London: The Earth's oldest fossils that show convincing evidence of cells and bacteria living in an oxygen-free world over 3.4 billion years ago have been discovered.
The discovery has been made by a team led by Dr David Wacey of the University of Western Australia and including Professor Martin Brasier of Oxford University, according to findings reported 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," says Professor Brasier of the Department of Earth Sciences at Oxford.
The Earth was still a hot, violent place at this time, with volcanic activity dominating the early Earth.
The sky was cloudy and grey, keeping the heat in despite the sun being weaker than today, a university release said.
The water temperature of the oceans was much higher at 40-50 degrees and circulating currents were very strong.
Any land masses were small, or about the size of Caribbean islands, and the tidal range was huge.
Significantly, there was very little oxygen present as there were no plants or algae yet to photosynthesise and produce oxygen.
The new evidence points to early life being sulphur-based, living off and metabolising compounds containing sulphur rather than oxygen for energy and growth.
"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," explains Professor Brasier.
The microfossils were found in a remote part of Western Australia called Strelley Pool.
They are very well preserved between the quartz sand grains of the oldest beach or shoreline known on Earth, in some of the oldest sedimentary rocks that can be found anywhere.
Professor Brasier said: "We can be very sure about the age as the rocks were formed between two volcanic successions that narrow the possible age down to a few tens of millions of years. That's very accurate indeed when the rocks are 3.4 billion years old".
The fossils are very clearly preserved showing precise cell-like structures all of a similar size.
They look like well known but much newer microfossils from 2 billion years ago, and are not odd or strained in shape.
The fossils suggest biological-like behaviour.
The cells are clustered in groups, are only present in appropriate habitats and are found attached to sand grains.
The fossils also show biological metabolisms.
The chemical make-up of the tiny fossilised structures is right, and crystals of pyrite (fool's gold) associated with the microfossils are very likely to be by-products of the sulphur metabolism of these ancient cells and bacteria.
Early fossils of life on Earth has been a controversial area.
In the past decade, the barriers that need to be overcome before claiming such evidence have been raised significantly, aided by new techniques for mapping the chemistry of rocks at fine scales.
The work has implications for looking for life on other planets, giving an indication of what evidence for such life might look like.
Should there be life elsewhere in our solar system on Mars or on the moons of Titan or Europa it is likely to be similar sorts of bacteria and cells living in similar environments.
So any fossils in rocks from these planets and moons ought to look like these Australian microfossils and pass the same evidence tests.
Professor Brasier said: "Could these sorts of things exist on Mars? It's just about conceivable. But it would need these approaches mapping the chemistry of any microfossils in fine detail and convincing three-dimensional images to support any evidence for life on Mars".
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