DUBLIN: The oldest well-preserved fossils on Earth have been discovered in 3.4-billion-year-old rocks in Western Australia.
The fossilised microbes were found alongside the mineral pyrite (an iron sulphide nicknamed fool’s gold), which the scientists say is a by-product of their consumption of sulphur compounds.
“The microscopic fossils show convincing evidence for cells and bacteria living in an oxygen-free world over 3.4 billion-years ago,” said David Wacey, a researcher at the University of Western Australia and lead author of the findings published today in Nature Geoscience.
The discovery, he said, provides the missing evidence to confirm previous theories that Earth’s earliest life employed sulphur-based metabolism. The Earth was still a hot, violent place at this time, with volcanic activity dominating the early Earth.
A world without oxygen
Most experts now accept that life existed back at this early period, though this consensus was only reached in the last decade. Back in 2002, the researchers involved in the present study were unsure that the fossils in the 3.4 billion-year-old Apex chert of Western Australia had a biological origin.
Wacey said direct evidence for early life was exceedingly rare and evidence for what type of life came first had proven elusive. Oxygen was rare or absent on the early Earth, as there were no plants or algae around yet to photosynthesise and produce oxygen. So life had to employ other means to survive. The new report points to early life living off compounds containing sulphur.
The microfossils were found in a remote part of the Pilbara region of Western Australia around three hours drive off road from Port Hedland. Various team members camped for up to a month at a time and climbed over about 15 km of jagged ridges to pinpoint the rocks which were most likely to contain life. Once back in the lab, a combination of electron microscopy and ion probe analysis was used to probe the fossils.
This is the first time that microfossils can be directly linked to a known metabolism, Wacey said. This ability to essentially ‘breathe’ sulphur compounds has long been thought to be one of the earliest stages in the transition from a non-biological to biological world.
Life prefers to use lighter isotopes as the chemical bonds then takes less energy to split. “So the pyrite containing high amounts of the light sulphur isotope (32S) are thought to have formed biologically,” Wacey said.
Some of the tubular microfossils resembled the protective outer casings of some modern filamentous bacteria and there were also small spheroidal microfossils which often occurred in clusters just like modern colonies of bacterial cells, according to the researchers. “They also behaved like modern microbes,” said Wacey. “Some of the microfossils can be seen attached to sand grains like modern microbial biofilms.”
Surviving in silica
The fossils were encased in the mineral silica and found in black-coloured sandstone from what is now the oldest record of life in shoreline or beach-like settings. “Silica is very strong which has helped these microfossils survive 3.4 billion years without being squashed or weathered away,” Wacey said.
Malcolm Walter, director of the Australian Centre for Astrobiology at the University of New South Wales, who was not involved in the study, said the presence of pyrite and the sulfur isotopic compositions are consistent with a sulfur-cycling ecosystem in an oxygen-poor environment.
“These are not the oldest reported microfossils or microbial deposits (stromatolites) from this region, those being around 3.5 billion years old, but they do add significantly to the already rich record of what some palaeobiologists regard as the oldest convincing record of life on Earth,” he added. The findings highlight a potential target in the search for life on Mars.
Original paper in Nature Geoscience
David Wacey homepage
Australian Centre for Astrobiology