The first eleven chapters of Genesis are fascinating, majestic, and tantalizing. As a story, they are incredibly dense, tight narrative that straddles realism and myth. They soar high above other literature of the Ancient Near East and offer a deep well of theological richness as well as a plethora of difficulties for biblical literalists.
The first five chapters are relatively straightforward on the face of the story. Adam and Eve are the first humans, directly created by God. They have two sons, Cain and Abel. Cain murders Abel in a fit of jealousy involving God himself and Cain is banished from the family. Cain is sent into the “land of Nod,” which means “wandering.” The problem comes with verse 17, Cain's wife, and verse 14, “whoever finds me will kill me.” Who did Cain marry and who are the people who will kill Cain when they find him?
Here are some of the possible answers:
1) The whole thing is a myth with not attempt at coherency.
2) A biblical editor has taken two traditions and combined them into one story (much of Arthurian legend is like that).
3) God created lots of people after he created Adam and Eve and Cain married one and then lived among them.
4) There were lots of people on the earth during the time of Adam and Eve that they were not “related to,” both during and before their time.
5) Adam and Eve were the first two actual “humans,” in the image of God (presumably with a spiritual sense), and Cain married and lived among pre-humans who became a human life with the inclusion/infusion of Cain. This is basically a Neanderthal hypothesis.
6) Cain married his sister and all people on the earth were, and are, descended from Adam and Eve.
Answer #1 – In one sense this is a logically valid hypothesis, one which non-Christians would embrace without a thought. The creation story becomes an aetiological tale, a “how humans came to be” story, not unlike Rudyard Kipling's “How the Elephant Got his Trunk.” For the Christian who believes in the divinity of Christ and the inspiration of the apostles this is not really an option, as both Jesus and Paul seemed to believe in the historicity of the Adam and Eve account (Matt 19:4-6; 1 Tim 2:13). Since the resurrection of Christ is a strongly established fact in most Christian's minds, one would have to argue that Jesus, who rose from the dead, was pretty naïve. On top of that, the account is not written as most Ancient Near Eastern myths and the Jews never read it that way (in contrast most educated Romans understood their gods and legends were fabrications).
Answer #2 – This answer is much like the first, except that it hypothesizes that there were originally two stories and that answers the question as to why someone would compose a story with such glaring narrative holes. Trying to discern hypothetical source stories is a favorite occupation of Bible scholars (liberal ones). Chopping up the Adam-Cain narrative doesn't work well, though. It doesn't lend itself to revealing any helpful separation and the literary tools used to do so are based on Western European forms. The Western European literary traditions are oral and the Ancient Near Eastern forms were mostly written. The hypothesis doesn't work very well.
Answer #3 – According the Bible narrative, God could have created more people after Adam and Eve if one reads the narrative that way. This would reduce the theology of the Fall to something fully federal. God made Adam the representative of the whole human race and he fell in their place. God then imputes, or credits, the sin of Adam to the whole human race. Or perhaps Adam's sin brings sin into the world and all humans are effected by it. It kind of works except that it involves a mechanism for sin that isn't very biblical. It doesn't seem that Adam's sin creates a radioactive sin cloud that settles on the earth. Instead, all of Adam's descendants, starting with Cain and Abel, inherit a corrupted nature. Jesus doesn't inherit the corrupted nature because he is a son of Eve and not a son of Adam (having no human father). Why would the "radioactive sin cloud" not effect Jesus?
Answer #4 – Simply taking the story as a story about real people, but not the first or only people, takes the story from being a story about the origins of humanity to the origins of Israel. However, that doesn't work very well because the story clearly intends to tell the story of all humanity and the story is understood throughout Scripture as a story of humanity. The story is told as if Adam and Eve are the parents of all humanity. In addition, the theology behind why Jesus had to die on the cross becomes a little shaky if Adam and Eve are not the first parents.
Answer #5 – The Neanderthal Hypothesis is very clever. Adam and Eve are the first two people with spirits and created in the image of God. This allows science and faith to co-exist, though not happily. There are a few Christian Scientists who like this kind of resolution, but their colleagues still think they are daft. I imagine that this idea will look pretty silly in a couple of decades. There's also nothing in the text to give the slightest hint that there were non-humans being married into humanity.
Answer #6 – The oldest answer is that Cain married his sister. There is the moral objection to this answer, but that isn't much of an objection because of the uniqueness of the situation, such strictures would come later when there were other people to marry. There do seem to be others around, but they could all be part of Adam and Eve's fruitful progeny. They lived a long time and had plenty of opportunity to have lots of kids. This doesn't change the general impression that the “others” don't seem to be a part of the family and it doesn't seem like there would have been others born before Cain and Abel were born. There are no easy answers here.
All of this to say, I don't know the answer. And what's more, I don't think the answer is knowable. I accept the Bible for what it says and have no idea about what it doesn't say. Clearly Adam, Eve, Cain and Abel are meant to be taken historically, and I do so. The Cain and Abel incident is a clear demonstration of the immediate outworking of sin, and I accept that as well. I acknowledge the difficulties with the unanswered questions and take comfort in the fact that the narrative made sense to the writer, even if all of it doesn't make sense to me. I receive what it teaches and shrug at what it doesn't. It makes no difference to my faith and even less difference to my life who Cain married.
When we start trying to answer a question like that we run into problems and create more than we solve. I think that is true even with option #6, the Cain's sister resolution. I prefer to respond to the question with an “I don't know.” At least it is honest.