https://phys.org/news/2019-01-habitable-planets-red-dwarf-stars.html

Put simply, life on Earth is thought to have emerged between 3.7 and 4.1 billion years ago (during the late Hadean or early Archean Eon), at a time when the planet's atmosphere would have been toxic to life today. Between 2.9 to 3 billion years ago, photosynthesizing bacteria began to appear and started to enrich the atmosphere with oxygen gas.

As a result, Earth experienced what is known as the "Great Oxidation Event" about 2.3 billion years ago. During this time, photosynthetic organisms gradually converted Earth's atmosphere from one composed predominantly of carbon dioxide and methane to one made up of nitrogen and oxygen gas (~78 percent and 21 percent, respectively).