English: The origin and evolutionary tree of life that is based on small-subunit RNA. The branches that perform oxygenic photosynthesis are labeled with 'O2'. The black arrow indicates the plastid endosymbiotic event that resulted in the origin of eukaryotic photosynthesis from cyanobacteria-like organisms, which ultimately became chloroplasts in algae and later in plants. However, while chloroplasts of the higher plants, glaucophytes, green and red algae are thought to be the result of the plastid (primary) endosymbiosis, all other groups of algae are assumed to have arisen due to the algal (secondary and tertiary) endosymbiosis (not shown), in which one eukaryotic alga was incorporated into another eukaryote. For details, see Olson and Blankenship (2004), Blankenship et al. (2007), Hohmann-Marriott and Blankenship (2011), Kim et al. (2011), Garcia-Mendoza et al. (2011). Only some branches of bacteria, eukarya, and archaea are displayed. Modified and adapted from Blankenship (2002) and Blankenship et al. (2007).