by Matthew Cobb
Unless you are a keen student of geology, you’ll probably have never heard of the Great Oxidation Event. And yet it was arguably the most important single thing that happened to that planet after the first appearance of life, 4 billion years ago. In Earth’s early history, the dominant gas was methane, produced by bacteria. Free oxygen was in short supply – it reacts so easily with other elements that it does not last long in a free state (think about how easily rust forms – this is a consequence of oxygen reacting with iron).
And yet for the world to look the way it does, oxygen had to be present in astounding quantities – about 21% of our current atmosphere is made up of oxygen. Without oxygen there would be no multicellular organisms, and even the continents would look different – oxidative weathering is an important process that has shaped the planet. The decisive change began around 2.4 billion years ago, in the Great Oxidation Event.
So where did the oxygen come from? Life. Or more precisely, blue-green algae (“cyanobacteria”). These organisms could photosynthesize, producing oxygen. They first appeared around 2.7 billion years ago, but the oxygen they produced was quickly consumed by the more numerous methane-producing bacteria.
We know that after the Great Oxidation Event (GOE), the cyanobacteria became dominant, paving the way for the development of life as we know it and, eventually, to the high levels of atmospheric oxygen found today, which are the result of plant and microbial respiration. But was the GOE a consequence of a slow growth of the cyanobacteria and weakening resistance from the methane-producing bacteria, or was there a more rapid appearance of the cyanobacteria?
A study in Nature this week, led by Kurt Konhauser from the University of Alberta, suggests that neither of these scenarios is right. Looking at the ratio of nickel and iron in sedimentary rocks found in what are called the “banded iron formations” (see picture below). They conclude that there was a massive decrease in oceanic levels of nickel around 2.7 billion years ago, because of changes in the Earth’s geological activity.
This had a catastrophic effect on the methane-producing bacteria, which require nickel to metabolise. This in set the stage for the rapid expansion of cyanobacteria and the accumulation of oxygen in the atmosphere and in the oceans. The GOE was on its way.
As well as offering an insight into the early evolution of life, this study also shows how geological cycles have played a decisive role in the evolution of the planet as we know it.
Citations (You or your institution will need a subscription to Nature to read more than the abstracts):
A Nature “News & Views” summary can be found here.