The Archean Eon is a geological time period 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 the earliest known life forms, primarily simple prokaryotic organisms like bacteria and archaea. During the Archean, the atmosphere was largely anoxic, meaning it lacked oxygen. The planet's surface was dominated by volcanic activity and the formation of greenstone belts, which are ancient rock formations that provide insights into early Earth conditions. This eon set the stage for the later development of more complex life.