These traces of bacteria “are the oldest fossils ever described. Those are our oldest ancestors,” said Nora Noffke, a biogeochemist at Old Dominion University in Norfolk who was part of the group that made the find and presented it last month at a meeting of the Geological Society of America.
Unlike dinosaur bones, the newly identified fossils are not petrified body parts. They’re textures on the surfaces of sandstone thought to be sculpted by once-living organisms. Today, similar patterns decorate parts of Tunisia’s coast, created by thick mats of bacteria that trap and glue together sand particles. Sand that is stuck to the land beneath the mats and thus protected from erosion can over time turn into rock that can long outlast the living organisms above it.
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 eon, 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 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.