Oldest known fossils discovered in Australia

22 August 2011 by Tamera Jones

Scientists have discovered the fossilised remains of what may be the oldest known life-forms on Earth in a 3·4 billion-year-old rock formation from Western Australia.

Fossilised cells

3·4 billion-year-old fossilised cells

They found the exceptionally well-preserved microscopic fossils in what is perhaps the oldest beach or shoreline known on Earth.

The fossils are the relics of primitive bacteria that lived during a violent period of Earth's history. The sky would've been cloudy and grey; and it would have been much hotter than today despite the sun being weaker. The moon was closer too, so tidal ranges must've been huge.

The atmosphere was most likely filled with methane and carbon dioxide and certainly devoid of oxygen. It was another billion years before the so-called Great Oxygenation Event - when oxygen started accumulating in the Earth's atmosphere.

"To us it would have seemed like a hellish place to live. To early life, this was paradise. A true Eden," says Professor Martin Brasier from the University of Oxford, co-author of the study, published in Nature Geoscience.

To us it would have seemed like a hellish place to live. To early life, this was paradise. A true Eden.

- Professor Martin Brasier, the University of Oxford

Instead of using oxygen to generate energy, these bacteria metabolised sulfur from volcanic gases and hydrothermal vents. Evidence for this comes from pyrite crystals (fool's gold) the researchers found in close association with the microscopic fossils.

Proof of life from so long ago is exceedingly rare, because erosion has destroyed most of the evidence contained in rocks of this age. Indeed, the earliest uncontroversial fossils that point to early life on Earth are 3·2 billion years old – 200 million years younger than this latest find – and come from South Africa.

Some scientists say there's chemical evidence for life at 3·8 billion years old, "but there's no indication of fossilised cells or metabolic pathway," says Brasier.

Others say they have evidence for 3·5 billion-year-old microfossils. "But these claims tend to be controversial: some of these fossils aren't very well preserved, others appear in rocks whose age is dubious, and others show no evidence for metabolism," says Dr David Wacey from the University of Western Australia, lead author of the study.

Location in Western Australia

The site in Western Australia where the researchers found the fossils

"We think three key criteria need to be satisfied before you can claim signs of ancient life. They need to look like cells; they need to suggest biological-like behaviour, such as being clustered; and crucially, they should show evidence of biological metabolism," says Brasier.

The microfossils satisfy all three conditions. They have precise cell-like structures all of a similar size, and they have carbon and nitrogen in their cell walls, like today's cells. Not just that, but they're clustered in groups - some are still attached to sand grains from the beach - and the fool's gold is a sure sign of metabolism.

"What we think was going on here is that there was a miniature ecosystem based around sulfur," says Wacey.

Despite the evidence pointing to early life being sulfur-based, this method of generating energy is still used today.

"Such bacteria are still common today. Sulfur 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 Brasier.

As for evidence for even earlier life on Earth, Wacey says: "There could still have been much earlier life on Earth, which you might expect to find evidence for in older and rarer rocks. But these older rocks are very deformed so the chances of finding intact microfossils is a long shot."

It's even conceivable that scientists could find similar fossils extra-terrestrially - on Mars, Saturn's moon Titan or Jupiter's moon Europa.

"The latest NASA studies suggest that deposits of comparable or even older age are widespread on Mars. It might help us answer the question - was there ever life on Mars and what was it like?" Brasier says.

'Microfossils of sulphur-metabolizing cells in 3·4-billion-year-old rocks of Western Australia - David Wacey, Matt R Kilburn, Martin Saunders, John Cliff and Martin D Brasier. Nature Geoscience, published online 21 August 2011, doi:10.1038/ngeo1238