Until Bitcoin along, paper credit was likely the biggest institutional invention by humanity. A piece of paper could represent a promise, and better yet, it was portable and could be endorsed for transfer. All the electronic systems we have today are digitalized versions of paper credit, including banknotes. The big problem with this system is that you needed to trust the issuers, and the notes could be fake. Bitcoin solved these problem in one fell swoop with proof of work and a novel application of cryptography.