The Archean is the second of four eons in Earth's geological history. It started 4 billion years ago and ended about 2.5 billion years ago. It is characterized by the first definitive evidence of life on Earth.
Derived from the ancient Greek arkhe, meaning "beginning," this eon marks the beginning of the fossil record. Life at this time consisted of simple, single-celled organisms like bacteria. Although Earth developed its first continental crust, it was largely submerged under oceans much deeper than those of today. The low-oxygen atmosphere wouldn't have supported most modern life forms. In fact, the eventual rise of oxygen — known as the Great Oxidation Event — marked the end of the Archean and the dawn of the Proterozoic eon.