Answer;
Oxygen
Before there was any life on earth, oxygen gas was absent from earth’s ancient atmosphere.
Explanation;
In the ancient atmosphere there was no oxygen gas.
Oxygen gas was introduced in the atmosphere by Early bacteria life.
Free oxygen was released through the process of photosynthesis by cyanobacteria and green algae.
Photosynthesis uses the carbon dioxide in the atmosphere and sun light to form simple sugars and release oxygen gas.