Scientists examined diamonds that were formed on Earth about 2  9 billion years ago  These diamonds had a higher-than-normal concentration of sulfur-33  This concentration can be explained only by certain chemical reactions that are stimulated by ultraviolet light  If there had been more than a trace of oxygen in Earth' s atmosphere 2
9 billion years ago, then not enough ultraviolet light would have reached Earth' s surface to stimulate the chemical reactions
Earth's atmosphere contained very little, if any, oxygen 2. 9 billion years ago.