When did oxygen appear on Earth?

* Recent research suggests that Oxygen would have appeared on Earth for the first time about 50 to 100 mn years before the Great Oxidation Event. The latter is believed to have occurred about 2.3 to 2.4 bn years ago. Today oxygen makes up about 21% of the atmosphere. Before the Great Oxidation Event occurred it was methane and ammonia which composed the atmosphere on the Earth.