Episode 3 of The Story of Life on Earth travels to Earth’s ancient oceans roughly 2.5 billion years ago, when the planet looked nothing like it does today. Rust-colored seas and an oxygen-free atmosphere were reshaped by the rise of microbial life, in a shift that changed the trajectory of the entire planet.
The episode traces how that microbial shift altered ocean chemistry and the atmosphere itself, setting the stage for the more complex life that would follow over the next two billion years.
Scientists call this shift the Great Oxidation Event. Photosynthesizing cyanobacteria began releasing oxygen into water and air that had almost none before, and the buildup was toxic to much of the existing anaerobic life. Dissolved iron in the oceans rusted out of solution into the banded iron formations still visible in the rock record today, marking the transition in layers of red and gray stone. The newly oxygen-rich planet eventually opened the door to more complex, oxygen-breathing life, though it took close to two billion more years for that potential to be realized.
It’s the same long arc of deep time that runs through Martin’s 2019 book, Life: The First Four Billion Years, illustrated by Grahame Baker-Smith, which follows Earth’s story from the Big Bang through the fossil record and the age of dinosaurs to the first mammals.
The Great Oxidation Event ranks among the largest extinction events in Earth’s history, though it rarely gets described that way, since almost nothing alive at the time had a skeleton or shell to leave behind as a fossil. Most of what survived did so by retreating into oxygen-free pockets of mud and sediment, where anaerobic microbes still live today. The banded iron formations left behind are mined commercially now as a major source of the world’s iron ore, which means a two-billion-year-old chemical crisis quietly underpins a lot of modern steel production.
Watch Episode 3 on YouTube, start from Episode 1, or see the full series on the YouTube Series page.