Scientists have found the oldest fossils yet, providing evidence that life existed 3.4 billion years ago in an oxygen-free environment. The fossils are of bacterial cells that appear to have subsisted on sulfur. The researchers' work has implications for looking for life on other planets, giving an indication of what bacteria and cells in other oxygen-free environments might look like.
At the time of these fossils' origin, volcanoes dominated the Earth, land was constrained mostly to small islands poking up through the hot seas and circulating currents were much stronger than today. The planet was the temperature of a hot bath, and the sauna-like atmosphere was heightened by an overcast sky that retained the Earth's heat although the sun was weaker than it is today.
There were no plants or algae to photosynthesize and produce oxygen, so the bacteria existing at this time lived off of sulfur-containing compounds. Today, such sulfur-metabolizing bacteria are found in hydrothermal vents, hot springs and other places that have little free oxygen.
“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,” David Wacey, a postdoctoral researcher at the University of Western Australia who led the study, told Popular Science.
Until now, Archaean rocks, the earliest-known ones from the Precambrian era, have shown evidence for sulfur-based life forms, but it has been difficult to find fossils of such organisms and even harder to verify that they were actually living.
The researchers from UWA and the University of Oxford say that their fossils satisfy three crucial tests indicating that the fossils show evidence of biological forms and are not the product of a mineralization process:
- They show precise cell-like structure all of a similar size and look like microfossils from two billion years ago.
- The fossils show biological behavior because they are clustered in groups, only present in appropriate habitats and are attached to sand grains.
- And most crucially, they show biological metabolisms. They have the right chemical makeup, and the pyrite (fool's gold) found with the microfossils are likely byproducts of sulfur metabolism.
The fossils were found in the Strelley Pool Formation (seen at left), an outcropping of rocks in Western Australia that represents the oldest beach on earth.
The researchers, who reported their finding in Nature Geoscience, acknowledge it is difficult to prove whether fossils from the Archaean era were once biological organisms, but they used three methods of analysis to show that the fossils do contain carbon-based material (as opposed containing carbon that resulted from later contamination).
The three techniques they used were Raman spectroscopy, high-resolution transmission electron microscopy and geochemical analysis.
The same Oxford team had previously studied ancient fossils from a site just 20 miles away and did not deem them to be of biological origin.
Implications for life on other planets
This finding could indicate what life on other planets might look like. It would likely be similar kinds of bacteria and cells living in similar environments that would need to pass the same tests.
In the press release, researcher Martin Brasier of Oxford University, says:
Could these sorts of things exist on Mars? It's just about conceivable 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.
photos: David Wacey