The Phanerozoic Eon is split into three eras: Paleozoic (541.0-252.2 MYA symbolized by PZ in the GRI GIS data), Mesozoic (252.2-66.0 MYA symbolized by MZ in the GRI GIS data), and Cenozoic (66.0 MYA-present symbolized by CZ in the GRI GIS data). The next subdivision of the timescale is the Phanerozoic Eon (541.0 MYA-present). Global evolutionary and tectonic events that occurred during the Precambrian include (organized from oldest to youngest and including the eon in which the event occurred): formation of the Earth 4,600 MYA (Hadean) formation of the Earth’s crust (Hadean) origin of life (Hadean) oldest known Earth rocks (Archean) early bacteria and algae (stromatolies Archean) simple multicelled organisms (Proterozoic) Kanektok Metamorphic Complex (oldest known rocks in Alaska Proterozoic) and complex multicellular organisms (Proterozoic). The Precambrian is split into three eons: Hadean (4600-4000 MYA), Archean (4000-2500 MYA), and Proterozoic (2500-541 MYA). The oldest subdivision of the time scale is the Precambrian (symbolized by PC, X, Y, or Z in the GRI GIS data). The following subdivisions and events are included on the time scale, from oldest to youngest. The time scale also shows the onset of major evolutionary and tectonic events affecting the North American continent and the Northern Cordillera (SCAK, south-central Alaska SEAK, southeast Alaska NAK, northern Alaska CAK central Alaska). Geologic time scale showing the geologic eons, eras, periods, epochs, and associated dates in millions of years ago (MYA). Today the recognition of formal subdivisions of geologic time is determined by international committees.
It grew by trial and error through the efforts of numerous geologists working independently. No one person or expert committee proposed the geologic time scale used today. The geologic time scale grew out of necessity: organizing the immensity of geologic time and correlating geologic events on a worldwide scale. One way to distinguish and define each segment of time is by the occurrence of major geologic events and the appearance (and disappearance) of significant life-forms, starting with the formation of Earth’s crust followed by the appearance of ever-changing forms of life on Earth.
For the purposes of geology, the “calendar” is the geologic time scale. They have created a tool for measuring geologic time, breaking it into useable, understandable segments. Humans subdivide time into useable units such as our calendar year, months, weeks, and days geologists also subdivide time.