Jobs II: A View From 30,000 Feet
whoever gives his labor for money sells himself and puts himself in the rank of slaves........Cicero, in De Officicus
A central problem, or maybe The Central Problem, of a civilization is figuring out how get a large group of people more interested in cooperating than killing each other. An early and crucial invention in that regard was the market economy. At some early point, selling ones labor became part of that market economy, though not a very prestigious part, as Cicero's remark indicates.
Today most of us have put "ourselves in the ranks of the slaves," but that's a moderately new phenomenon. In earlier times, workers were often independent artisans, or bound in quasi feudal relationships with those whose lands they worked. If most of us are to be suppliers of wage labor, then the supply of customers for that labor becomes a crucial ingredient of civilization.
Working for wages is really tough when slavery is permitted and slaves with valuable talents are available. The rapidly increasing talents of robots are threatening to bring that time back.