Ethereum has undergone four planned upgrades since its public mainnet launch in July 2015 (called “Homestead”). In order the four upgrades were: Homestead (March 2016), Metropolis Byzantium (October 2017), Metropolis Constantinople (February 2019), and Istanbul (December 2019). Together, these upgrades improved the functionality of the Ethereum 1.0 chain while setting the stage for Ethereum 2.0
More information about the people behind Ethereum 2.0: A Short History of Ethereum