Geological clock, illustration. This is a representation of the timeline of the history of the Earth, showing some of the major events and units of geological time. The Hadean eon occurred before the fossil record of life on Earth; from Earth's beginning around 4.54 Ga (billion years ago) to about 3.8 Ga. Other subdivisions reflect the evolution of life; the Archean and Proterozoic eons, and the Palaeozoic, Mesozoic and Cenozoic eras. The 2.5 million year period during which humans have evolved is too short to be visible at this scale.