2.33 billion BCE • Earth
"Earth’s atmosphere experienced the first significant, irreversible influx of oxygen as early as 2.33 billion years ago. This period marks the start of the Great Oxygenation Event, which was followed by further increases later in Earth’s history... scientists have also determined that this initial rise in atmospheric oxygen, although small, took place within just 1 to 10 million years and set off a cascade of events that would ultimately lead to the advent of multicellular life...
'It took another roughly 1.7 billion years for animals similar to those we have today to evolve. But the presence of molecular oxygen in the ocean and the atmosphere means that organisms that respire oxygen could thrive.'"
Source: "Study pinpoints timing of oxygen’s first appearance in Earth’s atmosphere." Jennifer Chu | MIT News. May 13. 2016.
Image Source: "The Top of the Atmosphere" NASA Earth Observatory