The third of Earth's four geologic eons, the Proterozoic spans from about 2.5 billion to 541 million years ago. It was a time of extreme transition due to the oxygenation of Earth's atmosphere.
Before the Great Oxidation Event that occurred early in the Proterozoic, there were only whiffs of oxygen in the atmosphere. This event wiped out the earliest life forms, but more complex cells with nuclei and organelles arose; near the end of this eon, simple multicellular organisms appeared. Supercontinents like Rodinia formed and then split apart. There were episodes of extreme glaciation, such as "Snowball Earth," when glaciers likely covered nearly the entire planet. The Proterozoic set the stage for the explosion of life in the next eon.