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The Geologic Time Scale

Eon Era Period Epoch Biology Geology Years Ago Duration
(millions of years)
Phanerozoic Cenozoic

"Age of Mammals"

Quaternary Period Holocene
Rise of humans, earliest writing in c. 3200 B.C., human-initiated extinction Much available fossil fuel is converted to atmospheric CO2 to power Industrial Revolution. .01 .01
Pleistocene Early human varieties & large mammals. The Ice Age. Glaciers such as N. America's Laurentide and Eurasia's Fenno-Scandia cover large areas. Continents have essentially reached their modern configurations. 2 2
Tertiary Pliocene First evidence of tool use 2.6 m.y.a.

Non-marsupial mammals form, including hoofed mammals, carnivorous mammals, and bats, variety of insects, trees (producing amber), man-like apes, modern grasses.

The island of India crashes into Asia, forming the Himalayan Mountains. 65 63

"Age of Reptiles"

Cretaceous Upper Dinosaurs at peak, flowering plants appear and provide food source for mammals and birds. Giant impact at Chicxulub, Mexico leads to mass extinction, eliminating dinosaurs.

Continents break apart from one another. Shallow oceans run through the centers of North America, Eurasia, Australia, and northern Africa.

135 70
Jurassic White Jurassic Huge ammonites, first birds and mammals, large dinosaurs, sea-going reptiles flourish. Pangea begins to break apart; Eurasia seperates entirely. 190 55
Brown Jurassic
Black Jurassic
Triassic Keuper Reptiles expand range, first dinosaurs appear. Moderate extinction occurs towards end of period.   225 35
Paleozoic Permian Upper Earliest mammal-like animals, conifers replace most coal-forests, reptiles enlarge & diversify. Asteroid impact ends Paleozoic Era in a massive extinction, destroying 80-90% of species.

Laurasia fuses to Gondwana, creating a single continent, Pangea.

280 55
Carboniferous Pennsylvanian Huge forests, insects proliferate, amphibian-like animals Various island continents merge with Old Red Sandstone continent, forming the new continent of Laurasia (north of Gondwana).

A large amount of organic material, which will gradually become fossil fuel, is buried.

345 65
Devonian Upper FIRST SIGNIFICANT LAND ANIMALS, first bony fishes, all fish proliferate, amphibian-like animals, first spiders & insects appear   395 50
Silurian Upper FIRST LAND PLANTS North America and Europe fuse to form the Old Red Sandstone Continent NW of Gondwana. 430 35
Ordovician Upper First vertebrates (these were fish-like organisms). Period ends in large extinction.   500 35
Cambrian Upper "Explosion" of life forms, multicellular life proliferates, first hard-shelled organisms (such as trilobites) proliferate. Land that will become Australia, India, Antarctica, Africa, and South America is joined in giant continent Gondwana. Other scattered continents lay to the north. 570 70
The time before the phanerozoic era is collectively called "Precambrian." It is not included on many geologic timescale charts because of the scarcity of life forms before the Cambrian Explosion.
Proterozoic Vendian   Eukaryotes evolve. In addition to unicelluar eukaryotes, multicellular eukaryotes appear. By the end of the Proterozoic Era, all four kingdoms of Eukaryotes had evolved: Protoctista (protists), Fungi (fungi), Plantae (plants), and Animalia (animals).

Atmospheric oxygen increases as a result of photosynthesis.

  620 50
Sturtian     800 180
Riphean   Nena breaks apart. The largest remaining continent is Rodinia. Massive ice sheets cover the continents.

Formation of supercontinent Nena, comprising all land except for Siberia.

1600 800
Animikean     2200 600
Huronian     2500 300
Archeozoic Randian   Prokaryotes evolve, including both eubacteria and archaebacteria (archea), the two subkingdoms of the bacteria kingdom.

The first unicellular LIFE EMERGES in Earth's waters.

  2800 300
Swazian   Largest continent, Ur, contains parts of what will be Africa, India, and Australia; other continents are seperate. 3500 700
Isuan     3800 300
Hadean   Solar system, including Earth, forms. Giant impact causes Moon to seperate from Earth. Oceans form. 4560 760
Times earlier than this are not subdivided into geologic eras. The earth formed with the rest of the solar system about 4.6 billion years ago. The Big Bang, the fantastic explosion in which space, time, the laws of physics, and existance itself first appeared, occurred about 13 billion years ago. The long timespan between these two events was marked by such events as the condensation of matter from subatomic particles into atoms and eventually into groupings as large as galaxies or larger, as well as the formation of stars and planets much older than the Sun and Earth.

Note: Sources differ as to the exact boundary dates between the various eons, eras, and periods, so view all numbers here as approximate. (This is because the opening and closing dates of many time units are tied to geological or biological events that have not been precisely and irrefutably dated.)

Time Almanac 2001, by Borgna Brunner ©2000 Information Please
World Almanac and Book of Facts 1997, by Robert Famighetti ©1996 K-III Reference Corporation
Fossils, by Frank H. T. Rhodes and Herbert S. Zim ©1962 Western Publishing Company
Geology, by Frank H. T. Rhodes ©1991 Western Publishing Company
Exhibitions at the Smithsonian Museum of Natural History
A Correlated History of Earth [poster], by Paul R. Janke ©2002 Pan Terra, Inc.

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