Scientists don't know how far back oxygen goes.
'Almost certainly biological'
All in all, the researchers suggest atmospheric oxygen levels 3 billion years ago were about 100,000 times higher than what can be explained by regular chemical reactions in Earth's atmosphere. "That suggests the source of this oxygen was almost certainly biological," Crowe said.
"It's exciting that it took a relatively short time for oxygenic photosynthesis to evolve on Earth," Crowe added. "It means that it could happen on other planets on Earth, expanding the number of worlds that could've developed oxygenated atmospheres and
complex oxygen-breathing life."
Future
research
can look for similarly aged rocks from other places, both on and outside Earth, to confirm these findings. "Research could also look at earlier rocks," Crowe said. "Chances are, if there was oxygen 3 billion years ago, there was likely oxygen production some time before as well. How far back does it go?"