Archean

The Archean ( /ɑrˈkiːən/, also spelled Archaean; formerly Archaeozoic /ɑrkiəˈzoʊɨk/), also spelled Archeozoic or Archæozoic) is a geologic eon before the Proterozoic Eon, before 2.5 Ga (billion years), or 2,500 million years ago. The Archean era is generally agreed to have started at 3.8 billion years ago, but this boundary is informal.