About 2.3 billion years ago, Earth underwent what is known as the Great Oxidation Event, as early lifeforms photosynthesized enough oxygen that the gas took hold as a dominant part of our atmosphere. This massive shift allowed for the evolution of pretty much all complex life... but its roots go back far earlier than scientists suspected.
That's the finding of an international team of researchers, who tested samples from some of the oldest soil found on Earth. This pristine soil, which dates back about 3 billion years and had spent most of those eons safely locked away in South African rocks, shows clear evidence of oxidative weathering, which — as the name does rather imply — needs oxygen in order to occur. The most obvious way for oxygen to be present in significant enough quantities to affect the soil would be for early organisms to have produced it via photosynthesis. That definitely isn't impossible, but it's an unexpected find, as it strongly suggests that our planet's earliest lifeforms were far more complex far earlier than we thought. As team member Dr. Sean Crowe of the University of British Columbia tells the BBC:
"Oxygenic photosynthesis is a very complicated metabolism and it makes sense
that the evolution of such a metabolism would take perhaps two billion years -
that we might not see its manifestation until the Great Oxidation Event. But now
that we see oxygen much earlier in the atmosphere, it tells us that even really
complex metabolisms can evolve very fast."
The researchers are hoping to find still more data to really pin down just when organisms started produced oxygen in significant quantities. As you might imagine, very little soil can be expected to endure undisturbed for 3 billion years, but the team is optimistic that some such samples might be found in places like Australia and Greenland. In the meantime, Professor Michael Bau of Germany's Jacobs University stresses that this doesn't push back the timing of the Great Oxidation Event itself — it just reveals that Earth may have had a few minor sneak previews of the main event:
"This is considerably more than people had estimated. There is some evidence also for a whiff of oxygen at around 2.6-2.7 billion years ago based on molybdenum isotope systems. But the important point about these older whiffs is that they probably represent episodic increases, and it is not until 2.3-2.4 billion years ago that we see an irreversible oxygenation of the atmosphere
Photo by Andrew Magill.