Which statement best describes the change in Earth's atmospheric oxygen content as the planet evolved over time?

• A. Oxygen was the dominant atmospheric gas before any life appeared.
• B. Oxygen levels have always been high since Earth's formation.
• C. Oxygen levels increased significantly after the evolution of photosynthetic organisms.
• D. Oxygen content steadily decreased as life evolved.

Answer: (C)

Verified step by step guidance

1. Understand the context of Earth's atmospheric evolution: Early Earth had a reducing atmosphere with very little oxygen, primarily composed of gases like methane, ammonia, and carbon dioxide.
2. Recognize the role of photosynthetic organisms: Cyanobacteria, which evolved around 2.5 billion years ago, were the first organisms to perform oxygenic photosynthesis, releasing oxygen as a byproduct.
3. Explain the Great Oxygenation Event: The oxygen produced by cyanobacteria began accumulating in the atmosphere, leading to a significant increase in oxygen levels approximately 2.4 billion years ago.
4. Discuss the impact of increased oxygen levels: Higher oxygen levels allowed for the evolution of aerobic organisms and more complex life forms, fundamentally changing Earth's biosphere.
5. Conclude by identifying the correct statement: The statement 'Oxygen levels increased significantly after the evolution of photosynthetic organisms' best describes the change in Earth's atmospheric oxygen content over time.