An analysis of a deep rock core from Australia indicates the presence of at least some oxygen 50 million to 100 million years before the great change when the life-giving element began rising to today's levels, according to two papers appearing in Friday's edition of the journal Science.

Previously, the earliest indications of oxygen had been from between 2.3 billion and 2.4 billion years ago when the "Great Oxidation Event" occurred. The cause of the event is still not known, but before that the atmosphere was dominated by methane and ammonia. Today oxygen makes up about 21% of the atmosphere. The discovery of traces of early oxygen was made in a study of a 3,000-foot-long rock core extracted in western Australia.

 Source : USA Today Science