Oxygen-producing photosynthesis, the sort of photosynthesis used by all modern cyanobacteria, may have evolved as early as three billion years ago. This is suggested by evidence for brief “whiffs” of increased oxygen levels even before the end of the Archean eon, two and a half billion years ago. But at first, any oxygen they released would have been quickly absorbed by iron or hydrogen sulfide or free hydrogen atoms, because oxygen is an electron thief and will combine eagerly with any element that has spare electrons. That is why atoms that have had their electrons stolen are said
