All air-breathing life on Earth depends on the energy produced by the respiration of carbon compounds (i.e. food) with oxygen. When Earth formed roughly 4.6 billion years ago there was no oxygen in its atmosphere. Now, the concentration of oxygen is maintained near 21% by a delicate balance of biological, environmental and geological processes. Understanding the rise of O2, and the feedbacks that keep it stable, constitute one of the great puzzles of geobiology.
Oxygen supports all complex (multicellular) life on our planet. However, there are good scientific reasons to believe there was no free O2 when the Earth formed about 4.6 billion years ago. This is because oxygen likes to form stable oxides with many other elements such as hydrogen, carbon, sulfur and iron. An iron oxide you may be very familiar with is rust, and another common molecule that contains oxygen is water! Any free O2 in contact with these elements would react in a relatively short time and, therefore, disappear from the atmosphere, unless it was replenished. Evidence from sediments formed before 2.5 billion years ago supports this theory. They contain rounded grains of minerals like pyrite - an iron sulfide with the chemical formula FeS. You may know pyrite by its common name of fool's gold. Because their rounded shapes show that they must have become smooth during transport in flowing water, such as a river, they are known as ‘detrital’ minerals. These detrital grains are unstable in the presence of oxygen - the FeS molecule would quickly break down into an iron oxide and sulfate. So, our oxygen-intolerant mineral grains are rounded, showing that they were tossed around in a stream at the surface of the Earth, and thus, expose to the atmosphere. This good evidence for Earth’s early atmosphere having no free O2 - otherwise, these detrital minerals would never be preserved in the sedimentary rock record!
All this changed at about 2.4 billion years ago and, to the best of our knowledge, it happened very suddenly. Instead of detrital grains of unstable minerals geologists observe that most common sediments, such as sandstones, contain an abundance of sand grains coated with iron oxides (rust). Their sudden appearance, and the knowledge that oxygen production during photosynthesis is a pivotal biological process, has led geologists and geobiologists to coin the term ‘Great Oxidation Event’ or GOE. This event is also marked by the peak production of iron-bearing sediments known as the Banded Iron Formations (BIF). These are literally mountains of ironoxides that are being mined for steel-making.
The geochemical changes we see at the GOE could happen with just a small amount oxygen, perhaps as little as 0.01% of what we have today. It very likely took another 2 billion or so years to reach levels of 21% that support air-breathing animals. Exactly why atmospheric oxygenation took so long is not known but it is likely due to feedbacks in the way biology interacts with the atmosphere-ocean-rock system. The time lag can be explained if iron and other reduced minerals in rocks acted as an oxygen ‘buffer’. Such a lag would also have allowed early life to develop mechanisms to adapt to oxygen’s toxic effects and to develop metabolisms that make use of its incredible chemical properties. Still, understand how this all works is one of the great outstanding puzzles in the Earth sciences.
We know that free oxygen is a byproduct of the splitting of water during oxygenic photosynthesis. This requires a sophisticated light-harvesting system that can capture high-energy photons in the spectrum of visible light. Today, this is not only found in green plants but in more primitive organisms such algae and cyanobacteria. Paleontology and genetics tell us that the cyanobacteria are the most ancient group that is capable of oxygenic photosynthesis.
You would not be reading this article were it not for the remarkable properties of oxygen. This is why understanding why we have an atmosphere with exactly the right amount to support intelligent beings is such a tantalizing problem.