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Finally, the Phanerozoic eon encompasses 541 million years of diverse abundance of multicellular life starting with the appearance of hard animal shells in the fossil record and continuing to the present. After about 2.5 billion years, oxygen generated by photosynthesizing single-celled organisms began to appear in the atmosphere marking the beginning of the Proterozoic. The first eon was the Hadean, starting with the formation of the Earth and lasting about 540 million years until the Archean eon, which is when the Earth had cooled enough for continents and the earliest known life to emerge. The largest catalogued divisions of time are intervals called eons. 3.4 Naming of geologic periods, eras and epochs.3.2 Establishment of primary principles.3 History and nomenclature of the time scale.The table of geologic time spans, presented here, agrees with the nomenclature, dates and standard color codes set forth by the International Commission on Stratigraphy (ICS). The time scale was developed through the study and observation of layers of rock and relationships as well as the times when different organisms appeared, evolved and became extinct through the study of fossilized remains and imprints. It is used by geologists, paleontologists, and other Earth scientists to describe the timing and relationships of events in geologic history. The geologic time scale ( GTS) is a system of chronological dating that classifies geological strata ( stratigraphy) in time. The three million year Quaternary period, the time of recognizable humans, is too small to be visible at this scale. 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 Hadean eon represents the time before the fossil record of life on Earth its upper boundary is now regarded as 4.0 Ga ( billion years ago). This clock representation shows some of the major units of geological time and definitive events of Earth history.