Atmospheric clues

Our planet probably formed with a good deal of oxygen; there’s plenty of it in the universe to go around. But oxygen is highly volatile and reactive, and it doesn’t really last long on its own in an atmosphere

– it either escapes into space or binds with other elements and turns into other things, like carbon dioxide or silicon dioxide. But around 2 billion years ago, a planet full of singlecelled photosynthetic organisms ate enough carbon dioxide and burped out enough oxygen to completely revamp Earth’s atmosphere, giving it substantially more oxygen than it would have in equilibrium. Life on Earth changed the very character of this planet’s atmosphere. And that’s detectable elsewhere.