World’s oldest fossils found

HOPE HOLBOROW, SCIENCENETWORK WA

The microfossils found in basal sandstone at Strelley Pool Formation, 60 km west of Marble Bar, are dated at 3.4 billion years old. Microfossils of sulphur-metabolizing cells in 3.4-billion-year-old rocks of Western Australia published in the journal Nature Geoscience provide convincing morphological evidence from sulphur isotope data that sulphur-metabolising extremophiles were thriving in an atmosphere devoid of oxygen. UWA Postdoctoral Research Fellow and senior author Dr David Wacey was part of the long-running research program led by Martin Brasier of Oxford University. “[What we found] were microfossils of sulphur-metabolising cells. The evidence for [the presence of] sulphur was that the microfossils were found right next to grains of pyrite—an iron/sulphur mineral,” Dr Wacey says. Dr Wacey says the microbes had the appearance of coating the grains within the sandstone and were extracting sulphur from them as well as from pyrite and silica. “It was the close association in space plus the isotopic data from the pyrite which led us to conclude that they were sulphur metabolising [microbes]. “There’s quite a lot of evidence there was very little to no oxygen [present] on the early earth but there was a lot of carbon dioxide and methane, so any present life had to metabolise alternatively.” According to the research, the discovery has been identified as microfossils of spheroidal and ellipsoidal cells and tubular sheaths demonstrating the organisations of multiple cells. “The cells are spheres and tubes of about 10 micrometers across and they cluster together just as you might expect modern bacteria to do,” Dr Wacey says. The fossils were found in what would have been a beach-like setting and were remarkably well preserved due to the high silica content in the Archaen oceans. “These [microbes] were preserved by silica which precipitated around them. Essentially sediment became rock, ensuring good preservation,” he says. The first structures were found in 2006 and have undergone about four years of testing and analysis through a variety of techniques including electron microscopy, ion probe analysis and mass spectrometry measuring isotopes within carbon and sulphur. Optical petrography, morphological analysis and fabric mapping were carried out on 30 μm and 100 μm thin sections under bright-field transmitted and reflected light using Nikon Optiophot-2 (biological), Optiophot-pol (polarizing) and Leica 2,500 DM microscopes. Editor's Note: Original news release can be found here.