It is now 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 4,000 and 5,000 million years ago.
Life on Earth originated about 2,000 million years ago, but there are no good fossil remains from periods earlier than the Cambrian, which began about 550 million years ago. The largely unknown past before the Cambrian Period is referred to as the Pre-Cambrian and is subdivided into the Lower (or older) and Upper (or younger) Pre-Cambrian - also called the Archaeozoic and Proterozoic Eras.
The known geological history of the Earth since the beginning of the Cambrian Period is subdivided into three eras, each of which includes a number of periods. They, in turn, are subdivided into subperiods. In a subperiod, a certain section may be especially well known because of rich fossil finds. Such a section is called a formation, and it is usually identified by a place name.
This era began 550 million years ago and lasted for 355 million years. The name was compounded from Greek palaios (old) and zoon (animal).
(from Cambria, Latin name for Wales)
Lower Cambrian, Middle Cambrian, Upper Cambrian
Invertebrate sea life of many types, proliferating during this and the following period.
(from Latin Ordovices, people of early Britain)
Lower Ordovician, Upper Ordovician
(from Latin Silures, people of early Wales)
Lower Silurian, Upper Silurian
First known fishes; gigantic sea scorpions.
(from Devonshire in England)
Lower Devonian, Upper Devonian
Proliferation of fishes and other forms of sea life, land still largely lifeless.
(from Latin carbo = coal + fero = to bear)
Lower or Mississippian, Upper or Pennsylvanian
Period of maximum coal formation in swampy forests; early insects and first known amphibians.
(from district of Perm in Russia)
Lower Permian, Upper Permian
Early reptiles and mammals; earliest form of turtles
This era began 195 million years ago and lasted for 135 million years. The name was compounded from Greek mesos (middle) and zoon (animal). Popular name: Age of Reptiles.
(from trias = triad)
Lower or Buntsandstein (from German bunt = colourful + sandstein = sandstone). Middle or Muschelkalk (from German muschel = clam + kalk = limestone). Upper or keuper (old miner's term)
(from Jura Mountains)
Lower or Black Jurassic, or Lias (from French liais = hard stone), Middle or Brown Jurassic, or Dogger (old provincial English for ironstone), Upper or White Jurassic, or Malm (Middle English for sand)
Many sea-going reptiles; early large dinosaurs; somewhat later, flying reptile (pterosaurs), earliest known birds.
(from Latin creta = chalk)
Lower Cretaceous, Upper Cretaceous
Maximum development of dinosaurs; birds proliferating; opossum-like mammals.
This era began 60 million years ago and includes the geological present. The name was compounded from Greek kainos (new) and zoon (animal). Popular name: Age of Mammals.
(originally thought to be the third of only three periods)
Palecene (from Greek palaios = old + kainos = new). Eocene (from Greek eos = dawn + kainos = new). Oligocene (from Greek oligos = few + kainos = new). Miocene (from Greek meios = less + kainos = new). Pliocene (from Greek pleios = more + kainos = new)
First mammals other than marsupials. Formation of amber; rich insect fauna; early bats; steady increase of large mammals. Mammals closely resembling present types; protohumans.
(from Greek pleistos = most + kainos = new) (popular name: Ice Age)
Four major glaciations, named Gunz, Mindel, Riss, and Wurm originally the name of rivers. Last glaciation ended 10,000 to 15,000 years ago
Various forms of early man.
(from Greek holos = entire + kainos = new)
The last 3,000 years are called history.