The first successful on-chain atomic swap between Bitcoin and Litecoin is executed on September 23, 2017, by developer Charlie Lee (Litecoin's creator). Using hash time-locked contracts (HTLCs) enabled by SegWit, the swap proved that trustless cross-chain trading is possible without intermediaries or centralized exchanges. Decentralized exchange moved from theory to reality. While atomic swaps never achieved widespread retail adoption, they demonstrated a fundamental cryptographic principle: you don't need to trust anyone to trade.
Wikipedia: Atomic swap