Synonyms for Proterozoic (same or very similar meaning)
WordNet sense 1 (from 2,500 to 544 million years ago; bacteria and fungi; primitive multicellular organisms):
Proterozoic aeon, Proterozoic eonWordNet sense 2 (at or near the beginning of a period of time or course of events or before the usual or expected time):
earlyFrom the ODE community, based on WordNetadd/edit