Final answer:
Oxygen produced by cyanobacteria was used in the process of oxidizing Earth's reduced materials, leading to the eventual oxygenation of the atmosphere after billions of years.
Step-by-step explanation:
For a billion years or more, oxygen being produced by cyanobacteria was completely consumed in the process of oxidizing reducing materials on the Earth's surface and in the oceans. Cyanobacteria, which appeared within one billion years of Earth's formation, started the Great Oxygenation Event about three billion years ago by producing oxygen via photosynthesis, causing a dramatic change in Earth's atmosphere and leading to the oxygenation of the atmosphere. This oxygen was initially used to oxidize reduced materials such as iron and therefore did not immediately accumulate in the atmosphere. It wasn't until these oxidizable substances were saturated that free oxygen began to build up in the atmosphere, paving the way for aerobic life forms. This oxygenation process transformed Earth's environment and led to significant biological evolution.