The primitive living organisms were extremely prone to oxidation, so oxygen was a kind of poison for the early organisms. The evidence for the absence of oxygen in the early atmosphere comes from the fact that the early organisms were able to survive long enough and evolve into more viable forms (which were able to exist). Had there been oxygen in the early atmosphere of earth, then all the primitive 'living organisms' would have been oxidized and hence destroyed.
We have, however, just studied that the early living organisms like blue-green algae could carry out the process of photosynthesis and release oxygen gas. Now, an important question arises that if there was no oxygen in the early atmosphere, then where did this oxygen produced during photosynthesis go ? This can be explained by saying that the oxygen given out by early organisms like blue-green algae during photosynthesis, reacted with the iron salts dissolved in oceans to form iron oxide which then turned into ironstone rocks.
The proof for the combination of oxygen with 'Yon in oceans comes from the existence of extensive deposits of banded iron stones which were formed between 3.5 and 2.0 billion years ago. From this discussion we conclude that though now we cannot imagine life to exist without oxygen, but there was no free oxygen in the early atmosphere of the earth. In fact, an atmosphere containing oxygen in the early days could have oxidized and killed the early organisms before they had a chance to invent answer organisms having a higher chance of survival in the changing environment me above discussed organization of the earth system that our earth took lead in ushering in biological revolution.