A UC Davis geologist has identified what may be Earth's oldest known complex communities -- elaborate structures built by simple microbes covering the ocean floor 2.5 billion years ago. In unusually old rocks from South Africa, assistant professor Dawn Y. Sumner recently found evidence that the single-celled organisms--probably bacteria--organized into two communities that interacted, yet had distinctly different architectures and effects on their environment. The millions of microbes in one community somehow crystallized calcite--the mineral that eons later would make up seashells and corals--from the surrounding sea water and used it to construct rigid walls. The other organisms nestled together in thin, gooey, layered mats draped horizontally between the walls. Sumner says the wall-builders might have used their constructions to move closer to beneficial light or other nutrients, while the mat-builders had different survival needs and so evolved different survival strategies. The communities' calcite remains were preserved as rocks, now called stromatolites. Other researchers have examined similarly ancient stromatolites, but they have seen only the simple mat-shaped structures without the associated walls. Sumner's findings, based on doctoral research at the Massachusetts Institute of Technology, were published in the August issue of the journal Palaios. At UC Davis, Sumner will continue to study the lives of very early organisms and their influences on the young Earth's environment.