Geologists from the University of Wisconsin-Madison have found the earliest known evidence of oxygen on Earth, in rocks that were formed on the ocean floor 3.2 billion years ago.
Cyanobacteria, which are ocean-dwelling bacteria that create their own food, are likely the first organisms that created oxygen on our planet. That oxygen left a footprint in rocks gathered in South Africa. The rocks originally formed at the ocean floor, and contain iron, which reacts with oxygen to form iron oxide. Find some iron oxide in 3.2 billion year old rock, and bam, you have evidence of oxygen, which means bacteria were around to create it.
The rocks are perfectly situated in the niche between when we know there was no oxygen, and the earliest evidence of oxygen that we had, before this find.
"Rock from 3.4 billion years ago showed that the ocean contained basically no free oxygen," says Clark Johnson, professor of geoscience at UW-Madison. "Recent work has shown a small rise in oxygen at 3 billion years. The rocks we studied are 3.23 billion years old... [W]e believe they show definite signs for oxygen in the oceans much earlier than previous discoveries."
The earliest evidence for life on Earth is currently placed at 3.5 billion years ago, so as Earth-time goes, it was a relatively quick passage of time between that and when we now know cyanobacteria began photosynthesizing and providing oxygen to our planet.
It used to be thought that oxygen was rare until about 2.4 to 2.2 billion years ago, so this places plentiful oxygen at this site at more like a billion years before that. However, all the samples studied were from one region in South Africa, so we can't be sure that oxygen was widespread across Earth at that time. Still, it likely developed rapidly once bacteria started catching onto making their own food from solar energy.
"Once life gets oxygenic photosynthesis, the sky is the limit," said Brian Beard, a co-author of the study and senior scientist at the UW-Madison. "There is no reason to expect that it would not go everywhere."