Meet BIF. Not the villain from Back to the Future, but a sedimentary rock known as Banded Iron Formation. The red bits were formed the last time an organism radically altered Earth’s atmosphere, changing the course of life on our planet. 
The stage: Planet Earth, 2.4 billion years ago. It was the Great Oxygenation Event. Tiny cyanobacteria, sometimes called blue-green algae, figured out how to get energy from the sun: photosynthesis! The oxygen they produced reacted with dissolved iron in the oceans, which rusted.
This build-up of oxygen eventually caused a mass extinction of almost all life on Earth, which was still single-celled, and restricted to the oceans. But this new, oxygen-rich world is also thought to have been the environment that fostered the evolution of multicellular life.
And once we had oxygen in our atmosphere, we had an ozone layer, which was necessary for the first life on land (go, plants!) some 470 million years ago. Without an ozone layer, we’d have nothing to protect us from radiation damage from the sun’s ultraviolet light.
The cyanobacteria formed some of the earliest known fossils: mounds and columns of layered calcium-rich mud and bacteria known as stromatolites. And they’re still around today! You can see stromatolites in shallow lagoons in Australia, Mexico, Canada, Turkey, and Belize.
We take it for granted that we live on a planet with oxygen, but it wasn’t always so. This was one of the many chance events leading us to this moment, over billions of years. So, hug a microbe today: we wouldn’t be here without bacteria.
You can read more about the Great Oxygen Catastrophe, as it’s also known, in this piece by @BadAstronomer: slate.com/technology/201…
