Final answer:
The Great Oxygenation Event took place about 2.5 billion years ago, due to the oxygen released by cyanobacteria through photosynthesis, leading to Earth's atmospheric oxygen increase and the formation of the ozone layer.
Step-by-step explanation:
The Great Oxygenation Event occurred about 2.5 billion years ago, fundamentally changing the Earth's atmosphere by significantly increasing its oxygen content. This event was a result of the photosynthetic activity of cyanobacteria, which released oxygen as a byproduct.This oxygen started to accumulate in the atmosphere, leading to a biological catastrophe for the anaerobic life present at the time. The increase in oxygen concentration allowed the evolution of other life forms and paved the way for the richness of life we see today. While initially posing a challenge to anaerobic life forms and causing the first-known mass extinction, the increased oxygen levels allowed more complex life forms to thrive and led to the evolution of aerobic respiration. The oxygenation also had the secondary effect of contributing to the formation of the Earth's ozone layer, which protects the planet from harmful ultraviolet radiation.