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6. Geologic Timelines - Background
The geologic time scale is a tool used by geologists to break up the history of Earth (all 4.6 billion years of it) into chunks that are more manageable. These divisions are determined by the major changes in biodiversity (and therefore the appearances of some fossils and the disappearance of others) that occurred throughout time. As new information is discovered, the geologic time scale is refined to reflect these new discoveries. The dates given in the handout are derived from a 2004 revision of the time scale endorsed by the International Commission on Stratigraphy. Several changes such as the phasing out of the Tertiary/Quaternary periods in favor of a Paleogene/Neogene division is included here.
The largest divisions are eons that define the most major developments in Earth’s history. The most ancient eon, the Hadean (from 4.6 – 3.8 billion years ago), finds the Earth as it coalesces and cools into a more stable planet. The first life appears in the form of ancient bacteria during the second eon, the Archean (from 3.8 – 2.5 billion years ago). Gradually, more complex eukaryotic life including algae and the first multicelluar organisms evolve during the third eon, the Proterozoic (from 2.5 – 0.54 billion years ago). These early photosynthetic organisms produced oxygen that accumulated in the Earth’s atmosphere. Finally, there is the Phanerozoic eon (from 542 million years ago to the present) encompassing the evolution of most complex life on Earth.
Eons are divided into eras that are in the range of hundreds of millions of years long. The major eras of the Pharnerozoic include the Paleozoic (from 542 – 251 million years ago), the Mesozoic (from 251 – 65 million years ago) and the Cenozoic (from 65 million years ago to the present). These can broadly be described as the “Age of Fish and Amphibians”, the “Age of the Dinosaurs”, and the “Age of Mammals”. Each era ended with a major extinction event. Following eras, the divisions of time are called: periods, epochs, and ages.