http://hn.vernccvbvyi5qhfzyqengccj7lkove6bjot2xhh5kajhwvidqafczrad.onion/stories/35175926
At various points in time, clams, beaver pelts, cowhides, knives, massive multi-tonne stones (< https://www.npr.org/sections/money/2011/02/15/131934618/the-... >), letters of credit, and cryptographic hashes have served as money. In mediaeval Europe, Roman coins were long used in trade, well after the Roman empire itself had fallen.