The history of life on Earth is marked by tragedy. Life as we know it only exists because more ancient life lost the fight for survival. In this episode, we discuss the most mysterious – and possibly the most tragic – extinction event in Earth history, the most important transformation our planet has experienced since its formation: the Great Oxidation Event.
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Early Earth: Before the Rise of Oxygen
Today, about 21% of Earth’s atmosphere is composed of oxygen in the form of O2. Oxygen is essential for most of the biological functions of plants, animals, bacteria and more. But when we analyze the most ancient rocks on our planet, we find chemical signatures that could only occur in the absence of atmospheric oxygen. For about two billion years, our planet’s atmosphere was practically oxygen-free, instead filled with other gases such as H2 (hydrogen) and CH4 (methane).
Yet there was life back then! In the Archean Eon, as far back as 3.5 billion years or more, fossil evidence includes stromatolites left behind by microbial mats as well as chemical evidence for biological processes (biomarkers). But most of this life must have been anaerobic – able to metabolize in the absence of oxygen.
For over a billion years, Earth was home to a global ecosystem of microbial life that did just fine without oxygen. And then…
The Great Oxidation Event: The Rise of Oxygen
Around 2.5-2.3 billion years ago, as the Archean Eon gave way to the Proterozoic Eon, the rock record shows signs of a major change. We see the disappearance of certain minerals and chemical signatures that could only survive in the absence of free oxygen, and at the same time we see the appearance of abundant oxidized minerals. Oxygen was on the rise. Life on Earth had changed.
Photosynthesis is a biological process that most commonly takes in carbon dioxide and releases oxygen. In the early days of Earth, the most common photosynthesizers were cyanobacteria. DNA evidence suggests that most of our modern cyanobacteria trace their ancestry back to a major diversification around 2.5 billion years ago. Around this same time, we also see the appearance of the earliest multicellular cyanobacteria and the earliest eukaryotic (complex-celled) organisms.
The Great Oxidation Event seems to have been the tipping point where photosynthesis began filling the air with free oxygen. And the result was a bloom of oxygen-loving life (including more photosynthesizers), the beginnings of life as we know it today. Life produced oxygen, and as oxygen levels rose, life bloomed and produced more oxygen. Some research estimates that oxygen levels at the time equaled or exceeded modern levels.
Also known as “The Oxygen Catastrophe”
Lots of oxygen sounds great … for us. But for pre-oxygen life, it was tragic. There’s hardly any fossil evidence from so far in the past, but as oxygen filled the world and oxygen-loving life took over global ecosystems, there must have been a mass extinction of microbes. Today, anaerobic microbes live mainly in some of the harshest and most secluded environments.
This rise in atmospheric oxygen also pushed a lot of the methane out of the air, and since methane is a powerful greenhouse gas, this resulted in major global cooling. There’s evidence from this time of a major glaciation event, the Huronian glaciation, which may have covered most of the planet in ice: a Snowball Earth.
But after a few hundred million years or so, this bloom of new life appears to have exhausted its resources. At the end of the Oxidation Event, there’s biomarker evidence of a major decline in life processes, as much as 80% or more. In what’s been called a feast-to-famine event, life rose and crashed spectacularly. And with it, oxygen levels plummeted as well.
For almost another billion years, oxygen levels remained fairly low and the fossil record yields few surprises. But the Oxidation Event set the stage for the world as we know it. Oxygen eventually rose again to fill the oceans and atmosphere, with another dramatic rise around 600 million years ago, also associated with another major glaciation and the diversification of the earliest animals … but that’s another episode.
Some technical links:
The first two billion years on Earth
The Huronian Glaciation
Evidence for a major die-off at the Great Oxidation Event. (Paper)
Two great overviews of the Great Oxidation Event, both paywalled: one, two