Are you annoyed by how happily our species has marched towards climate change? Well, meet some old friend of mine.

As we trundle along towards the end of semester in a mad panic, it seems fitting to talk endings; in this case, extinctions. Not the little ones, either: major extinction events.

These are the big ones where there is a major loss of biodiversity on a planetary scale; where around 70 per cent of species die out and there is a major shift in the way life looks once the survivors have again evolved into a diverse range of plants, animals, insects and bacteria.

An artist’s depiction of a meteor impact. Image: NASA / Wikimedia Commons.

There have been quite a few in our planet’s history. The one everyone knows is the Cretaceous–Paleogene extinction event. This was the end of the dinosaurs (or at least the big ones that didn’t eventually evolve into chickens on our dinner table and other birds).

Some major extinction events happened like this, when meteors or volcanoes threw up dust and debris which blocked out sunlight and caused firestorms and acidification of the oceans. Others may have been the result of huge rushes of gas escaping from under the surface of the sea or earth during the movement of the continental plates. The link between all extinction events were changes to our atmosphere and climate. In most cases, this was due to geological changes no one would see coming.

But I’m going to tell you about a different sort of extinction event: one caused by excess. By one type of being consuming too much, creating too much waste, polluting the atmosphere and choking the rest of the planet… No, not man-made climate change. This was the Great Oxygenation, the first major extinction event.

Earth under an alien atmosphere

In the first half of our planet’s existence, there was very little oxygen in the atmosphere. One of the first forms of life that emerged around 3.5 to 4 billion years ago was anaerobic bacteria, which didn’t rely on oxygen for growth – in fact, it was poisonous to most of them. But the bacteria happily muddled along in this oxygen-free environment, generating its food from whatever nutrients it could find and creating methane as waste, which trapped heat from the sun and kept the planet nice and warm.

But things began to change around 2.5 billion years ago. Cyanobacteria emerged. We often call them blue-green algae; these were probably the first organisms to photosynthesise – create energy from sunlight like plants do. Oxygen was the byproduct of this process, their waste.

At first, this wasn’t a problem for the anaerobic bacteria. Oxygen quickly reacts with hydrogen to form water, iron to form rust, and many other examples. But cyanobacteria were successful lifeforms, and in a few hundred million years, they were creating more oxygen than iron, hydrogen and the other sinks could store. It began to collect in the atmosphere.

A bloom of cyanobacteria in a freshwater pond in Germany. Yes, it survives to choke waterways today, with environmental effects that we don’t have time to go into now. Photo: Christian Fischer / Wikimedia Commons.

This poisoned the anaerobic bacteria and confined them to the dark and miserable places you can find them today. It was the Great Oxygenation. Countless species that once sludged around on our planet were extinguished. Some survived; today some anaerobic bacteria can cope with mild amounts of oxygen, others with a full dose from our atmosphere. But the cyanobacteria were king. And that’s when things got really bad.

Climate change and excess

The planet was kept warm by the methane atmosphere the anaerobic bacteria created. Methane is really efficient as a greenhouse gas and did a great job keeping the planet warm. But when the oxygen the cyanobacteria created mixed with this methane, it formed carbon dioxide. Despite it being the headline greenhouse gas today, carbon dioxide does not trap as much heat in our atmosphere as the same quantity of methane. This is why we worry about the methane agriculture creates today, despite it being generated in relatively small quantities compared to carbon dioxide.

As the oxygen diluted the methane from the atmosphere, the planet cooled, and then froze in what is called the Huronian glaciation. This ice age lasted 300 million years. Another mass extinction occurred as the cyanobacteria became the victims of their own success and excess, dying out in droves in the longest ice age our planet has ever suffered.

I think any moral lesson here is a bit too obvious to point out.

Eventually, species emerged who were able to use the oxygen now present in the atmosphere for their own growth. These were the ancestors of the plants and animals we know today – give or take another few mass extinctions.

Life as a whole has a way of escaping its mistakes. Species do not.