The primary source for free oxygen introduced into the atmosphere during the Proterozoic was through photosynthesis.

Question

Quizplus · Accepted Answer

This is a well-established scientific fact. Cyanobacteria, which evolved during the Proterozoic, were the first organisms to carry out photosynthesis and release oxygen as a byproduct. This eventually led to the oxygenation of the atmosphere and enabled the evolution of aerobic organisms.

The Primary Source for Free Oxygen Introduced into the Atmosphere