"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 organization 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 specialized 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 percent 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 percent. 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.