Earth's atmosphere has gone through "dramatic" composition changes over millions of years. A well-known example is the Great Oxidation Event. Currently the atmosphere is 78% nitrogen, 21% Oxygen, 1% Argon, and 0.04% Carbon Dioxide.
Atmospheric composition is changing because of human activity. Is it also changing because of natural forces? For example, have oxygen levels stabilised or will they continue to change because of oxidation cycles from life on Earth? Is volcanic activity, solar activity, or other natural factors slowly changing the atmospheric composition?
CODA: The motivation of my question is the fact that we humans evolved in and adapted to the current atmospheric composition, but we might not be adapted for future compositions if over millions of years the atmosphere continues to change.