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 percent 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 of how it came about has eluded scientists for years.
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 a group of oxygen-producing microbes to gain the ascendancy. The trigger for this event was a fall in a trace metal called nickel.
The role of nickel in the story of atmospheric oxygen is new. If 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 was 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.
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 methane gas produced by 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 environments of swamps and wetlands, 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 3.8bn years ago. They found there was a marked fall in nickel between 2.7bn and 2.5bn 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," Papineau said.
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. - Independent Foreign Service