The Oxygen Revolution refers to the rise of oxygen in Earth's atmosphere and oceans primarily due to

• A. Increases in atmospheric oxygen due to volcanic emissions
• B. Oxygen release from oxides during meteor impacts
• C. Photosynthesis by cyanobacteria and algae increasing atmospheric O2
• D. Consumption of oxygen by reduced minerals in the ocean

Answer: (C)

The Oxygen Revolution is driven by oxygenic photosynthesis carried out by cyanobacteria and later algae. These organisms use light energy to split water and release oxygen as a byproduct. In Earth's early oceans, produced oxygen was largely consumed by reduced minerals and dissolved substances, so atmospheric oxygen stayed low. As photosynthesis continued and sinks were overwhelmed, free oxygen began to accumulate in the atmosphere and oceans. This rise transformed global chemistry, enabled aerobic respiration, and supported more complex life. Volcanic emissions don’t create a lasting increase in oxygen; meteor impacts can release some oxygen from minerals but only transiently; and consumption by reduced minerals would remove oxygen rather than build it up.