The fossilised remains of the oldest known lifeforms on Earth have been discovered in samples of rock collected near a remote watering hole in the middle of the Australian Outback.
Scientists said that the microscopic fossils belonged to primitive bacteria that lived more than 3.4 billion years ago, when the Earth had emerged from a period when it was probably too hostile for life. The primitive microbes used sulphur instead of oxygen to generate energy from food and, the scientists said, they may be the closest that science will ever get to the mysterious origin of life on Earth.
The fossils were found in rocks that were originally formed in shallow seas near a coastline and suggest that beaches may have been the key habitat where the Earth's first lifeforms thrived, said David Wacey, of the University of Western Australia.
"The environment in which the microfossils were found is important – it extends the record of life in shoreline or beach-like environments by about 200 million years. This suggests that beaches could have been the setting for the origin of life itself," he said.
"The discovery gives good solid evidence for life over 3.4 billion years ago. It confirms there were bacteria at this time, living without oxygen."
The Earth is estimated to be about 4.5 billion years old but the planet's hostile, meteorite-bombarded environment is thought to have been too inhospitable for life to get going until about 3.8 billion years ago. Previous studies have indicated the presence of similar microfossils in 3.5 billion-year-old rocks but these claims have been disputed. The latest microfossils have been subjected to an exhaustive series of tests which have confirmed that they were once living cells, not merely the product of non-living chemical reactions. They were discovered in rock that was sandwiched between layers from two well-dated volcanic eruptions, which narrowed the fossils' date of origin to within a few tens of millions of years.
"That's very accurate when the rocks are 3.4 billion years old," said Professor Martin Brasier of the University of Oxford, co-leader of the study, published in the journal Nature Geoscience. "At last we have good solid evidence for life over 3.4 billion years ago. It confirms there were bacteria at this time, living without oxygen. Such bacteria are still common today. Sulphur bacteria are found in smelly ditches, soil, hot springs, hydrothermal vents – anywhere where there's little free oxygen and they can live off organic matter."
Oxygen appeared in significant quantities in the atmosphere only after the evolution many millions of years later of plant-like microbes which could use sunlight for photosynthesis, producing oxygen as a by-product. Until that point, life on Earth had to make do with sulphur, which can be metabolised in a similar way to obtain energy from food. "I believe we are as close as we have ever been to the very first microbes here... The problem we now have is that there are very few rocks older on Earth in which to search for anything more primitive," Dr Wacey said. The scientists found the microfossils in the Pilbara, a remote region of Western Australia with a harsh environment of spiky vegetation and red dust.