Rubisco, the most abundant enzyme on the Earth and responsible for all photosynthetic carbon fixation, is often thought of as a highly conserved and sluggish enzyme. Yet, different algal Rubiscos demonstrate a range of kinetic properties hinting at a history of evolution and adaptation. Here, we show that algal Rubisco has indeed evolved adaptively during ancient and distinct geological periods. Using DNA sequences of extant marine algae of the red and Chromista lineage, we define positive selection within the large subunit of Rubisco, encoded by rbcL , to occur basal to the radiation of modern marine groups. This signal of positive selection appears to be responding to changing intracellular concentrations of carbon dioxide (CO ₂ ) triggered by physiological adaptations to declining atmospheric CO ₂ . Within the ecologically important Haptophyta (including coccolithophores) and Bacillariophyta (diatoms), positive selection occurred consistently during periods of falling Phanerozoic CO ₂ and suggests emergence of carbon-concentrating mechanisms. During the Proterozoic, a strong signal of positive selection after secondary endosymbiosis occurs at the origin of the Chromista lineage (approx. 1.1 Ga), with further positive selection events until 0.41 Ga, implying a significant and continuous decrease in atmospheric CO ₂ encompassing the Cryogenian Snowball Earth events. We surmise that positive selection in Rubisco has been caused by declines in atmospheric CO ₂ and hence acts as a proxy for ancient atmospheric CO ₂ .