In Earth's history, the most extreme episode of 13C-depletion in sedimentary organic matter occurred about 2.7 billion years ago. This isotopic excursion is widely interpreted as resulting from increased burial of organic carbon derived from microbial fixation of 13C-depleted methane. Modern microbial physiologies involve either oxygen or sulfate as electron acceptors in such processes. Until now, the isotopic signal has been exclusively assigned to growth of aerobic, methanotrophic bacteria and consequently interpreted as an indication for the presence of molecular oxygen in Late Archaean surface environments. The alternative anaerobic pathway involving sulfate as the electron acceptor was dismissed because this process has been considered inefficient in producing biomass sufficient to account for significant fractions of 13C-depleted carbon in some Archaean rocks. Calculations based on recent findings at two modern sites now show, however, that anaerobic processes could account for the Archaean signal.