Modern life on earth depends on the presence of oxygen in the atmosphere. Now it is about 21 percent of the atmosphere, following nitrogen, which is the most plentiful gas. But there was a time in earth’s history when there was very little oxygen in the atmosphere. How did we get the current oxygen-rich atmosphere we depend on? The American Museum of Natural History has a concise article that describes what the early atmosphere of the earth was and how it changed over time to our current conditions. You can read it here.