These are "our oldest ancestors," said Nora Noffke, a biogeochemist at Old Dominion University, who was part of the group that made the discovery. Similar traces are found today along parts of Tunisia's coast, created by thick mats of bacteria that trap and glue together sand particles. The ancient and pristine Pilbara landscape was once shoreline during the Archean eon, which ended 2.5 billion years ago.

Many of the textures seen in the Australian rocks had already shown up in 2.9 billion-year-old rocks from South Africa, found by Noffke and colleagues in 2007. "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," says said Maud Walsh, a biogeologist atLouisiana State University.