Kudos to all of you who recognized this week’s fabulous fossil as a lovely stromatolite—especially Dan Coleman who was the first to get it. I can tell you almost nothing about this particular specimen (Minda Berbeco snapped the photo at a conference), but I can tell you a great deal about stromatolites in general. First thing to know: These things are cool. Second thing: They’re still around! Third thing: Without them, we probably wouldn’t be here. Why? Because stromatolites are composed of matted layers of cyanobacteria, Earth’s first photosynthesizers. Throughout the Archaean and Proterozoic Eons, cyanobacteria within stromatolites pumped out enough oxygen to completely transform Earth’s atmosphere. Without an oxygen-rich atmosphere, the tape of life would have played a different tune.