Oxygen is toxic to obligately anaerobic organisms, which were the dominant form of early life on Earth until began to accumulate in the atmosphere about 2.5 billion years ago during the Great Oxygenation Event, about a billion years after the first appearance of these organisms.
Oxygen is toxic to obligately anaerobic organisms. Obligately anaerobic organisms were the dominant form of early life on Earth until began to accumulate in the atmosphere about 2.5 billion years ago during the Great Oxygenation Event after the first appearance of these organisms. The Great Oxygenation Event was about a billion years.