India, Dec. 5 -- Dinosaurs leave behind unmistakable physical remains as fossils. A femur the size of a human adult or a skull serrated with teeth is hard to miss. But for the first 90 percent of Earth's history, life was microscopic, soft and squishy. When ancient microbes died, they did not leave skeletons though some have argued controversially that colonies left ancient microstructures. Microbes left behind chemical signatures, trace amounts of lipids and amino acids trapped in mud that eventually turned to stone.

The problem is that chemistry fades. Over billions of years, heat and pressure scramble these molecules until they look indistinguishable from abiotic carbon, the dead and non-living matter found in meteorites. For decades,...