The most likely source of early Earth's atmospheric oxygen was
A) volcanic eruptions.
B) lightning striking water vapor.
C) the spontaneous breakdown of fatty acids.
D) photosynthesis by ancient cyanobacteria (blue-green algae).
E) the decay of fossil organisms.

Question

Quizplus · Accepted Answer

The most accepted theory for the source of early Earth's atmospheric oxygen is photosynthesis by ancient cyanobacteria (blue-green algae). These organisms were able to convert solar energy into chemical energy and release oxygen as a byproduct. This process began about 2.4 billion years ago and gradually increased the oxygen levels in the atmosphere to levels suitable for complex life forms. Other options listed do not have a significant impact on atmospheric oxygen levels.

The Most Likely Source of Early Earth's Atmospheric Oxygen Was