Ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO) is arguably one of the most abundant proteins in the biosphere and a key enzyme in the global carbon cycle. Although RubisCO has been intensively studied, its evolutionary origins and rise as Nature's most dominant carbon dioxide (CO2)-fixing enzyme still remain in the dark. In this review we will bring together biochemical, structural, physiological, microbiological, as well as phylogenetic data to speculate on the evolutionary roots of the CO2-fixation reaction of RubisCO, the emergence of RubisCO-based autotrophic CO2-fixation in the context of the Calvin-Benson-Bassham cycle, and the further evolution of RubisCO into the ‘RubisCOsome’, a complex of various proteins assembling and interacting with the enzyme to improve its operational capacity (functionality) under different biological and environmental conditions. For the full article see here.