According to a report in The Washington Post yesterday, scientists analysing Australian rocks have discovered traces of bacteria that lived a record-breaking 3.5 billion years ago – a billion years after the Earth was formed.Old Dominion University biogeochemist Nora Noffke said the traces of bacteria were the oldest fossils ever described.“Those are our oldest ancestors," Dr Noffke told a meeting of the Geological Society of America. The Washington Post that unlike dinosaur bones, the newly identified fossils were not petrified body parts.They are textures on the surfaces of sandstone thought to be sculpted by once-living organisms, Dr Noffke said.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, according to The Washington Post article.The ancient Pilbara region was once shoreline and rocks made from sediment piled up billions of years ago are now exposed and available for examination.Maud Walsh, a biogeologist at Louisiana State University in Baton Rouge, told The Washington Post that while there were older rocks on Earth, the Pilbara find was the “best-preserved sedimentary rocks we know of”.“They are the ones most likely to preserve the really tiny structures and chemicals that provide evidence for life," she said.Last year scientists published the discovery of 3.4 billion-year-old fossils in the Pilbara’s Strelley Pool."It’s not just finding this stuff that’s interesting," Alan Decho, a geobiologist at the University of South Carolina’s Arnold School of Public Health, told The Washington Post. "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.