The world was an unrecognizable place more than 2.45 billion years ago. But for most of the small organisms that lived on the planet at the time, things were fine; at least until the cyanobacteria start to multiply.
As a result, these photosynthetic bacteria began to produce oxygen—so much so that the tiny microbes caused “Great Oxidative Event” that put the gas into our atmosphere.
“It could be argued that this is the biggest poisoning in the history of the Earth,” says Simon Poulton, a biogeochemist at the University of Leeds in the UK.
The event proved fatal for countless species on the planet anaerobic bacteriacapable of metabolize their food without oxygen, resulting in mass extinction. But on the infusion of oxygen was essential for humans and a large number of animals living 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 colleagues published in Nature in 2021. The Great Oxidation Event came in spurts and lasted a bit longer than researchers previously thought.
A breath of fresh air
Scientists still debate about the appearance of the first “breath of oxygen”, as well as when the gas became more stable on our planet. Evidence for small traces of ancient oxygen is scarce today because there aren’t that many accessible, surviving rocks that bear definitive evidence of its presence.
The Great Oxidative Event began about 2.45 billion years ago during the Paleoproterozoic Era. Previously, researchers thought this period lasted only about 113 million years, ending 2.32 billion years ago.
Read more: Scientists have discovered an ancient source of life-sustaining oxygen
This period was chaotic, characterized by several glaciations where the entire planet was frozen in what is sometimes called “Snowball Earth,” Poulton says. “If you go back in a time machine and end up in one of these freezes, you’re going to be unbearably cold.”
Heavy periods of glaciation are likely due to rising oxygen levels, Poulton says. But once the oceans were covered with ice, the productivity of the cyanobacteria in the oceans would drop again, which meant less oxygen. Eventually, the build-up of greenhouse gases in the atmosphere from volcanic activity melted much of the ice. These intermittent post-glacial periods were probably even warmer than our world is today.
Oxygen pulses
Poulton and his colleagues began working on ancient rocks in Transvaal Supergroup in South Africa. These rocks date from the period when researchers believed the Great Oxidation Event stopped and after. They measured multiple types of sulfur isotopes from cores drilled into the rocks.
“What we got was really cool and a little surprising,” he says. Sulfur isotopes show several fluctuations that suggest the atmosphere was far from stable until approximately 2.22 billion years ago. This meant that the Great Oxidation Event may have lasted 100 million years longer than previously thought.
“It’s a really extended period,” Poulton says of the event. And although its name suggests there was only one event, he adds, there were actually multiple events involved in the Great Oxidation.
Once all was said and done, the amount of oxygen became relatively stable after that, although it still took a long time for more complex life forms to develop. Only at the end of the Precambrian period — around 541 million years ago— that Earth experienced a second significant rise in oxygen levels after a long era that Poulton and other geologists call “the boring billion” when life evolved much more slowly.