Molecular clock analysis header Image (OIST). Credit: Kaori Serakaki (OIST) Microbial organisms dominate life on Earth, but tracing their early history and evolution is difficult because they rarely fossilize. Determining when exactly a particular group of microbes first appeared is especially hard. However, ancient sediments and rocks hold chemical clues of available nutrients that could support the growth of bacteria. A key turning point was when oxygen accumulated in the atmosphere around 2.3...