Researchers have discovered fossils of microbial organisms that were living on Earth 3.4 billion years ago.

Earth's oldest fossils have been found in Australia by a team from the University of Western Australia and Oxford University. The microscopic fossils show convincing evidence for cells and bacteria living in an oxygen-free world over 3.4 billion years ago.

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 Martin Brasier of the Department of Earth Sciences at Oxford.

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.

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, says Professor Brasier. That's very accurate indeed when the rocks are 3.4 billion years old.

At that time, the Earth was still a hot, violent place, with frequent volcanic activities.  The water temperature of the oceans was much higher at 40-50 degrees - the temperature of a hot bath - and circulating currents were very strong. Any land masses were small, or about the size of the Caribbean islands, and the tidal range was huge, researchers say.

The new evidence points to early life being sulphur-based, living off and metabolizing 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, said Professor Brasier.

The chemical make-up of the tiny fossilized 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 have 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.

In 2002, the same Oxford group suggested that well-known microfossils from the Apex chert in Australia were not the preserved forms of ancient bacteria after all. They argued that the context, shape and mineralogy of the forms were all wrong for them to be of biological origin.

They believe the current fossils, found just 20 miles away, satisfy all criteria for judging such finds.

The researchers are now using the techniques and approaches they used in this study to re-examine other fossil finds that have been proposed to contain evidence for life on Earth at these extremely early times.

We're now making detailed comparisons with all other early microfossils, and we're very optimistic for future finds, says Professor Brasier.

The discovery could also throw light on what life on other planets might look like.

Should there be life elsewhere in our solar system - on Mars or Titan or Europa - it is likely to be similar kinds 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.

Could these sorts of things exist on Mars? It's just about conceivable, said Professor Brasier. 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.