GEN 004:001 And Adam knew Eve his wife; and she conceived, and bare Cain, and said, I have gotten a man from the LORD.
GEN 004:002 And she again bare his brother Abel. And Abel was a keeper of sheep, but Cain was a tiller of the ground.
GEN 004:017 And Cain knew his wife; and she conceived, and bare Enoch: and he builded a city, and called the name of the city, after the name of his son, Enoch.
At this time Adam, Eve and Cain should have been the only humans on earth (Abel was slain by Cain). So where did Cain's wife come from? God does not explain that he made more humans. Did he leave this part out? If so, why?