It is generally assumed that planets are formed by the accretion of gas and dust in a cosmic cloud, but there is no way of estimating the length of this process. Our Earth acquired its present size, more or less, between 4 billion and 5 billion years ago. Life on Earth originated about 2 billion years ago, but there are no good fossil remains from periods earlier than the Cambrian, which began about 490 million years ago.
The known geological history of Earth since the Precambrian Time is subdivided into three eras, each of which includes a number of periods. They, in turn, are subdivided into epochs and stage ages. In an epoch, a certain section may be especially well known because of rich fossil finds.
In March 2004, geologists added a new time period to Earth's chronology—the Ediacaran Period. The Ediacaran Period lasted about 50 million years, from 600 million years ago to about 542 million years ago. It was the last period of the Precambrian's Neoproterozoic Era. Multicelled organisms first appeared during this time. This period is the first new one added in 120 years.
The Precambrian's lower limit is not defined, but ended about 542 million years ago. The Precambrian encompasses about 90% of Earth's history.
|Archaean (Greek archaios = ancient )||2,500?||Eoarchean (Greek eos = dawn + archaios = ancient)|
Paleoarchean (Greek palaios = old)
Mesoarchean (Greek mesos = middle)
Neoarchean (Greek neo = new)
|Formation of oceans, |
atmosphere, and continents;
|Proterozoic (Greek proteros = earlier + zoön = animal)||c. 2,000||Paleoproterozoic (Greek palaios = old)|
Mesoproterozoic (Greek mesos = middle)
Neoproterozoic (Greek neo = new)
|Oxygen build-up; |
This era began 542 million years ago and lasted about 291 million years. The name was compounded from Greek palaios (old) and zoön (animal).
|Cambrian (Cambria, Latin name for Wales)||54||Lower Cambrian|
|Invertebrate sea life proliferating during this and the following period|
|Ordovician (Latin Ordovices, people of early Britain)||45||Lower Ordovician|
|Diverse marine life, including vertebrates; vascular plants|
|Silurian (Latin Silures, people of early Wales)||28||Lower Silurian|
|Coral reefs; giant scorpions; first jawed fish|
|Devonian (Devonshire in England)||57||Lower Devonian|
|Numerous fishes, other sea life; many plants, first trees; wingless insects|
|Carboniferous (Latin carbo = coal + fero = to bear)||60||Upper, Middle, and |
Upper, Middle, and
|Maximum coal formation in swampy forests; insects, amphibians, reptiles; fishes, clams, crustaceans|
|Permian (district of Perm in Russia)||48||Lower Permian|
|Large reptiles, amphibians; most species become extinct|
This era began 251 million years ago and lasted about 186 million years. The name was compounded from Greek mesos (middle) and zoön (animal). Popular name: Age of Reptiles.
|Triassic (trias = triad)||51||Lower Triassic|
|Early dinosaurs, crocodiles, turtles; first mammals|
|Jurassic (Jura Mountains)||54||Lower Jurassic|
|Many seagoing reptiles; early large dinosaurs; later, flying reptiles (pterosaurs), earliest known birds|
|Cretaceous (Latin creta = chalk)||80||Lower Cretaceous|
|Dinosaurs and other reptiles dominate; seed-bearing plants appear|
This era began 66 million years ago and includes the geological present. The name was compounded from Greek kainos (new) and zoön (animal). Popular name: Age of Mammals.
|Paleogene (Greek palaios = old + genes = born)||42||Paleocene (Greek palaios = old + kainos = new). Eocene (Greek eos = dawn). Oligocene (Greek oligos = few).||Rich insect fauna, early bats, increasingly diverse varieties of mammals and birds|
|Neogene (Greek neo = new + genes = born)||23||Miocene (Greek meios = less + kainos = new). Pliocene (Greek pleios = more). Pleistocene (Greek pleistos = most) (popular name: Ice Age). Holocene (Greek holos = entire), the last 10,000 years to the present.||Further development of mammals and birds. Various forms of humans, including Homo sapiens|
See also Geologic Timescale.
|The Periodic Table||General Science||Rocks|