From Adam and Eve's other children. Genesis says plainly they had many sons and daughters, so the early human family included the marriages an emerging population required. It's a smaller puzzle than it looks, and it isn't a threat to the story.
The Grace Answer
This is one of the oldest challenges leveled at Genesis, and it feels like a trap: if Adam, Eve, Cain, and Abel were the only people, where did Cain possibly find a wife? The honest answer is less dramatic than the objection assumes, and it comes straight from the text a few verses later.
Genesis tells us the first family was much larger than the handful of named characters. “After he begot Seth, the days of Adam were eight hundred years; and he had sons and daughters.” The named figures are the ones who carry the storyline; they are not a full census. Across long lifespans, the early family grew into a population, and Cain married within it. The account is not claiming there were only four people ever. It is following one family line while a wider family fills in around it.
What Genesis does and doesn't say
It is worth being honest about the limits here, because overclaiming is how apologetics loses credibility. Genesis never names Cain's wife, never says he met her in Nod, and never pauses to explain the surrounding population. It also says nothing about genetics, so the popular defense that early bloodlines were biologically “pure” is speculation the text does not offer. What we can say plainly is that close-relative marriage was not yet forbidden; the specific prohibitions came much later, in the law given to Moses. Beyond that, Genesis simply leaves the mechanics unexplained, because it is telling the story of humanity's alienation and rescue, not answering modern questions about population and DNA.
The deeper point is the one the genealogy is actually protecting. Paul told the philosophers in Athens that God “has made from one blood every nation of men.” The unity of the human family is not trivia; it is the backbone of the gospel, because Scripture's concern is headship, not biology. Adam stands at the head of the old humanity marked by sin and death, and Christ stands at the head of the new humanity marked by righteousness and life: “by one man's disobedience many were made sinners, so also by one Man's obedience many will be made righteous.” That is what the story is guarding, and it does not rise or fall on how we picture the early population.
So you can hold Genesis with confidence rather than embarrassment, and without pretending to certainty the text withholds. A question that looks like it undermines the story turns out to sit right next to the truth the whole rescue depends on. Cain's wife is a footnote. The one head who could stand in for us all is the point.