Evolution of photosynthesis C photosynthesis is the oldest and most common form. C3 is a plant that uses the calvin cycle for the initial steps that incorporate CO2 into organic material. C4 is a plant that prefaces the calvin cycle with reactions that incorporate CO2 into four-carbon compounds. CAM is a plant that uses crassulacean acid metabolism, an adaptation for photosynthesis in arid conditions. C4 and CAM Plants have special adaptations that save water. The biochemical capacity to use water as the source for electrons in photosynthesis evolved in a common ancestor of extant cyanobacteria. The geological record indicates that this transforming event
Evolution of photosynthesis as an electron donor. Green nonsulfur bacteria used various amino and other organic acids. Purple nonsulfur bacteria used a variety of nonspecific organic molecules. Fossils of what are thought to be filamentous photosynthetic organisms have been dated at 3.4 billion years old. More recent studies, reported in March 2018, also suggest that photosynthesis may have begun about 3.4 billion years ago. The main source of oxygen in the atmosphere is oxygenic photosynthesis, and its first appearance is sometimes referred to as the oxygen catastrophe. Geological evidence suggests that oxygenic photosynthesis, such as that in cyanobacteria, became important during the Paleoproterozoic