The ancestors of modern cyanobacteria invented photosynthesis over 3 billion years ago, or I should say they assembled it from processes obtained through horizontal gene transfer from at least three different ancient bacteria. (See "The first plants") Earlier bacteria that harvested sunlight created carbohydrate, but the cyanobacterial process also releases releases oxygen as a byproduct, and therein lies the most important part of the story. For the first 2 billion years of their existence,...