How did the atmosphere change over time?
Almost all living creatures on Earth depend on oxygen, but has the atmosphere always been able to support life?
Earth’s earliest atmosphere had almost nothing in common with the one that we live in today. The first atmosphere was a thin mixture of the very light gases of hydrogen and helium. The light gases quickly disappeared into space and were replaced by a brand new atmospheric make-up of methane, ammonia, nitrogen, CO , and lots of water vapour. In this cocktail of gases, the first life on Earth began to develop, but the air did still not contain any oxygen, as oxygen molecules would immediately react chemically with methane, etc.
Earth’s present atmosphere is a continuation created by photosynthetic aquatic life known as cyanobacteria. They produced so much oxygen that the chemical reactions could not use all of it, so there was a surplus.
The oxidation of the atmosphere began about three billion years ago, and now, most life forms depend on oxygen.