Earth's early atmosphere was a suffocating mix of methane, ammonia, and carbon dioxide, containing almost no free oxygen. This changed roughly 2.4 billion years ago during the Great Oxidation Event (GOE). The catalyst was the emergence of cyanobacteria, microscopic organisms capable of photosynthesis. These bacteria utilized sunlight to split water molecules, releasing oxygen as a byproduct. Initially, this oxygen was absorbed by minerals and the oceans, visible today in banded iron formations. However, once these mineral 'sinks' were saturated, oxygen began to accumulate in the atmosphere. This shift was catastrophic for the anaerobic life forms that dominated the planet, as oxygen was toxic to them. Yet, for the evolution of complex, multicellular life, the GOE was a prerequisite. The resulting ozone layer also provided a shield against ultraviolet radiation, allowing life to eventually migrate from the safety of the oceans onto the desolate landmasses.