Which event is known as a major turning point that increased Earth's atmospheric oxygen?

• A. Great Oxygenation Event
• B. Cambrian explosion
• C. Plate tectonics
• D. Mass extinctions

Answer: (A)

The event highlights a watershed in Earth’s history when oxygen began to accumulate in the atmosphere because photosynthetic organisms, like cyanobacteria, were producing it as a byproduct of photosynthesis. As this oxygen entered the air, it gradually built up—first reacting with reduced minerals and solutions in oceans, then persisting in the atmosphere. Over time, oxygen levels rose enough to support aerobic (oxygen-using) metabolism and to form the ozone layer, which shields the surface from harmful UV radiation. This shift fundamentally changed the chemistry of the planet and set the stage for more complex life.

The Cambrian explosion is about a rapid diversification of multicellular life long after oxygen began to rise, not the initial increase in atmospheric oxygen. Plate tectonics describes the movement of Earth's plates and its long-term effects on climate and geography, not the specific surge in oxygen. Mass extinctions are events of widespread biodiversity loss, not the rise of oxygen levels.