WordNet sense 1 (the time from 3,800 million years to 2,500 million years ago; earth's crust formed; unicellular organisms are earliest forms of life):

Archaeozoic, archeozoic, Archaeozoic aeon, Archean aeon, Archean eon, Archeozoic eon

WordNet sense 2 (used of the earliest known rocks):

archaean

WordNet sense 3 (at or near the beginning of a period of time or course of events or before the usual or expected time):

early