Researchers have found what could be the oldest microbial fossils yet documented. The traces, discovered in 3.4-billion-year-old Australian rocks, might help to resolve the question of when cellular life arose, and how it produced energy.

Martin Brasier, a palaeobiologist at the University of Oxford, UK, and his collaborators found the cell-like fossils in black sandstone of the Strelley Pool Formation in Western Australia, an ancient beach that is now inland. Their work is published in Nature Geoscience.

Chemical analysis of rocks hints that life was around as long ago as 3.5 billion years, but physical evidence of it is hard to come by, because it is difficult to prove that fossils that resemble cells are truly signs of life.

For example, structures thought to be fossilized cyanobacterial mats, found in the 1980s in the 3.5-billion-year-old Apex Chert Formation, 30 kilometres from Strelley Pool, were this year shown to have an inorganic origin (see Filamentous figments in the Apex Cherts). Such uncertainty has made the question of when life arose hugely controversial.

Brasier had been critical of the Apex Chert fossils, but the latest findings should cheer researchers in the field, he says. "This goes some way to resolving the controversy over the existence of life forms very early in Earth's history. The exciting thing is that it makes one optimistic about looking at early life once again."

Signs of life
The fossils' sizes, shapes and carbon-containing cell walls are characteristic of bacterial colonies. The traces range between 5 and 80 micrometres in diameter, and take the shapes of spheres, ellipsoids and rods.

The cell walls are of uniform thickness, unlike the highly variable carbonaceous layers found in inorganic traces formed by geological processes. The fossils are also depleted in carbon-13 - the heavier form of carbon found in the atmosphere. This is a sign of biological activity, because living organisms preferentially use the lighter form, carbon-12, in their biological processes.

"The authors have demonstrated as robustly as possible, given current techniques and the type of preservation, the biological origin of these microstructures," says Emmanuelle Javaux, a palaeobiologist at the University of Liege in Belgium. However, she remains cautious, adding: "Maybe one day we will come up with a non-biological explanation for this type of microstructure - only time will tell."

Micrometre-sized crystals of iron sulphide - pyrite, or fool's gold - were found in and around the fossils' cell walls. This pattern of pyrite deposition also occurs in modern bacteria that power their metabolism by reducing sulphur-containing particles called sulphates to sulphides, although it is not conclusive evidence that the fossil cells also produced energy in this way.

The possibility of linking a microfossil to its metabolism is the most exciting aspect of the work, says Javaux. "This could be a new way to identify microfossils," she says.

Reference: Wacey, D., Kilburn, M. R., Saunders, M., Cliff, J. and Brasier, M. D.Nature Geosci. http://dx.doi.org/10.1038/NGEO1238 (2011).

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