I was listening to Tom Hartman on Air America Radio (the liberal network that is on XM Satellite radio) and he brought up the subject of Cain's wife. He said that the Bible says that Cain got his wife from a neighboring town and that proves that the Genesis Creation story was intended only to explain the first man and woman who were the descendants of what was to become the Hebrews. He said that each of the many religions has its own Creation story, and that all are somewhat or very different, and that all only claim to explain the origin of their ancestors. In other words, all assume there were separate Creations of all the other races.
That explanation did not make sense to me so I looked it up and found what appears to be the official Christian answer. Adam and Eve were the first humans, Adam Created by God and Eve by God from Adam's rib. They bore many children (perhaps 33 sons and 23 daughters) over their 900-plus year lives. So, Cain married one of his sisters (or perhaps a niece).
What about the Judeo-Christian prohibition against marrying close relatives? Well, that rule was not proclaimed until Moses' time, so it would not apply to earlier generations.
Of course, as a non-literal believer who accepts the whole Evolution and Natural Selection explanation, I don't believe in a literal Adam and Eve. However, there is still a scientific question here. We believe that the last common ancestor of humans and the other great apes lived about 5 million years ago. It was the common ancestor of what was to become the chimps and what was to become we humans. We believe the first hominids with big brains around our size evolved about 100,000 years ago. The first humans, like their great ape and early hominid ancestors, were hunter-gatherers. Their language was much less developed than ours. They basically voiced threats, warnings, and requests with little nuance. They had limited ability for abstract thinking and had not yet invented metaphoric terms.
The first humans who developed the capability for full-fledged language and the ability for abstract reasoning and metaphoric thinking may have evolved around 6000 years ago, several thousand years after the development of agriculture which made larger societies possible. The estimate of 6000 years is based on studies of the rate of spread of genes in humans. Certain brain structures appear to be more recently evolved in some groups of humans than others.
I believe the Adam and Eve story is true to the extent that it is giving names and stories to the first humans who had the ability to communicate in a fully-human way and think abstractly - and thereby Create a vision of "God". More probably "gods" because monotheism did not arise until thousands of years later.
So, giving the names "Adam" and "Eve" to the first humans with metaphoric thinking capability, and the first inkling of spiritual belief, and the names "Cain" and "Able" and "Seth" for their children, along with unknown names to their other chldren, we can conclude that Cain either married one of his sisters (or nieces) -or- that he married one of the other, not fully metaphoric-thinking humans in the neighborhood. Thus, the genes and memes for metaphoric thinking spread to the whole of humanity.