Complex sentence: 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.
Split into simpler sentences: Oxygen is toxic to obligately anaerobic organisms, which were the dominant form of early life on Earth until began to accumulate in the atmosphere. Free elemental only 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.