How long did it take for oxygen to build up in the Earth's atmosphere to its present levels?
A) 545 million years
B) 360 million years
C) 2 billion years
D) 3.5 billion years

Question

Quizplus · Accepted Answer

It took approximately 2 billion years for oxygen to build up in the Earth's atmosphere to its present levels. This process, known as the Great Oxygenation Event, began around 2.4 billion years ago and was caused by the evolution of cyanobacteria that produced oxygen as a byproduct of photosynthesis. This oxygen slowly built up in the atmosphere over the next few hundred million years until it reached its current levels of around 21%.

How Long Did It Take for Oxygen to Build Up