Where Does Oxygen On Earth Come From? Moss Played Important Role, Plant Evolution Study Reveals

There was once a time when oxygen was not part of the makeup of the Earth’s atmosphere. Roughly 2.4 billion years ago, the current form of the chemical element materialized after the Great Oxidation Event, when tiny organisms called cyanobacteria first conducted photosynthesis. But it was not until 400 million years ago that oxygen molecules appeared in the atmosphere at current levels.