22 August 2011, by Tamera Jones
Scientists have discovered the fossilised remains of what are almost certainly the oldest known life-forms on Earth in 3.4 billion-year-old rock formations from Western Australia.
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 sulphur from volcanic gases and hydrothermal vents. Evidence for this comes from sulphate and pyrite (fool's gold) crystals 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 only other 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 and Texas.
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 all these claims are 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,.
'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 and sulphate are sure signs of metabolism.
'What we think was going on here is that there was a miniature ecosystem based around sulphur,' says Wacey.
Despite the evidence pointing to early life being sulphur-based, this method of generating energy is still used today.
'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,' 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.'
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.