AbstractCellulases catalyze the hydrolysis of cellulose. Improving their catalytic efficiency is a long-standing goal in biotechnology given the interest in lignocellulosic biomass decomposition. Although methods based on sequence alteration exist, improving cellulases is still a challenge. Here we show that Ancestral Sequence Reconstruction can “resurrect” efficient cellulases. This technique reconstructs enzymes from extinct organisms that lived in the harsh environments of ancient Earth. We obtain ancestral bacterial endoglucanases from the late Archean eon that efficiently work in a broad range of temperatures (30–90 °C), pH values (4–10). The oldest enzyme (~2800 million years) processes different lignocellulosic substrates, showing processive activity and doubling the activity of modern enzymes in some conditions. We solve its crystal structure to 1.45 Å which, together with molecular dynamics simulations, uncovers key features underlying its activity. This ancestral endoglucanase shows good synergy in combination with other lignocellulosic enzymes as well as when integrated into a bacterial cellulosome.