The tl;dr is that humanity has at least an 80,000 year history of goods which are fungible, collectible, portable, scarce, and made to an exact standard, traded between people who may not speak the same language for any other sort of trade good. The familiar example is wampum, but the practice predates the colonization of the Americas by many multiples.
Debt is where state money comes from. But shell and hunk money is where states got it from, and the systems coexisted into the late 19th century.