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This timeline of natural history summarizes significant geological and biological events from the formation of the Earth to the arrival of modern humans. Times are listed in millions of years, or megaanni (Ma).
The geologic record is the strata (layers) of rock in the planet's crust and the science of geology is much concerned with the age and origin of all rocks to determine the history and formation of Earth and to understand the forces that have acted upon it. Geologic time is the timescale used to calculate dates in the planet's geologic history from its origin (currently estimated to have been some 4,600 million years ago) to the present day.
Radiometric dating measures the steady decay of radioactive elements in an object to determine its age. It is used to calculate dates for the older part of the planet's geological record. The theory is very complicated but, in essence, the radioactive elements within an object decay to form isotopes of each chemical element. Isotopes are atoms of the element that differ in mass but share the same general properties. Geologists are most interested in the decay of isotopes carbon-14 (into nitrogen-14) and potassium-40 (into argon-40). Carbon-14 aka radiocarbon dating works for organic materials that are less than about 50,000 years old. For older periods, the potassium-argon dating process is more accurate.
Radiocarbon dating is carried out by measuring how much of the carbon-14 and nitrogen-14 isotopes are found in a material. The ratio between the two is used to estimate the material's age. Suitable materials include wood, charcoal, paper, fabrics, fossils and shells. It is assumed that rock exists in layers according to age, with older beds below later ones. This is the basis of stratigraphy.
The ages of more recent layers are calculated primarily by the study of fossils, which are remains of ancient life preserved in the rock. These occur consistently and so a theory is feasible. Most of the boundaries in recent geologic time coincide with extinctions (e.g., the dinosaurs) and with the appearances of new species (e.g., hominids).
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In the earliest Solar System history, the Sun, the planetesimals and the giant planets were formed. The inner Solar System aggregated more slowly than the outer, so the terrestrial planets were not yet formed, including Earth and Moon.
The Proterozoic (from c. 2500 Ma to c. 539 Ma) saw the first traces of biological activity. Fossil remains of bacteria and algae.
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Period | Started | Root word | Meaning | Reason for name |
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Siderian | c. 2500 Ma | Greek sideros | iron | ref. the banded iron formations |
Rhyacian | c. 2300 Ma | Gk. rhyax | lava flow | much lava flowed |
Orosirian | c. 2050 Ma | Gk. oroseira | mountain range | much orogeny in this period's latter half |
Statherian | c. 1800 Ma | Gk. statheros | steady | continents became stable cratons |
Calymmian | c. 1600 Ma | Gk. calymma | cover | platform covers developed or expanded |
Ectasian | c. 1400 Ma | Gk. ectasis | extension | platform covers expanded |
Stenian | c. 1200 Ma | Gk. stenos | narrow | much orogeny, which survives as narrow metamorphic belts |
Tonian | c. 1000 Ma | Gk. tonos | stretch | The continental crust stretched as Rodinia broke up |
Cryogenian | c. 720 Ma | Gk. cryogenicos | cold-making | In this period all the Earth froze over |
Ediacaran | c. 635 Ma | Ediacara Hills | stony ground | place in Australia where the Ediacaran biota fossils were found |
Cambrian | c. 538.8 Ma | Latin Cambria | Wales | ref. to the place in Great Britain where Cambrian rocks are best exposed |
Ordovician | c. 486.85 Ma | Celtic Ordovices | Tribe in north Wales, where the rocks were first identified | |
Silurian | c. 443.1 Ma | Ctc. Silures | Tribe in south Wales, where the rocks were first identified | |
Devonian | c. 419.62 Ma | Devon | County in England in which rocks from this period were first identified | |
Carboniferous | c. 358.86 Ma | Lt. carbo | coal | Global coal beds were laid in this period |
Permian | c. 298.9 Ma | Perm Krai | Region in Russia where rocks from this period were first identified | |
Triassic | c. 251.902 Ma | Lt. trias | triad | In Germany this period forms three distinct layers |
Jurassic | c. 201.4 Ma | Jura Mountains | Mountain range in the Alps in which rocks from this period were first identified | |
Cretaceous | c. 143.1 Ma | Lt. creta | chalk | More chalk formed in this period than any other |
Paleogene | c. 66 Ma | Gk. palaiogenos | "ancient born" | |
Neogene | c. 23.04 Ma | Gk. neogenos | "new born" | |
Quaternary | c. 2.58 Ma | Lt. quaternarius | "fourth" | This was initially deemed the "fourth" period after the now-obsolete "primary", "secondary" and "tertiary" periods. |