Comment by M95D

> Humans (likely) started out by trading favors. Then upgraded that with barter. Barter is pretty inefficient, so money was invented. A complex society cannot run on favors.

That's not exactly how it was. I recommend reading "Debt: The First 5,000 Years" by David Graeber. https://en.wikipedia.org/wiki/Debt:_The_First_5,000_Years