[2026] PTE Academic Reading - Multiple Choice, Multiple Answers

The Great Oxidation Event, occurring approximately 2.4 billion years ago, was a period when Earth's atmosphere experienced a significant rise in free oxygen.Before this event, the atmosphere was primarily anaerobic, composed of gases like methane and carbon dioxide.The primary driver of this transformation was the emergence of cyanobacteria, which utilized photosynthesis to produce oxygen as a byproduct.Initially, this oxygen was absorbed by 'sinks' such as iron-rich rocks on the seafloor, creating banded iron formations.Once these minerals were saturated, oxygen began to accumulate in the atmosphere, leading to the extinction of many anaerobic organisms for whom oxygen was toxic.This biological crisis, often called the Oxygen Catastrophe, cleared the way for the evolution of more complex, aerobic life forms.The surge in oxygen also reacted with atmospheric methane, a potent greenhouse gas, causing a massive drop in global temperatures.This cooling triggered the Huronian glaciation, one of the longest ice ages in the planet's history.While it was a catastrophic event for the dominant life forms of the time, the Great Oxidation Event was essential for the eventual appearance of animals and humans.