The geologic clock below depicts to scale the major units of geological time along with pivotal milestones of the history of Earth and life. The Hadean eon is now officially by the International Commission on Stratigraphy taken to begin at 4.0 Ga (billion years ago). Other geological period major events in the evolution of life on Earth; the Archean and Proterozoic are both eons, the Palaeozoic, Mesozoic and Cenozoic are eras of the Phanerozoic eon. The two million year Quaternary period, the time of recognizable humans, is too small to be visible at this scale.