Anatoly Yakovenko's Proof of History whitepaper
Solana Labs' own history of the project says that in November 2017 Anatoly Yakovenko published a whitepaper describing Proof of History, a way for computers that do not trust each other to keep time. No copy dated November 2017 has been found. The earliest dated trace is the whitepaper repository, whose first commits on December 12, 2017 hold a draft titled "Loom: High performance blockchain", and by early February 2018 loomprotocol.com hosted a v0.8.5 PDF titled "Loom: A new architecture for a high performance blockchain".