How did oxygen (O₂) get into Earth's atmosphere?
A) It was released by life through the process of photosynthesis.
B) It was captured from the solar nebula.
C) It was outgassed from volcanoes.
D) It came from chemical reactions with surface rocks.

Question

Quizplus · Accepted Answer

Oxygen was released by life through the process of photosynthesis, specifically by early cyanobacteria. This began to occur roughly 2.4 billion years ago, and over time, oxygen levels increased in the atmosphere.

How Did Oxygen (O₂) Get into Earth's Atmosphere