Direct fossil evidence for early microbial life on Earth is rare. Microbiological data indicate that sulfate and iron reduction are both among the earliest forms of microbial respiration, and direct evidence for the early origin of sulfate reduction comes from sulfur isotopic anomalies in ancient sediments. Fe isotope geochemistry potentially provides a new way of identifying microbial iron reduction early in Earth's history. We present Fe isotopic data for sedimentary pyrite from the 2.7 Ga Belingwe sedimentary basin in Zimbabwe. Isotopically light Fe and a remarkable covariation between Fe and S isotopes provide strong evidence for coexisting Fe and S reduction. Our results are consistent with an early origin for sulfate reduction and provide direct geochemical evidence for the antiquity of bacterial Fe reduction. The covariation of Fe and S isotopes may provide a useful new tracer of microbial evolution early in Earth history.