If the early atmosphere of the earth was composed of carbon dioxide, why does the present atmosphere contain oxygen molecules?

The atmosphere of the early earth had high concentrations of carbon dioxide due to the volcanic activity at this time in the earth's development. The atmospheric concentration of oxygen only began to increase with the appearance of the first types of photosynthetic organisms. These organisms used energy from the sun, water and carbon dioxide to make food. Oxygen was a byproduct of this reaction.