Major Units of Geological Time
Definitive Events in Biological and Geophysical History
The Hadean eon represents the time before fossil record of life on Earth; its upper boundary is now regarded as 4.0 Ga (billion years ago).
Other subdivisions reflect the evolution of life:
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.
Some people say that depicting geologic time and evolutionary history using a clock face design trivializes the truly incomprehensible time frame being depicted, and so is misleading.
The time involved is in fact something very hard to get your mind around, but doesn’t that make any depiction, even on a really long linear time line, misleading? Probably.