In case you haven’t seen this on Reddit already, Cunicula posted a thread about the history of proof-of-stake and why it matters. He was around when the concept of pos was first introduced and realized that it was the solution to Bitcoin’s “Tragedy of the Commons” problem. It’s a great read. He also links to older threads on BitcoinTalk where pos was first introduced.
It seems after QuantumMechanic first posted about pos, Sunny King began designing Peercoin 2-3 months later, in October 2011. PPC’s design process starting date was in the recent CNBC article.