The conventional fossil record is built of hard parts—bones, shells, and decay-resistant organic tissues buried in the sediments that accumulate on floodplains, in lakes, and on the seafloor. In the 1950s, geologists first began the routine application of radioactive decay to problems of geologic age. The geologic record of microbial life is preserved in four distinct ways. First, bacteria and protists leave what we can consider an extension of the conventional fossil record: cell walls and extracellular envelopes preserved directly in sedimentary rocks. Second, microorganisms also leave molecular fossils that complement the record of morphology. Sediments transported across the seafloor interact physically with microbial mat communities, providing a third and distinctly different biological signature in sedimentary rocks. Finally, microbial populations can actually influence the composition of seawater, providing a distinct chemical signature in minerals precipitated from ancient oceans. One can hazard only broad guesses about the biological properties of early microorganisms, but one can make one key statement with confidence: early cells lived without oxygen. The plants and animals so conspicuous in our own world are evolutionary latecomers, intercalated into ecosystems that were already 3 billion years old when sponges first gained a foothold on the seafloor. The author suspects that the correct explanation will not point to physical or biological processes acting alone but, rather, will emphasize the interactions between Earth and life.