What is the Precambrian atmosphere composed of?

The Precambrian atmosphere, which existed before the emergence of complex life on Earth, was primarily composed of nitrogen, carbon dioxide, and water vapor, with very little oxygen. During this era, volcanic outgassing contributed to the atmospheric composition, while the absence of significant photosynthetic life meant that oxygen levels remained low. It wasn't until the Great Oxygenation Event, around 2.4 billion years ago, that oxygen began to accumulate in the atmosphere due to the photosynthetic activity of cyanobacteria. This shift significantly altered the composition and chemistry of the atmosphere, paving the way for aerobic life forms to evolve.