Geological Time Scale

The history of the earth has been divided into a geological time scale consisting of eons, eras, periods, epochs, and ages. Major periods generally correspond to significant changes in the earth's climate or fossil record (e.g. appearance or extinction of various life forms). The following table is a summary of times (mya = millions of years ago) and some significant events - not all subdivisions are listed. Time estimates are "start times" based on International Commission on Stratigraphy (ICS 2009) information - different sources vary somewhat for dates.

I. Hadean Eon			4570 mya	Formation of the earth. Age estimate based on asteroid and moon rock analysis and assuming the earth was formed about the same time.
II. Archean Eon
	Eoarchean Era		3800 mya	Appearance of prokaryotic (non-nucleic) cells - e.g. bacteria.
	Paleoarchean Era		3600 mya	First oxygen-producing bacteria and oldest definitive microfossils appear. Atmosphere is mostly carbon dioxide, water vapor, sulphur dioxide, and nitrogen. Small "continents" form and split due to movements from higher heat of earth.
	Mesoarchean Era		3200 mya	Photosynthetic cyanobacteria (stromatolites) appear. Landmasses grow in size forming continental cratons.
	Neoarchean Era		2800 mya	Continental landmasses formed from early rock including gneiss, granite, greenstone, komatiites, and various sedimentary rocks. Mineral deposits formed including copper, zinc, nickel, and gold.
III. Proterozoic Eon
	Paleoproterozoic Era		2500 mya	Landmasses begin to form relatively stable proto-continents. Appearance of eukaryotic (containing membrane-bound structures) single cells about 2000 mya. Photosynthetic activity increases atmosphere oxygen concentration. With increased oxygen, a UV blocking ozone layer begins to form. Quarz-rich sandstone, shale, and limestone (a photosynthetic product) were deposited on continents. Rock and mineral deposit formation included hematite (iron oxide), chromium, lead, uranium, and gold. About 1700 mya iron-containing rock oxidized by atmosphereic oxygen appears.
	Mesoproterozoic Era		1600 mya	Stromatolites and Protozoa (single celled animals) dominate.
	Neoproterozoic Era		1000 mya	Appearance of multi-celled eukaryotes - e.g. algae. Stromatolites begin to decline. The supercontinent Rodina, containing at least most of the earth's landmass forms and later breaks up (c. 750 mya).
			630 mya	Appearance of multicellular Ediacara (soft-body, wormlike animals). Landmasses recombine to form another supercontinent, Pannotia (c. 600 mya).
IV. Phanerozoic Eon
	A. Paleozoic Era
		1. Cambrian Period	542 mya	Appearance of various marine invertebrates (e.g. sponges, worms, 'moss animals'), animals with mineral skeletons ('armored jawless fish', trilobites, primitive mollusks), and starfish. Plants were only marine including seaweed. The supercontinent Pannotia splits forming the continents of Laurentia (pre-N. America), centered near the equator, Baltica (pre-northern Europe), and Siberia along with the supercontinent of Gondwana (including parts of present-day Africa, S. America, southern Europe, Middle East, India, Australia, and Anarctica).
		2. Ordovician Period	488 mya	Appearance of vertebrates (primitive fish), chordates, corals, clams and primitive land plants. Continental movements occur including an initial combining of Lauentia, Baltica, and Avalonia (a later small continent split-off from Gondwana) to form the supercontinent of Euramerica (or Laurussia). Volcanic eruptions and earthquakes create mountains (e.g. proto - Urals, Norwegian, Appalachians). Gondwana moves to the South Pole. Extinction of ~ 57 % of genera (~ 440 mya).
		3. Silurian Period	444 mya	Appearance of fish with jaws, coral reefs, air-breathing scorpions and vascular leafless land plants. The supercontinent of Euramerica further established (with Avalonia).
