Doug Durno February’20

The Great Oxygenation Event

By Doug Durno

If you were asked where the oxygen we breathe comes from, you’d likely say from plants. That is certainly true, but land-based plants have ‘only’ been around for 420 to 450 million years – and yet, oxygen has existed in our atmosphere for about 2.7 billion years. How then, did oxygen originate?

Let’s go back to the beginning. Not long after the Earth formed about 4.5 billion years ago, the atmosphere was up to 70% carbon dioxide (CO2). Life as we know it couldn’t survive breathing in anywhere near that amount of CO2 (it’s only 0.04% of the air we breathe today).

Gradually, the CO2 dissolved in the oceans and solidified as carbon. Carbon is very important as we couldn’t exist without it. It’s called ‘an essential building block of life’ because so much of the solid parts of our bodies are carbon-based. So, when carbon became available in the oceans 3.5 billion years ago, life emerged in the form of single-celled organisms.

Since oxygen did not appear in our atmosphere for another 800 million years, new life was anaerobic; meaning these organisms didn’t use oxygen to produce the energy needed to keep them functioning. They used fermentation, a process that extracts energy from carbohydrates without using oxygen, but fermentation can’t produce enough energy to fuel multicellular organisms. Along came a special type of bacteria called cyanobacteria. They were still single-celled, but they learned to harness the sun to produce the energy they needed to exist. This is the process we call photosynthesis.

Now we come to the good part for us. When cyanobacteria produced energy to keep themselves going, they produced something else in the process – oxygen. They didn’t require oxygen, so they got rid of it by releasing it into the atmosphere. Even better, photosynthesis produced enough energy in the cyanobacteria that they could evolve into multicellular organisms, and multicellular organisms produce greater quantities of oxygen.

This wasn’t so fortunate for anaerobic life, which didn’t require oxygen to live. In fact, oxygen was poisonous to them and they began to die off. This left more resources for the cyanobacteria, and they thrived. Still, it took a while for oxygen to build up in the atmosphere since at first it combined with dissolved iron in the oceans. Gradually, oxygen increased to make up about 10% of the atmosphere (compared to 21% today). The consequences were momentous:

  1. Extinction – Oxygen caused the first mass extinction on Earth. As mentioned, most anaerobic life couldn’t survive the presence of oxygen.
  2. Global cooling – Though we’re constantly told that methane in the atmosphere is bad because increasing levels contribute to global warming, that same presence enabled and continues to enable life on Earth. Without its warming properties, the Earth would be too cold for any life to exist. When oxygen was introduced into the atmosphere it messed with the methane, transforming it into the weaker greenhouse gas, CO2. As a result, the greenhouse effect weakened and the Earth got cold enough that glaciation took over the planet for up to 300 million years This event killed off most life and almost wiped out the cyanobacteria.
  3. Biodiversity – After glaciation, some multicellular organisms had a unique opportunity to evolve to use oxygen to produce energy. Since oxygen produces about 18 times more energy than fermentation, organisms gained the ability to develop more complex structures; driving life to evolve into the biodiversity we see today.
  4. Mineral explosion – With a more oxygenated atmosphere, elements could be transformed into new minerals. Of the 4500 minerals present on Earth, the Great Oxygenation Event is responsible for about 2500.

Oxygen changed the world. It enabled humans to eventually inhabit the Earth, and now we’re changing the world. As we increase the levels of carbon dioxide and methane in the atmosphere, we’re warming it to a degree that could dramatically affect our way of life.