The oxygenation events which paved the way to intelligent life roaming Earth may have been more spontaneous than believed, according to a new study.
Up until 2.4 billion years ago, the planet had no oxygen in the atmosphere, when the first of three major oxygenation events took place.
Currently theories suggest that a biological revolution, or tectonic changes causing a shift in volcanic activity and make-up of the crust could be behind these oxygenation events.
But scientists at the University of Leeds think that it was simply a matter of time before oxygen levels would be capable of supporting complex life.
Their new theory suggests breathable air came about as a result of internal feedbacks in phosphorus, carbon and oxygen cycles - three of the six essential elemental ingredients for life - without the requirement for extensive external forcing.
They say this also increases the chances of other high-oxygen planets out there.
"This research really tests our understanding of how the Earth became oxygen rich, and thus became able to support intelligent life," said Lewis Alcott, postgraduate researcher and lead author of the paper, which appears in the Science journal.
"Based on this work, it seems that oxygenated planets may be much more common than previously thought, because they do not require multiple - and very unlikely - biological advances, or chance happenings of tectonics."
Senior author Dr Benjamin Mills added: "The model demonstrates that a gradual oxygenation of Earth's surface over time should result in distinct oxygenation events in the atmosphere and oceans, comparable to those seen in the geological record.
"Our work shows that the relationship between the global phosphorus, carbon and oxygen cycles is fundamental to understanding the oxygenation history of the Earth.
"This could help us to better understand how a planet other than our own may become habitable."
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