Until roughly 2.4 billion years ago, the world had a very different atmosphere that was almost completely lacking in oxygen, and animals and leafy plants were entirely absent. Instead, anaerobic bacteria covered the planet, and were happily converting their food into energy without needing to use oxygen.
However, a zoological revolution was imminent. Photosynthetic bacteria (in the form of sea-drifting algae) had been around for perhaps as long as a billion years beforehand – and these little critters were slowly converting the carbon dioxide into oxygen.
Most of this oxygen was initially absorbed by iron compounds, but when surface-level, iron-rich rocks could no longer absorb any more, the oxygen was permitted to freely float around in the air. On a geological timescale, this happened rather suddenly.
As oxygen was toxic to most forms of life on Earth at this point, it killed them all off, leaving the photosynthesizing bacteria behind. This is known as the Great Oxygenation Event (GOE) by some, but it’s also appropriately referred to as the Oxygen Catastrophe. In effect, it was a complete regime change, and the planet’s first mass extinction event.
However, oxygen didn’t continue to simply accumulate. As seafaring photosynthetic microbes flourished, more of them rained down onto the sea floor as they died. They were converted into carbon-rich rocks, which were later exhumed to the surface. These reacted with the free oxygen so quickly that just 400 million years after the GOE, oxygen levels were just 0.01 percent of current levels.
Many thought it wasn’t until 1.8 billion years after the GOE – around 600 million years ago – that levels were once again high enough for multicellular life to flourish. This study kicks this date back around 215 million years, and upends our understanding of life on Earth as a result. There is now a “second GOE,” one that, again, appears quite suddenly – and tantalizingly, no one knows what caused it.
Previously modeled ocean-atmospheric oxygen proportions (dashed line) over time (millions of years) compared to this study’s actual measurements (green bar). What caused this secondary oxygenation event? Blamey et al./Geology