The Earth's Atmosphere Contained Little to No Oxygen Until About

Question

The Earth's Atmosphere contained little to no oxygen until about:
A) 4.5 billion years ago
B) 2.5 billion years ago
C) 1 billion years ago
D) Five hundred million years ago
E) One hundred million years ago

Accepted Answer

The Earth's atmosphere contained little to no oxygen until about 2.5 billion years ago, a period known as the Great Oxidation Event, when oxygen levels began to rise significantly due to photosynthetic processes.