A major evolutionary breakthrough occurred when some microbes developed the ability to use sunlight to split water and release oxygen. This oxygenic photosynthesis was carried out by cyanobacteria. Over hundreds of millions of years the oxygen released slowly accumulated in the oceans and atmosphere - wikipedia 
Rise of Oxygen: 3.5–2.4 Billion Years Ago
HTML5 mp4 https://david.hitchhikers.earth/assets/history-of-the-earth/History%20of%20the%20Earth%20%5BQ1OreyX0-fw%5D.mp4#t=97 Rise of Oxygen: 3.5–2.4 Billion Years Ago
A critical tipping point occurred around 2.4 billion years ago during the Great Oxygenation Event. Atmospheric oxygen levels rose sharply, transforming global chemistry. Many anaerobic organisms died out, while others adapted. Oxygen also reacted with methane, causing a cooling period that may have led to widespread glaciation.