Hashcash

In the '90s, Jonathan MacDonald, founder of SELF and ENTIRETY, and inventor of the Proof of AI consensus mechanism, started following the work of Tim May, Eric Hughes, and John Gilmore.

“I felt that there was a rebellious streak to what was being discussed - it felt revolutionary. Our online chat rooms with a dozen folk became hundreds, then thousands. A lot of smart people got involved, and new concepts began to appear”.

In those environments, a concept called "Hashcash" was proposed in 1997 by Adam Back (again, a PoW system used to limit email spam and denial-of-service attacks). Shortly after, PoW was published formally in a 1999 paper by Markus Jakobsson and Ari Juels.

These concepts were then adapted to digital tokens by Hal Finney in 2004 through the idea of "RPoW" (Reusable Proofs Of Work). RPoW became the core element of Bitcoin, which was introduced in 2009, specifically to eliminate digital double-spending.