Archean

also Ar·chae·an (är-kēˈən)

1. Of or belonging to the earlier of the two divisions of Precambrian time, from approximately 3.8 to 2.5 billion years ago, marked by an atmosphere with little free oxygen, the formation of the first rocks and oceans, and the development of unicellular life. See Table at geologic time.
2. Of or relating to the oldest known rocks, those of the Precambrian Eon, that are predominantly igneous in composition.