Oxygen is an indicator of life, but not a strong one, as there are many processes which could create an oxygen atmosphere. What is a strong indicator is the pre

Oxygen is an indicator of life, but not a strong one, as there are many processes which could create an oxygen atmosphere. What is a strong indicator is the presence of both oxidizing (oxygen, ozone) and reducing (hydrogen, methane, dimethyl sulfide) molecules, in sufficient quantities. That suggests something driving the equilibrium away from all-oxidizing or all-reducing atmospheres.