Strelley Pool in the Pilbara, where 3.4 billion-year-old fossils have been found. Credit:David Wacey Finding the earliest remnants of this process required a long, hard look at some of the planet's oldest rocks, located in Western Australia's Pilbara region. This ancient landscape was once shoreline. Rocks made from sediment piled up billions of years ago are now exposed and available for examination. Relatively pristine in condition, such outcrops, along with others in South Africa, have long been a popular place to look for traces of life from the Archean aeon, which ended 2.5 billion years ago. There are older rocks on Earth, said Maud Walsh, a biogeologist at Louisiana State University in Baton Rouge. "But these are the best-preserved sedimentary rocks we know of, the ones most likely to preserve the really tiny structures and chemicals that provide evidence for life." Last year, another team of researchers published the discovery of microscopic fossils in Pilbara's Strelley Pool Formation, about 3.4 billion years old. "It's not just finding this stuff that's interesting," says Alan Decho, a geobiologist at the University of South Carolina's Arnold School of Public Health. "It's showing that the life had some organisation to it." Ridges that crisscross the rocks like strands in a spider web hint that primitive bacteria linked up in sprawling networks. Like their modern counterparts, they may have lived in the equivalent of microbial cities that hosted thousands of kinds of bacteria, each specialised for a different task and communicating with the others via chemical signals. Many of the textures seen in the Australian rocks had already shown up in 2.9-billion-year-old rocks from South Africa, reported on by Noffke and colleagues in 2007.

Still, old Australian rocks have proved deceptive before. As early as 1980, rippling layers within the Strelley Pool were thought to be the handiwork of bacteria. But such stromatolites, which are different from the structures that Noffke studies, can also be the work of natural, non-living processes. For instance, water flowing along a seafloor can create similar structures under the right conditions. So can spraying jets of liquid loaded with particles onto a surface, as scientists at Oxford University demonstrated in laboratory experiments. That's why Noffke and her colleagues corroborated their story by measuring the carbon that makes up the textured rocks. About 99 per cent of carbon in non-living stuff is carbon-12, a lighter version of the element than the carbon-13 that accounts for most of the remaining 1 per cent. Microbes that use photosynthesis to make their food contain even more carbon-12 and less carbon-13. That bias, a signature of "organic" carbon that comes from a living being, showed up in the Australian rock. "It's always nice to have a number of different lines of evidence, and you definitely want to see organic carbon," says geomicrobiologist John Stolz of Duquesne University in Pittsburgh. What wasn't preserved: any proteins or fats or body fossils that would clinch the case for life and identify what types of bacteria left behind this organic carbon. Most microbial mats today contain lots of photosynthetic cyanobacteria, which make the food that sustains the other bacteria. Named after the blue-green pigment they use for this process, called phycocyanin, cyanobacteria also make oxygen and are given the credit for creating Earth's atmosphere about 2.4 billion years ago. Cyanobacteria living in microbial mats nearly 3.5 billion years ago could shake up the history of the air we all breathe.

"Studying this kind of past life is really about learning how the Earth got to be the way it is today," says Michael Tice, a geobiologist at Texas A&M University. Ultimately, the fossils found on Earth could help those looking for the building blocks of life on Mars, where NASA's Curiosity rover has recently found evidence for ancient waterways. Remnants of life on the Red Planet might even be better preserved than they are here on Earth, says Harvard University paleontologist Andrew Knoll. That's because old terrestrial rocks tend to get banged around by the movement of tectonic plates and cooked by the extreme heat of the planet's depths. Mars, a planet that's nearly dead geologically, lacks such tectonic activity. Though no signs of ancient Martian microbes have been found, fossil hunters may now have something new to start looking for. Washington Post