Oldest Fossils on Earth
Australian and English researchers may have stumbled upon the world's oldest fossil, presumed to be at least 3.4 billion years old. D. Wacey/UWA

A set of 3.4-billion-year-old rocks that scientists think contain the oldest known microbial fossils may show that life on Earth preceded oxygen.

The cell-like fossils were discovered by Martin Brasier, a paleobiologist at Oxford University, and his colleagues in black sandstone at the Strelley Pool Formation in Western Australia. It was an ancient beach, but is now inland. Because of the stromatolites preserved there, the Strelley Pool is very popular among paleontologists.

According to the research report published in Nature Geoscience, the sizes, shapes and carbon-containing cell walls of the fossils are characteristic of bacterial colonies. The traces discovered in the rock are 5 and 80 micrometres in diameter and are in different shapes of spheres, ellipsoids and rods.

Unlike the highly uneven carbonaceous layers that were spotted in inorganic traces caused by geological processes, the cell walls are of uniform thickness. The fossils are depleted in the heavier form of carbon called carbon-13. Scientists said the newly found structures are biological rather than mineral in origin, because living organisms generally use the lighter form of carbon, carbon-12, according to a report in Nature.

Scientists have also found iron sulphide around the fossils' cell walls, reflecting pyrites distribution surrounding modern bacteria. Since pyrite deposition powers the metabolism of modern bacteria by reducing sulphur-containing particles called sulphates to sulphides, scientists speculate that the fossil cells also produced energy in the same way, but this is not established.

If the Strelley Pool structures used metabolized sulphur, instead of oxygen, as a source of energy, the findings may advance the search for signs of life on other planets such as Mars or the moons of Jupiter and Saturn.

At last we have good solid evidence for life over 3.4 billion years ago, said Brasier. It confirms there were bacteria at this time, living without oxygen. ... Could these sorts of things exist on Mars? It's just about conceivable.