Answer:
A eruption of volcanoes
Explanation:
Scientists predict that this event occurred around 4.5 billion ago, so it is hard to collect enough data to make the verdict with 100% conviction. But they believe that Volcanic activity might play a role in the formation of the atmosphere.
Every time volcanoes erupted, they omitted a composition of gasses that consisted mainly of water vapor, carbon dioxide, Sulfur, Nitrogen, and Hydrogen. Over the time, this eruption slowly created layers on top of the earth that caused the early formation of the Earth's atmosphere.