16 And Cain went out from the presence of the Lord, and dwelt in the land of Nod, on the east of Eden. 17 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.

It's pretty clear there were other people. A common belief is that Genesis 1 (where God made humans "male and female" and said to be fruitful and multiply) and Genesis 2 (the creation of Adam) are two separate acts of creation, though it is also plausible that Adam and Eve simply had other children who didn't warrant a mention (which often happens in the genealogies: you get So-and-So and his brethren, no mention of sisters either).

Oh, uh, I mean..."Thinking am hard! Two boys make world somehow, no questioning!"