The ability to generate oxygen through photosynthesis—that helpful service performed by plants and algae, making life possible for humans and animals on Earth—evolved just once, roughly 2.3 billion years ago, in certain types of cyanobacteria. This planet-changing biological invention has never been duplicated, as far as anyone can tell. Instead, according to endosymbiotic theory, all the “green” oxygen-producing organisms (plants and algae) simply subsumed cyanobacteria as organelles in their cells at some point during their evolution.