What was the role of oxygen in evolution?
The Earth's atmosphere has undergone many changes since formation. About 2 billion years ago there was very little atmospheric oxygen, the primary gases being carbon dioxide, methane, and ammonia. What life that existed had geared its metabolic processes to such an atmosphere, one which almost all life on Earth today would find poisonous. But as plant life evolved first in the oceans, and then on land, they began to produce oxygen as a waste product from their metabolic process of photosynthesis. A tipping point occurred about 550 million years ago, when the atmospheric concentration of oxygen changed to the more familiar oxygen-nitrogen atmosphere. This change in the atmosphere was sufficient to support the evolution of the metabolic processes of animal life. See more details at the links: