https://thenextweb.com/syndication/2020/06/08/brrr-how-oxygen-played-a-role-in-the-earths-deep-freeze/

Around two and a half billion years ago the Earth was an alien world that would have been hostile to most of the complex life that surrounds us today. This was a planet where bacteria reigned, and one kind of bacteria in particular – cyanobacteria – was slowly changing the world around it through photosynthesis.

The atmosphere of the early Earth lacked oxygen. This began to change during what’s known as the Great Oxidation Event, or GOE. At its broadest definition, the GOE refers to a series of chemical changes that geologists and geochemists have observed in rocks that are between 2.5 and 2.3 billion years old. These changes were the result of oxygen given off by ancient cyanobacteria. Communities of this bacteria lived in shallow seawater and were preserved in rocks as structures called stromatolites.