Why do we assume that oxygen was not present in the original atmosphere?

Answer 1

In general,

1. There were no plants!
2. Oxygen is very reactive so if any formed from chemical reactions it would not remain in the atmosphere very long
3. Reacts with iron to form rust or hydrogen to form water
4. Atmospheric oxygen only increased when there were more plants to take up CO₂ and release O₂
5. Over time the oxygen built up creating the atmosphere we have today

Answer 2

Inorganic molecules couldn't have formed in the presence of oxygen becsue oxygen would have taken the place of the other elements in every and all chemical reactions due to it being a highly reactive element.