Carbon dioxide is the current villain in the story of atmospheric gases, but billions of years ago the bad guy was our friend oxygen. Although it's weird to think that we wouldn't be here today were it not for a global catastrophe, the Great Oxygenation Event actually wiped out most life on Earth at the time. This ramped up about 2.5 billion years ago, but now scientists have discovered signs of the oldest known "oxygen oasis" in South Africa, showing that the process started almost half a billion years earlier.

Our current atmosphere contains a healthy oxygen concentration of about 21 percent, but way back before the Great Oxygenation Event, Earth's air only contained about 100,000th of that amount. And that was just the way anaerobic organisms liked it – until cyanobacteria species started appearing and messing everything up. These were among the first photosynthesizing organisms, meaning they absorbed sunlight and emitted oxygen, and as they spread across the Earth over many millions of years they pushed ocean oxygen levels up so high it began escaping into the atmosphere in huge amounts.

This story has been pieced together through studies of ancient sediments. Certain sulfur signatures, which can only form in a low-oxygen environment, begin to vanish from the rock layers after a certain point, indicating increased oxygen in the atmosphere.

Although the process might have hit a tipping point around 2.5 billion years ago, the new study discovered the earliest signals of this large-scale oxygen production. From analysis of 2.97-billion-year old sediments in the Pongola Basin, South Africa, researchers from Universitaet Tübingen found high levels of sulfur isotopes – particularly sulfate – which indicates that the region was home to the earliest known oxygen-producing organisms.

"Sulfate is a form of oxidized sulfur," says Ronny Schönberg, co-author of the study. "A higher concentration of sulfate in the water indicates that sufficient free oxygen must have been present in the shallow sea of the Pongola Basin."

Although this area wouldn't have been the sole cause of the transition to an oxygen-rich atmosphere, the team says that the Pongola Basin is now the earliest known oxygen oasis – an ancient haven in the middle of a choking world.

The research was published in the journal Nature Geoscience.