Volcanic gases from the terrestrial mantle oxygenated the atmosphere 2,400 - 1,800 million years ago. This is at least what the latest research says. Oxygen-producing bacteria already existed at least 300 million years earlier and over the years enriched the atmosphere with oxygen. Although cyanobacteria produced and emitted oxygen, everything was rapidly reduced. However, Basalto iron was oxidized with water, releasing hydrogen into the atmosphere. Oxygen rushed to the seabed, which were enriched with oxygen and rising. Finally the volcanoes released this oxygen again and "filled" the atmosphere with oxygen.