I am missing something here. The article that led me to my quandary actually covers how we will search exoplanets for signs of life - how to know which planets are worth the search.
We get back as far as the cyanobacteria producing oxygen. I find this at Wiki: "The appearance of free oxygen in Earth's atmosphere led to the Great Oxygenation Event. My confusion: Did this "free oxygen in Earth's atmosphere result from something about the cyanobacteria itself which led to oxygenation of the atmosphere? Or was the "free oxygen" already in the atmosphere and cyanobacteria utilized this free oxygen to convert to Earth's use? Which came first?
Have I made it confusing enough? Sorry.