This clock representation shows some of the major units of geological time and definitive events of Earth history. The informal Hadean eon represents the time before fossil record of life on Earth, its upper boundary is now regarded as 4.0 Ga .Other subdivisions reflect the evolution of life; the Archean and Proterozoic are both eons, the Paleaozoic, Mesozoic and Cainozoic are eras of the ...