The emergence of photosynthesis is a fundamental step in the evolution of eukaryotes and therefore of life, as it has profoundly modified terrestrial ecosystems. Although molecular clocks (a technique used by biologists to date the temporal distance between two species from their common ancestor) predict this emergence during the Proterozoic (third Precambrian eon from -2.5 billion to -541 million years ago), scientists have found very few unambiguous microfossils of photosynthetic eukaryotes. The detection of metabolic by-products in situ in individual microfossils is the key to the direct identification of their metabolisms, but until now it has remained elusive.