http://www.sciencemag.org/news/2021/02/first-organism-use-oxygen-may-have-appeared-surprisingly-early

The first organisms to “breathe” oxygen—or at least use it—appeared 3.1 billion years ago, according to a new genetic analysis of dozens of families of microbes. The find is surprising because the Great Oxidation Event, which filled Earth’s atmosphere with the precious gas, didn’t occur until some 500 million years later.

“I was pretty thrilled to see this paper,” says Patrick Shih, an evolutionary biologist at the University of California (UC), Davis. The advent of proteins that can use oxygen, Shih and others say, marks a key step in the emergence of aerobic microbes, which are those able to harness oxygen. “The transition from a world that was mostly anaerobic to one that was mostly aerobic was one of the major innovations in life,” says Tim Lyons, a biogeochemist at UC Riverside.