2.4 billion years ago
For the first half of Earth’s history, there was hardly any oxygen in the air. But then some bacteria began harnessing sunlight to make sugar from carbon dioxide and water, just like green plants today. These microbes pumped out oxygen as a waste product, creating the oxygen-rich atmosphere we have today. But the first oxygen may have caused the entire planet to freeze over into a ‘Snowball Earth’, by stripping the greenhouse gas methane from the air.
See other: History of Life