at the time of Moses.
Today, brothers and sisters (and half-brothers and half-sisters, etc.) are not currently permitted by law to marry and have children.
Now it is true that children produced in a union between brother and sister have a greater chance to be deformed. As a matter of fact, the closer the couple are in relationship, the more likely it is that any offspring will be deformed. It is very easy to understand this without going into all the technical details.
Each person inherits a set of genes from his or her mother and father. Unfortunately, genes today contain many mistakes (because of sin and the Curse), and these mistakes show up in a variety of ways.
The more closely related two people are, the more likely it is that they will have similar mistakes in their genes, inherited from the same parents. Therefore, brother and sister are likely to have similar mistakes in their genetic material. If there were to be a union between these two that produces offspring, children would inherit one set of genes from each of their parents. Because the genes probably have similar mistakes, the mistakes pair together and result in deformities in the children.
Conversely, the further away the parents are in relationship to each other, 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 some of the pairs of genes containing only 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 have only crooked ones. (Overall, though, the human race is slowly degenerating as mistakes accumulate generation after generation.)
However, this fact of present-day life did not apply to Adam and Eve. When the first two people were created, they were perfect. Everything God made was “very good” (
Genesis 1:31). That means their genes were perfect—no mistakes. But when sin entered the world because of Adam (
Genesis 3:6), God cursed the world so that the perfect creation then began to degenerate, that is, suffer death and decay (
Romans 8:22). Over a long period of time, this degeneration would have resulted in all sorts of mistakes occurring in the genetic material of living things.
But Cain was in the first generation of children ever born. He, as well as his brothers and sisters, would 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. In that situation, brother and sister could have married (provided it was one man for one woman, which is what marriage is all about,
Matthew 19:4–6) without any potential to produce deformed offspring.
By the time of Moses (about 2,500 years later), degenerative mistakes would have accumulated to such an extent in the human race that it would have been necessary for God to bring in the laws forbidding brother-sister (and close relative) marriage (
Leviticus 18–20).
Christians have made many attempts over the years to harmonize the Genesis account of creation with accepted geology and its teaching of billions of years for the age of the earth. Examples of such attempts include the views of theistic evolution, 
