Hypothetical evolutionary scenarios of oxygenic photosynthesis in prokaryotes. A) Hypothetical evolutionary pathways of oxygenic photosynthesis. B) Hypothetical single origin of PSs/RCs. C) Hypothetical multiple originations of PSs/RCs. Gene gain, loss, and transfer events are labeled with the capital letters “G,” “L,” and “T,” respectively. The hypothetical extinct lineages are depicted in dashed branches, and the capital letter “X” denotes extinction.