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Oxygen is third most abundant element in the universe. Like most elements it is formed as a byproduct of a star exploding into a supernova, one of the most incredible phenomena in the universe.

It’s a major component of Earth’s atmosphere and supports a large proportion of the life on the planet, although it was not always this way.

Though the Earth always contained oxygen, it wasn’t always in the atmosphere. It was trapped within water, as much of it still is today (Oxygen is the O in H2O).

Way before dinosaurs, even before animals walked on land, all of life was simple and single-celled, like bacteria or algae. Their cells floated through the oceans, fed on carbohydrates, and absorbed the hydrogen in the surrounding water molecules. When the cell used the hydrogen, they expelled remaining oxygen as waste, which gradually diffused into the atmosphere.

While shallow oceans broiled with life, the continents remained barren like the surface of Mars. The land was pocked with rocky outcrops, many of which contained iron bands. These rocks acted as a β€˜sink’ that absorbed almost all of the atmospheric oxygen as it was produced. Over a time period of millions of years, the cells grew in population and the rocks became fully saturated, and the concentration of oxygen in the atmosphere rapidly and irreversibly swelled.

This was the cause of what is called the ‘Oxygen Holocaust’, and it almost send all life on the Earth extinct before it had even started. To explain why this happened, we need to understand what oxygen does.

Oxygen is one of the most highly reactive elements. It’s in perpetual need of electrons, and will rip them from previously stable compounds, leaving them as an electron-hungry radical that will repeat oxygen’s theft elsewhere.

Rocks that contain iron will react with oxygen, creating a red glow. Many of them are from the era preceeding the Oxygen Holocaust.

Once free in the atmosphere, oxygen began to attack the free-floating carbohydrates, the food source that early life relied on. When they ran out, oxygen attacked the early cells themselves by tearing apart their cell membranes. Almost all of life went extinct, victim to the toxic effect of its own waste.

Evolution saved life. A species of cyano-bacterium evolved that was not just destroyed by oxygen, it could harness its power and use it in its own metabolism. This species invented respiration, the process taking place within your lungs at every breath you take. This cyano-bacterium repopulated the oceans, and is one of our ancestors.

The oxygen producing life are the ancestors of all plants. They continue to produce oxygen, which is consumed by animals in a sustainable cycle.

Plants releasing bubbles of oxygen into the atmosphere.

In modern times, we are used to the ‘side effects’ of having a high concentration of oxygen in the atmosphere. One is that certain materials can, with just a spark, burst into flame. These flames are oxygen acting upon a material, ripping it apart. Things like bushfires, furnaces, and the combustion engine would not be possible in a low oxygen atmosphere.

A forest, in the process of being ‘oxidised’.

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Ben McCarthy

Ben McCarthy

Ben is the Founder of Discover Earth and the author of the Big Ideas Network.