Proterozoic
adjective
Pro·te·ro·zo·ic ˌprä-tə-rə-ˈzō-ik 
ˌprō-
: of, relating to, or being the eon of geologic time or the corresponding segment of rocks that includes the interval between the Archean and Phanerozoic eons, exceeds in length all of subsequent geologic time, and is marked by rocks that contain fossils indicating the first appearance of eukaryotic organisms (such as algae) see Geologic Time Table
Proterozoic noun
Word History
Etymology
Greek proteros former, earlier (from pro before) + -zoic entry 2 — more at for
First Known Use
1887, in the meaning defined above