About 2.5 billion years ago, though consensus on that date hasn’t yet been reached, one of the greatest calamities to hit life on Earth occurred. That event would have profound implications on the development of life from then on. Ultimately, it allowed complex life to arise on earth, and after more mass extinctions, eventually led to us. This great planetary calamity came from an unlikely source, indeed it is today one of the hallmarks of earth’s atmosphere and we die very quickly without it. It was oxygen. But oxygen as we know and breathe it, O2, was not always present in the earth’s atmosphere in high concentration and while we all know the reason it’s there, photosynthesis by plants, it was also deadly poisonous to some of earth’s earliest life.
Sometime before 2.3 billion years ago multicellular organisms evolved called cyanobacteria or what used to be called blue-green algae though that term is out of date. These oceanic organisms were the first life on planet earth to be capable of photosynthesis. The byproduct of photosynthesis is, of course, oxygen. Initially, this was not an issue. Oxygen is reactive and likes to bond with other elements. It would do so with elements such as iron early in the Earth’s history and that would keep it safely locked up. At some point, however, the planet’s oxygen sinks became saturated which allowed a build up of oxygen in the ocean and atmosphere. This was toxic to the anaerobic bacteria living in the ocean and they began to die off.
Earth was warm during this period of history because of methane, which is an effective greenhouse gas. Trouble is, when you start involving oxygen with methane, they react and one of the products of that is carbon dioxide. That interaction increased the levels of CO2 in the earth’s atmosphere. Now, we tend to think of carbon dioxide as a greenhouse gas in and of itself, especially in the context of human production of it and the atmospheric effects of that. And it is a greenhouse gas, but it’s nowhere near as efficient as methane when trapping heat. As a result, the earth almost froze solid in what is known as the Huronian glaciation and a mass extinction resulted. These related events threatened all life on earth including the cyanobacteria and hit the anaerobes very hard, though their descendents do still live in environments with poor oxygen on earth and some can now tolerate or even use oxygen, though they don’t need it. The point is, life on earth nearly committed suicide just because one type of organism started producing our life-giving oxygen. If things had been just a little worse during the glaciation, our planet would not be what it is today. But because oxygen is so reactive, that makes it useful to life that can incorporate it into its make up. Organisms that could do so suddenly had significantly more energy available to them and that allowed them to thrive. We descend, albeit very distantly, from those organisms.
This seemingly life-caused mass extinction isn’t the only one. Fast forward to the dawn of humankind. We ourselves are linked to thousands of extinctions from amphibians to megafauna. One could argue that these extinctions are due to the advent of technology, but at its most basic level we have merely changed the conditions of the planet like our cyanobacteria forbearers did. Perhaps someday we can bring those species back through a mastery of genetics and halt the Great Holocene extinction event. But it makes me wonder how often things like the great oxygenation event happen to habitable planets in the universe. Does life often develop on other worlds only to routinely destroy itself at the microbial level? Are there worlds where oxygen creation never have happened for some reason? Might these anaerobic worlds harbor complex anaerobic life? We have seen such things on our own planet, albeit rarely and it’s hard to envision anaerobic organisms ever getting too complex. But it’s well worth wondering about.