		4. Devonian Period	416 mya	Various forms of fish and mollusks evolve. Appearance of ammonites, sharks, lungfish, vertebrates (aquatic tetrapods), and wingless insects. Plants include mosses, horsetails, ferns and woody plants. Seed-ferns appear. Extinction of ~ 50 % of genera (~ 367 mya).
		5. Carboniferous Period
		Mississippian Subperiod	359 mya	Appearance of fern forests and land-living vertebrates (tetrapods). Climate was tropical forming thick peat layers which eventually became layers of sedimentary rock containing bands of coal. Siberia combines with Euramerica (Laurussia) to form the supercontinent of Laurasia.
		Pennsylvanian Subperiod	318 mya	Insects (including dragonflies) and amphibians flourish. Appearance of land snails, reptiles and seed plants (e.g. ginko trees, conifers). Laurasia and Gondwana move towards each other.
		6. Permian Period	299 mya	Reptiles spread and evolve. Laurasia and Gondwana (along with the other proto-continents) combine to form the supercontinent of Pangea. In the process, ancestral North America is pushed north off the equator and up against Siberia creating mountains (Ural, Appalachian, etc). Extinction of ~ 83 % of genera (~ 251 mya).
	B. Mesozoic Era
		1. Triassic Period	251 mya	Appearance of dinosaurs (including winged lizards) and earliest mammals. Ginkos, conifers, and palms (cycads) dominate. Extinction of ~ 48 % of genera (~ 205 mya).
		2. Jurassic Period	200 mya	Appearance of birds. Pangea starts to split between proto-North America and the adjacent lands of South America and Africa forming the initial stages of the Gulf of Mexico and the Atlantic. The split results in creating a reformed Laurasia continent in the north. The remaining "Gondwana" moves south of the equator and includes present-day Australia, Anarctica, India, Africa, and S. America. An eastern block (Eurasia) of Laurasia begins turning clockwise (south). A unit including Australia and Anarctica splits.
		3. Cretaceous Period	146 mya	Appearance of primates (e.g. marsupials), deciduous trees (e.g. oak, maple) and flowering plants. Also appearing are snakes and meat-eating crustaceans (shrimp, crab, lobster). North America splits off from Laurasia. The western movement creates the Rocky Mountains. India splits and joins Laurasia creating the proto-Himalayas. Africa and S. America separate. Extinction of ~ 50 % of genera including dinosaurs (~ 65 mya).
	C. Cenozoic Era
		1. Paleogene Period
		a. Paleocene Epoch	65 mya	Early mammals, primates, rodents, and insectivores evolve. Large herbivores and carnivores appear. Greenland separates from North America. Australia starts to separate from Anarctica.
		b. Eocene Epoch	55.8 mya	Appearance of bats, early horses, cattle, pigs, deer, monkeys, whales, early cats and dogs, modern birds, etc. First grasses appear. Gaps between Greenland / Europe and Australia / Anarctica widen.
		c. Oligocene Epoch	33.9 mya	Appearance of modern mammalian groups including early mastodons and apes. Grasslands expand. Himalayas and Alps are further uplifted
		2. Neogene Period
		a. Miocene Epoch	23.0 mya	Gradual cooling climate punctuated with ice ages. Mastodons and other mammals evolve. Mountain uplifting continues.
		b. Pliocene Epoch	5.3 mya	Grazing hoofed animals spread and diversify. Early humans appear in Africa. Grasslands dominate. Climate starts to become cooler.
		3. Quaternary Period
		a. Pleistocene Epoch	2.6 mya	Abundance of large mammals (e.g. buffalo, elephants, mammoths, mastodons). Humans evolve. Ice age begins. Lake Toba supervolcano erupts c. 71000 ya. Warming begins at the end of the epoch to melt glaciers (last maximum c. 30000 ya). Mammoths and mastodons become extinct.
		b. Holocene Epoch	11700 ya	Last glacial period ends introducing the current interglacial period. Appearance of human civilizations.
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