Oxygenic photosynthesis evolved at least 2.4 Ga; all oxygenic organisms use the ribulose bisphosphate carboxylase-oxygenase (Rubisco)—photosynthetic carbon reduction cycle (PCRC) rather than one of the five other known pathways of autotrophic CO 2 assimilation. The high CO 2 and (initially) O 2 -free conditions permitted the use of a Rubisco with a high maximum specific reaction rate. As CO 2 decreased and O 2 increased, Rubisco oxygenase activity increased and 2-phosphoglycolate was produced, with the evolution of pathways recycling this inhibitory product to sugar phosphates. Changed atmospheric composition also selected for Rubiscos with higher CO 2 affinity and CO 2 /O 2 selectivity correlated with decreased CO 2 -saturated catalytic capacity and/or for CO 2 -concentrating mechanisms (CCMs). These changes increase the energy, nitrogen, phosphorus, iron, zinc and manganese cost of producing and operating Rubisco—PCRC, while biosphere oxygenation decreased the availability of nitrogen, phosphorus and iron. The majority of algae today have CCMs; the timing of their origins is unclear. If CCMs evolved in a low-CO 2 episode followed by one or more lengthy high-CO 2 episodes, CCM retention could involve a combination of environmental factors known to favour CCM retention in extant organisms that also occur in a warmer high-CO 2 ocean. More investigations, including studies of genetic adaptation, are needed.