The Great Oxygenation Event was a time in the middle Proterozoic when
A) organisms that produce oxygen first originated.
B) oxygen began to accumulate as a gas in the atmosphere.
C) the oxygen concentration in the atmosphere reached its present concentration of
21 percent.
D) excessively high concentrations of oxygen in the atmosphere caused a mass extinction.

Question

Quizplus · Accepted Answer

The Great Oxygenation Event was a time when oxygen began to accumulate as a gas in the atmosphere. It was caused by the emergence of cyanobacteria, which were capable of photosynthesis and produced oxygen as a byproduct. The increase in atmospheric oxygen allowed for the evolution of new forms of life that were reliant on oxygen for survival. However, the oxygen concentration did not reach its present-day level of 21 percent until much later in Earth's history. There is no evidence to suggest that excessively high concentrations of oxygen caused a mass extinction.

The Great Oxygenation Event Was a Time in the Middle