Briefly explain the evolution of oxygen on earth.

The answer is tiny organisms known as cyanobacteria, or blue-green algae. These microbes conduct photosynthesis: using sunshine, water and carbon dioxide to produce carbohydrates and, yes, oxygen. ... "What it looks like is that oxygen was first produced somewhere around 2.7 billion to 2.8 billon years ago.  

Explanation:

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...

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