The concept of proof of work emerges

1993 marked the introduction of a concept called Proof of Work (PoW), invented by Moni Naor and Cynthia Dwork as a way to deter denial-of-service attacks and other forms of abusive behavior such as spam transactions by requiring some amount of processing power by a computer.

PoW is a form of cryptographic proof in which one party (the prover) proves to others (the verifiers) that a certain amount of computational work has been carried out. Verifiers can subsequently confirm that this is true, with minimal effort on their part.