Relics of the dawn of life 3.4 billion years ago have been unearthed in Australia.

The microscopic fossils are the oldest ever discovered and date back to a time when the earliest forms of bacteria survived in an oxygen-free world.

“At last we have good solid evidence for life over 3.4 billion years ago,” said Martin Brasier, professor from the department of earth sciences at Oxford University.

“It confirms there were bacteria at this time, living without oxygen.”

When the bugs were alive, the world was still a hot, violent place shaken by volcanic activity.

Thick clouds filled the sky, producing a “greenhouse effect” that made the oceans as warm as a hot bath. Any land masses that existed were small, around the size of Caribbean islands.

During this time, before the photosynthesising action of green plants and algae, there was hardly any oxygen on earth.

The new fossil evidence, described in the journal Nature Geoscience, points to early life living off compounds containing sulphur rather than oxygen.

“Such bacteria are still common today,” said Prof. Brasier, who was part of the Anglo-Australian team that made the discovery.

“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.”

The microfossils were found at a remote site in western Australia called Strelley Pool, preserved in grains of quartz sand on the oldest shoreline known on earth.