The Archean is a geological eon that lasted from about 4.0 to 2.5 billion years ago. It is characterized by the formation of the Earth's first stable continental crust and the emergence of early life forms, primarily simple microorganisms like bacteria and archaea. The atmosphere during this time was largely anoxic, meaning it lacked oxygen. During the Archean, the planet's surface was dominated by volcanic activity and the formation of greenstone belts, which are ancient rock formations. The eon is crucial for understanding the early conditions of Earth and the origins of life, as it set the stage for the more complex biological processes that developed in later eons.