When did oxygen-producing life forms first appear on our planet? It was a bit earlier than expected. Scientists have discovered that these life forms were actually present about 60 million years earlier than previously thought.

The findings come from a somewhat unusual source-rocks. Scientists found evidence for chemical weathering of rocks that led to soil formation that occurred in the presence of O2. Using the naturally occurring uranium-lead isotope decay system, the researchers found that this weather due to oxygen occurred at least 3.02 billion years ago, which means that organisms that produced oxygen must have been present at this time.

In fact, the pattern of chemical weathering preserved in the paleosol is compatible with elevated atmospheric O2 levels at the time. These oxygen levels could have only been produced by organisms that were converting light energy and CO2 to oxygen and water; in other words, the organisms were utilizing photosynthesis.

"This is a very exciting finding, which helps to fill a gap in our knowledge about the evolution of early Earth," said Quentin Crowley, one of the researchers, in a news release. "This paleosol from India is telling us that there was a short-lived pulse of atmospheric oxygenation and this occurred significantly earlier than previously envisaged."

The findings reveal a bit more about the evolutionary history of our planet. More specifically, it shows that organisms that could utilize photosynthesis were present far earlier than expected. The widely-held notion was that these organisms didn't rise until about 2.4 billion years ago during something called the "Great Oxidation Event," which enriched the atmosphere and oceans with O2. Yet it seems as if some of these organisms were active 3 billion years ago.

"Our research gives further credence to the notion of early and short-lived atmospheric oxygenation," said Crowley. "This particular example is the oldest known example of oxidative weather from a terrestrial environment, occurring about 600 million years before the Great Oxidation Event that laid the foundations for the evolution of complex life."

The findings are published in the journal Geology.