It was one of the most important changes to have happened to the Earth's atmosphere and it was the reason why today we can breathe life-giving oxygen. And yet the Great Oxidation Event has remained a mystery – until now.
Without oxygen, life on Earth would not exist as we know it. It has provided the supercharged air that has fuelled an explosion in the diversity and size of all living organisms, from the smallest shrimp to the biggest dinosaur.
About 21 per cent of air is oxygen, a vital ingredient for living organisms to carry out the most efficient method of converting food into energy using aerobic respiration. Yet an oxygen-rich atmosphere did not always exist, and the explanation for how it came about has eluded generations of scientists.
Now a team of researchers led by Kurt Konhauser of the University of Alberta in Edmonton, Canada, has come up with a convincing explanation for why oxygen suddenly began to accumulate in the early atmosphere of the Earth about 2.7 billion years ago, when life consisted of nothing more complex than single-cell microbes.
The Great Oxidation Event happened, they believe, when one group of oxygen-destroying microbes began to die off, leaving another group of oxygen-producing microbes to gain the ascendancy. The trigger for this event was a fall in a trace metal called nickel, which led to the inexorable rise of oxygen – and life – on Earth.
The role of nickel in the story of atmospheric oxygen is new. If Professor Konhauser and his colleagues are right then it could explain not just the explosive evolution of life, but how the Earth itself was shaped, because it the erosive power of oxygen that was so crucial to the sculpturing of rocks, the formation of rivers and the carving out of the coastlines.
"The Great Oxidation Event is what irreversibly changed surface environments on Earth and ultimately made advanced life possible. It was a major turning point in the evolution of life on our planet, and we are getting closer to understanding how it occurred," said Dominic Papineau of the Carnegie Institution in Washington.
Oxygen as a molecule is so reactive that it soon disappears unless it is being constantly produced. The concentration of oxygen in the atmosphere today is maintained by plants carrying out photosynthesis – the conversion of sunlight into chemical energy and oxygen.
The first photosynthetic microbes, the "blue-green" algae or cyanobacteria, are thought to have evolved about 300 million years before the Great Oxidation Event 2.5 billion years ago. But the oxygen they produced was quickly destroyed by the methane gas produced by the far more numerous methanogenic bacteria, which could breathe without oxygen using the less efficient method of anaerobic respiration.
These methanogenic bacteria, which still live in the waterlogged, oxygen-starved environment of swamps and wetlands, crucially need nickel to survive. Without a rich supply of nickel, the vital enzymes of these methane-producing microbes are fatally undermined.
The scientists found that by analysing a type of sedimentary rock known as banded-iron formations they could monitor levels of nickel in the oceans of the early Earth dating as far back as 3.8 billion years ago. They found there was a marked fall in nickel between 2.7 billion and 2.5 billion years ago – the same time as the Great Oxidation Event.
"The timing fits very well. The drop in nickel could have set the stage for the Great Oxidation Event. And from what we know about living methogens, lower levels of nickel would have cut back methane production," Dr Papineau said.
"The nickel connection was not something anyone had considered before. But our study indicates that it may have had a huge impact on the Earth's environment and on the history of life," he said.
Professor Konhauser said that the study, published in Nature, supports the idea that these methane-producing microbes prevented oxygen from accumulating in the early atmosphere for hundreds of millions of years.
The scientists believe that nickel levels fell because the Earth's crust had cooled down during this period, which meant that there was less nickel being ejected from volcanic eruptions into the ocean.
"We're certain from looking at the rocks in banded-iron formations that nickel dropped about 2.5 billion years ago to about half its previous value. The issue is how the methane-producing microbes responded to this decrease in nickel. We think they died off," Professor Konhauser said.
Although the Great Oxidation Event did not lead to a sudden rise of oxygen to levels like those experienced today, it did cause a significant rise that has never been reversed.