Scientists say they've found the world's oldest fossils, the remains of tiny creatures that lived 3.4 billion years ago in a pre-oxygen world.
‘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 Earth was hotter than today, and dominated by volcanic activity. The oceans were as warm as a bath, and circulating currents were very strong. Land masses were small and the tidal range was huge.
And without plants or algae to photosynthesise and produce oxygen, very little oxygen was present. The early life appears to have been based on 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,’ says Brasier.
The microfossils were found in a remote part of Western Australia called Strelley Pool - the remains 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 Brasier. "That’s very accurate indeed when the rocks are 3.4 billion years old."
The team says there's plenty of evidence that the structures are biological in origin.They're very clearly preserved, showing precise cell-like structures, all of a similar size, and look like well-known but much newer microfossils from two billion years ago.They're also clustered in groups, are only present in appropriate habitats and are found attached to sand grains.
And, crucially, they show biological metabolisms. The chemical make-up of the tiny fossilised structures is right, and associated crystals of pyrite are likely to be by-products of the sulphur metabolism of these ancient cells and bacteria.
"We’re now making detailed comparisons with all other early microfossils, and we’re very optimistic for future finds," says Brasier.
There are also implications for looking for life on other planets.
"Could these sorts of things exist on Mars? It’s just about conceivable," says 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."