They were building a city. Meaning the world by this time was well populated. Put it this way, they were far more advanced then Adam's family. Read what they were doing in chapter 4.
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. (Gen 4:17)
As you say, they were an ancient people.