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The first geologic time scale was proposed in 1913 by the British geologist Arthur Holmes (1890 - 1965). This was soon after the discovery of radioactivity, and using it, Holmes estimated that the Earth was about 4 billion years old - this was much greater than previously believed.

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For more information on continental drift and plate tectonics, click here.

EON	ERA	PERIOD		EPOCH	PIVOTAL EVENTS
P h a n e r o z o i c E o n "Visible Life" Organisms with skeletons or hard shells. 540 mya through today. P h a n e r o z o i c E o n "Visible Life" Organisms with skeletons or hard shells. 540 mya through today. P h a n e r o z o i c E o n "Visible Life" Organisms with skeletons or hard shells. 540 mya through today. P h a n e r o z o i c E o n "Visible Life" Organisms with skeletons or hard shells. 540 mya through today.	Cenozoic Era "The Age of Mammals" 65 mya through today	Quaternary Period "The Age of Man" 1.8 mya to today		Holocene 11,000 ya to today	Human civilization
				Pleistocene The Last Ice Age 1.8-.011 mya	The first humans (Homo sapiens) evolve. Mammoths, mastodons, saber-toothed cats, giant ground sloths, and other Pleistocene megafauna. A mass extinction of large mammals and many birds happened about 10,000 years ago, probably caused by the end of the last ice age.
		Tertiary Period 65 to 1.8 mya	Neogene 24-1.8 mya	Pliocene 5-1.8 mya	First hominids (australopithecines). Modern forms of whales. Megalodon swam the seas
				Miocene 24-5 mya	More mammals, including the horses, dogs and bears. Modern birds. South American monkeys, apes in southern Europe, Ramapithecus.
			Paleogene 65-24 mya	Oligocene 38-24 mya	Starts with a minor extinction (36 mya). Many new mammals (pigs, deer, cats, rhinos, tapirs appear). Grasses common.
				Eocene 54-38 mya	Mammals abound. Rodents appear. Primitive whales appear.
				Paleocene 65-54 mya	First large mammals and primitive primates, plesiadapiforms.
	Mesozoic Era "The Age of Reptiles" 248 to 65 mya	Cretaceous Period 146 to 65 mya		Upper 98-65 mya	High tectonic and volcanic activity. Primitive marsupials develop. Continents have a modern-day look. Minor extinction 82 mya. Ended with large extinction (the K-T extinction) of dinosaurs, pterosaurs, ammonites, about 50 percent of marine invertebrate species, etc., probably caused by asteroid impact or volcanism.
				Lower 146-98 mya	The heyday of the dinosaurs. The first crocodilians, and feathered dinosaurs appear. The earliest-known butterflies appear (about 130 million years ago) as well as the earliest-known snakes, ants, and bees. Minor extinctions at 144 and 120 mya.
		Jurassic Period 208 to 146 mya			Many dinosaurs, including the giant Sauropods. The first birds appear (Archaeopteryx). The first flowering plants evolve. Many ferns, cycads, gingkos, rushes, conifers, ammonites, and pterosaurs. Minor extinctions at 190 and 160 mya.
		Triassic Period 248 to 208 mya			The first dinosaurs, mammals, and crocodyloformes appear. Mollusks are the dominant invertebrate. Many reptiles, for example, turtles, ichthyosaurs. True flies appear. Triassic period ends with a minor extinction 213 mya (35% of all animal families die out, including labyrinthodont amphibians, conodonts, and all marine reptiles except ichthyosaurs). This allowed the dinosaurs to expand into many niches.
	Paleozoic Era 540 to 248 mya Paleozoic Era 540 to 248 mya	Permian Period "The Age of Amphibians" 280 to 248 mya			"The Age of Amphibians" - Amphibians and reptiles dominant. Gymnosperms dominant plant life.The continents merge into a single super-continent, Pangaea. Phytoplankton and plants oxygenate the Earth's atmosphere to close to modern levels. The first stoneflies, true bugs, beetles, and caddisflies, The Permian ended with largest mass extinction. Trilobites go extinct, as do 50% of all animal families, 95% of all marine species, and many trees, perhaps caused by glaciation or volcanism.
		Carboniferous Wide-spread coal swamps, foraminiferans, corals, bryozoans, brachiopods, blastoids, seed ferns, lycopsids, and other plants. Amphibians become more common. 360 to 280 mya	Pennsylvanian Period 325 to 280 mya		First reptiles. Many ferns. The first mayflies and cockroaches appear.
			Mississippian Period 360 to 325 mya		First winged insects.
		Devonian Period "The Age of Fishes" 408 to 360 mya			Fish and land plants become abundant and diverse. First tetrapods appear toward the end of the period. First amphibians appear. First sharks, bony fish, and ammonoids. Many coral reefs, brachiopods, crinoids. New insects, like springtails, appeared. Mass extinction (345 mya) wiped out 30% of all animal families) probably due to glaciation or meteorite impact.
		Silurian Period 438 to 408 mya			The first jawed fishes and uniramians (like insects, centipedes and millipedes) appeared during the Silurian (over 400 million years ago). First vascular plants (plants with water-conducting tissue as compared with non-vascular plants like mosses) appear on land (Cooksonia is the first known). High seas worldwide. Brachiopods, crinoids, corals.
		Ordovician Period 505 to 438 mya			Primitive plants appear on land. First corals. Primitive fishes, seaweed and fungi. Graptolites, bryozoans, gastropods, bivalves, and echinoids. High sea levels at first, global cooling and glaciation, and much volcanism. North America under shallow seas. Ends in huge extinction, due to glaciation.
		Cambrian Period "The Age of Trilobites" 540 to 500 mya			"Age of Trilobites" -The Cambrian Explosion of life occurs; all existent phyla develop. Many marine invertebrates (marine animals with mineralized shells: shell-fish, echinoderms, trilobites, brachiopods, mollusks, primitive graptolites). First vertebrates. Earliest primitive fish. Mild climate. The supercontinent Rodinia began to break into smaller continents (no correspondence to modern-day land masses). Mass extinction of trilobites and nautiloids at end of Cambrian (50% of all animal families went extinct), probably due to glaciation.
Proterozoic Eon 2.5 billion years ago to 540 mya	-	Vendian/Ediacaran Period 600 to 540 Million Years Ago			Vendian biota (Ediacaran fauna) multi-celled animals appear, including sponges. A mass extinction occurred. The continents had merged into a single supercontinent called Rodinia.
		-			First multicellular life: colonial algae and soft-bodied invertebrates appear. Oxygen build-up in the Mid-Proterozoic.
Archeozoic Eon (Archean) 3.9 to 2.5 billion years ago	-	-			"Ancient Life" - The first life forms evolve - one celled organisms. Blue-green algae, archaeans, and bacteria appear in the sea. This begins to free oxygen into the atmosphere.
Hadean Eon 4.6 to 3.9 billion years ago	-	-			"Rockless Eon" - The solidifying of the Earth's continental and oceanic crusts.

Good time scale links (and related topics):

• The Cladogram of Life.
• Definitions of the divisions of geologic time.
• The Tree of Life from the University of Arizona.
• University of California's Museum of Paleontology Web Lift for Geologic Time.
• Atlas of Paleo-vegetation from the Oak Ridge National Laboratory.
• Extinction information from the Univ. of Arizona.
• Time-lines from Talk.Origins Archive.

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