Archaean Eon

The Archaean Eon is a period in Earth's history that lasted from about 4.0 to 2.5 billion years ago. During this time, Earth’s crust had cooled enough for continents to form, and the earliest known life—simple bacteria—appeared in the oceans. The atmosphere was mostly methane and other gases, with little oxygen. This eon marks the beginning of Earth's solid surface and the origins of life, setting the stage for the complex biological and geological developments that followed. It’s a foundational chapter in Earth's long history.