The world was an unrecognizable place more than 2.45 billion years ago. But for most of the tiny organisms that lived on the planet at the time, things were just fine; at least, until the cyanobacteria started to proliferate.   As a result, these photosynthetic bacteria began to produce oxygen — so much of it that the tiny microbes sparked the “Great Oxidation Event” that infused the gas into our atmosphere. “You could argue that that’s the biggest poisoning event of Earth’s history,” says Simon Poulton, a biogeochemist at the University of Leeds in the U.K. The event proved fatal for the planet’s countless species of anaerobic bacteria, capable of metabolizing their food without oxygen, resulting in a mass extinction. But the infusion of oxygen was essential for humans and a great numbers of animals alive today. “We’re talking about probably the most significant environmental event in the history of our planet,” adds Poulton.   Read More: Earth’s First Whiff of Oxygen Now, according to research Poulton and his colleagues published in Nature in 2021, the Great Oxidation Event came in pulses, and lasted a little longer than researchers previously believed.  A Breath of Fresh Air  Scientists still debate about the appearance of the first “whiff of oxygen”, as well as when the gas became more stable on our planet. Evidence of small traces of ancient oxygen is scarce today, since there aren’t that many accessible, surviving rocks that carry definitive evidence about its presence.   The Great Oxidation Event began around 2.45 billion years ago during the Paleoproterozoic era. Previously, researchers had believed that this period lasted only about 113 million years, ending 2.32 billion years ago.   Read More: Scientists Find An Ancient Source of Oxygen Supporting Life This period was chaotic, characterized by several glaciations where the whole planet was frozen over in what is sometimes referred to as “Snowball Earth,” says Poulton. “If you went back in a time machine and landed in one of those glaciations, you would be unbearably cold.”  The strong glaciation periods likely came about due to rising oxygen levels, Poulton says. But once the oceans were covered in ice, the productivity of the cyanobacteria in the oceans would have dropped again, meaning less oxygen. Eventually, the buildup of greenhouse gases in the atmosphere from volcanic activity melted much of the ice. These intermittent periods after glaciations were likely even warmer than our world is today.   Pulses of Oxygen  Poulton and his colleagues began working on ancient rocks in the Transvaal Supergroup in South Africa. These rocks date back to the period when researchers believed that the Great Oxidation Event stopped and beyond. They measured multiple types of sulfur isotopes from cores drilled into the rocks.   “What we got was really cool and a little bit surprising,” he says. The sulfur isotopes showed several fluctuations that suggested the atmosphere was far from stable until roughly 2.22 billion years ago. This meant the Great Oxidation Event may have lasted for 100 million years more than previously believed.   “It’s really an expanded period,” Poulton says of the event. And although its name suggests there was just a single occurrence, he adds, there were actually multiple events involved in the Great Oxidation . Once said and done, the amount of oxygen became relatively stable afterwards, though it still took a long time for more complex lifeforms to evolve. It wasn’t until the end of the Precambrian period — around 541 million years ago — that the Earth experienced a second major rise in oxygen levels, after a long era Poulton and other geologists call “the boring billion,” when life evolved at a much slower pace.