Scientists have claimed that the bacteria which produce oxygen may have evolved hundreds of millions of years earlier than previously thought. An international team, led by the University of New South Wales, has based its findings on an analysis of ancient rock formations in Western Australia. Well-preserved fossils of stromatolites in the Tumbiana Formation, in the Pilbara region, have been dated as 2.72 billion years old, more than 270 million years older than the previous oldest evidence of oxygenic photosynthesis, says lead scientist David Flannery. According to the scientists, it's accepted 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 atmosphere has been uncertain. Team member Professor Malcolm Walter added: "The new findings may suggest the process not only started earlier but was more extended and gradual than previously thought. "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." The findings have been presented their findings at the Fifth International Archean Symposium.