Greg Mankiw refers to this as “Economic Principle Number 5: Trade can make everyone better off”:
Trade is important because, without it, it would be pretty difficult for some of us to survive only consuming what we produce well, especially those of us who haven’t figured out how to eat economics lessons yet. Technically, however, trade only gets us part of the way to what we would consider capitalism, since direct trade (i.e. barter) still requires a double coincidence of wants. The guys in the example above are fortunate in that each wants what the other has (hence double), but this isn’t usually the case, since, as far as I know, I don’t see a plethora of farmers and house builders consuming my economics lessons. (Holler if you’re out there, though.) Therefore, I’m guessing things really started taking off when these guys figured out how to use some of the smaller rocks as money.