World’s oldest fossils reveal earliest life on Earth

It can hardly be called a clement clime: the Earth around the time of the early Archaen eon was a thoroughly inhospitable place for life as we know it, but not for primitive sulphur-eating bacteria, traces of which have been discovered in the world’s oldest fossils in Western Australia.

The finding confirms that life emerged surprisingly shortly after the Earth finished being pulverised by asteroids during the Late Heavy Bombardment, which concluded around 3.8 billion years ago.

Only a few hundred million years later – around 3.4 billion years ago – saw the first primitive life emerge, including sulphur-eating bacteria.

Fossilised traces of these organisms have been uncovered by a team from the University of Western Australia and Oxford University in the UK in rocks from the early Archaen era found in the Strelley Pool Formation in Western Australia.

“At last we have good solid evidence for life over 3.4 billion years ago,” said Professor Martin Brasier of the Department of Earth Sciences at Oxford. “It confirms there were bacteria at this time, living without oxygen.”

Our planet was still a hot and violent place at this time, with volcanic activity dominating the early Earth. The sky was cloudy and grey, keeping in the heat despite the Sun being less radiant than it is today.

The water temperature of the oceans was much higher at 40-50 degrees and circulating currents were very strong. Any land masses were small, or about the size of the Caribbean islands, and the tidal range was huge.

There was also very little oxygen in the atmosphere, which only emerged much later in the Earth’s history as a by-product of respiration by algae and plants.

As such, these most primitive bacteria had to make do metabolising compounds containing sulphur instead of the more reactive oxygen. Similar bacteria are still found today loitering around hydrothermal vents along the ocean floor.

“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,” said Brasier.

The fossils were very well preserved between the quartz sand grains of the oldest beach known on Earth in some of the oldest sedimentary rocks that can be found anywhere.

The team scrutinised the fossils very closely, as it’s far from easy to accurately distinguish between early fossilised life and natural geological formations.

In the past decade the barriers that need to be overcome before claiming evidence for life have been raised significantly, aided by new techniques for mapping the chemistry of rocks at fine scales.

In 2002, the same Oxford group suggested well-known microfossils from the Apex chert in Australia were not the preserved forms of ancient bacteria after all. They argued that the context, shape and mineralogy of the forms were all wrong for them to be of biological origin.

In contrast, the microfossils from this study satisfy three crucial tests that the forms seen in the rocks are biological and have not occurred through some mineralisation process.

The fossils are very clearly preserved showing precise cell-like structures all of a similar size; they look like well known but much newer microfossils from 2 billion years ago; and are not odd or strained in shape.

And, crucially, they show biological metabolisms. The chemical make-up of the tiny fossilised structures is correct, and crystals of pyrite (fool’s gold) associated with the microfossils are very likely to be by-products of the sulphur metabolism of these ancient cells and bacteria.

Leading author Dr David Wacey, from the University of Western Australia, said direct evidence for early life in the form of microfossils was exceedingly rare and evidence for what type of life came first had until now proved elusive.

“Our research helps to answer the question: how did these microbes survive?” he said. “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.

“By showing the intimate association of these 3.4 billion-year-old microfossils with the mineral pyrite (FeS₂), we have now provided the earliest direct evidence of microorganisms employing a sulfur-based metabolism.”

The work has implications for looking for life on other planets, giving an indication of what evidence for such life might look like.

Should there be life elsewhere in our solar system – on Mars or on the moons of Titan or Europa – it is likely to be similar sorts of bacteria and cells living in similar environments. So any fossils in rocks from these planets and moons ought to look like these Australian microfossils and pass the same evidence tests.

“Could these sorts of things exist on Mars? It’s just about conceivable,” said 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.”

The finding was published in the journal Nature Geoscience.