Remarkable that Jakobsson and Juels, in 1999 wrote a paper 'Proof of work and Bread Pudding Protocols', building on work by Rivest and Shamir shortly before(?), that came pretty close to Bitcoin's design (see PIPOW and Section 4, quote: 'We show how to partition this task into a collection of POWs, enabling minting to be distributed among a collection of low power, untrusted entities'.