Bug that produces oxygen may have evolved hundreds of millions of years earlier than previously thought.

Well-preserved fossils of stromatolites, a rock formation in the Pilbara region of Western Australia, have been dated as 2.72 billion years old, more than 270 million years older than the previous oldest evidence of oxygenic photosynthesis.

Doctoral student David Flannery and colleagues from the University of New South Wales' Australian Centre for Astrobiology and the Macquarie University presented these findings at the Fifth International Archean Symposium in Perth, Australia.

They note that most scientists accept that the earth's atmosphere became oxygenated -- and thus habitable for other forms of life -- during a period known as the Great Oxidation Event around 2.45 and 2.32 billion years ago.

Blue-green algae, or cyanobacteria, are thought to have been the first organisms to do so and they lived in colonies that left behind the stromatolite fossils.

But when oxygen-producing organisms first evolved and how long it took the resulting oxygenation of the atmosphere has been uncertain, said a New South Wales statement.

The new findings may suggest the process not only started earlier but was more extended and gradual than previously thought, according to co-author Malcolm Walter, director of the Astrobiology Centre.

'The formerly neat story of the Great Oxidation Event now seems not to be so neat after all. The idea that the Earth's atmosphere suddenly became oxygenated about 2.45 billion years ago now seems too simple,' Walter says.