Australian and English researchers may have stumbled upon the world's oldest fossil, presumed to be at least 3.4 billion years old.

The research teams, from the University of Western Australia and Oxford University in England, came across the fossils in Western Australia in a remote part of the state called Strelley Pool. The microscopic fossils include evidence that cells and bacteria lived in an oxygen free world 3.4 billion years ago, when the Earth was still in its early stage of existence.

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, said Professor Martin Brasier of the Department of Earth Sciences at Oxford. Brasier led the team with David Wacey of the University of Western Australia.

The Earth was a much different place 3.4 billion years ago. It was hot, violent and had much volcanic activity. Land masses were few and far between, most were the size of the Caribbean islands. There was also very little oxygen present during this early stage of the Earth's existence. Most scientists believe life on Earth during this time was sulphur-based, living off and metabolizing compounds containing sulphur rather than oxygen for energy and growth.

On the early Earth, where free oxygen was rare or absent, evolving life had to employ other means to survive. Using a combination of electron microscopy and ion probe analysis, we were able to show that these particular microbes had a metabolism that was based on the use of sulfur. This ability to essentially 'breathe' sulfur compounds has long been thought to be one of the earliest stages in the transition from a non-biological to biological world, Wacey said.

The rocks are well preserved between quartz sand grains of the oldest beach or shoreline known on Earth. Brasier says the rocks were formed between two volcanic successions . These successions helped them determine the 3.4 billion age period.

That's very accurate indeed when the rocks are 3.4 billion years old, he said.

The rocks also passed a test which confirms they are biological and haven't occurred through some kind of mineralization process. They also contain biological metabolisms, such as pyrite, or fool's gold, that likely came from sulphur metabolism.

In the past, research surrounding these kind of early fossils has generated controversy. The Oxford research team in 2002 refuted another report that well-known microfossils from the Apex chert in Australia weren't the preserved forms of ancient bacteria. They said it didn't have the right context, shape and mineralogy, whereas this most recent finding does.

The findings, which were reported in a recent issue of the journal Nature Geoscience, generate hope for future discoveries.

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

Follow Gabriel Perna on Twitter at @GabrielSPerna