The Archean (also spelled Archaean, formerly called the Archaeozoic (/ɑrkiəˈzoʊɪk/), also spelled Archeozoic or Archæozoic)'s a geologic eon before the Proterozoic and Paleoproterozoic, before 2.5 Ga (billion years ago, or 2,500 ). Instead of being based on stratigraphy, this date's defined chronometrically. The lower boundary (starting point) hasn't been officially recognized by the International Commission on Stratigraphy, but it's usually set to 3.8 Ga, at the e… (More on Archean)