18.12: Geologic Eras - Biology

18.12: Geologic Eras - Biology

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History of life as revealed by the fossil record

With help from molecular phylogenies:

ErasPeriodsEpochsAquatic LifeTerrestrial Life
With approximate starting dates in millions of years ago in parentheses. Geologic features in green
Cenozoic (66)
The "Age of
Quaternary (2.6)HoloceneHumans in the new world
PleistocenePeriodic glaciationFirst humans
Continental drift continues
Neogene (23)PlioceneAtmospheric oxygen reaches today's level (21%)Hominids
MioceneAdaptive radiation of birds, continued radiation of mammals
Paleogene (66)OligoceneAll modern groups present
Mesozoic (251)
The "Age
of Reptiles"
Cretaceous (146)Still attached: N. America & N. Europe; Australia & Antarctica; Mass extinction of both aquatic and terrestrial life at the end
Modern bony fishesExtinction of dinosaurs and pterosaurs; first snakes
Extinction of ammonites, plesiosaurs, ichthyosaursRise of angiosperms
Africa & S. America begin to drift apart
Jurassic (200)Plesiosaurs, ichthyosaurs abundant; first diatomsArchaeopteryx; dinosaurs dominant but mammals (Eutheria) begin to diversify
Ammonites again abundantFirst lizards
Skates, rays, and bony fishes abundantAdaptive radiation of dinosaurs
Pangaea splits into Laurasia and Gondwana; atmospheric oxygen drops to ~13%
Triassic (251)Mass extinctions at the end.Mass extinctions at the end.
First mammals
Adaptive radiation of reptiles: thecodonts, therapsids, turtles, crocodiles, first dinosaurs
Ammonites abundant at first
Rise of bony fishes
Paleozoic (542)Permian (299)Periodic glaciation and arid climate; atmospheric oxygen reaches ~30%. Volcanic eruptions killed off 90% of marine species at end.
Extinction of trilobitesReptiles abundant. Cycads, conifers, ginkgos
Pennsylvanian (320)Warm, humid climate
the Pennsylvanian
and Mississippian
make up the
also called the
"Age of Amphibians"
Ammonites, bony fishesFirst reptiles
Coal swamps
Mississippian (359)Adaptive radiation of sharksForests of lycopsids, sphenopsids, and seed ferns
Amphibians abundant
Adaptive radiation of the insects (Hexapoda)
Atmospheric oxygen begins to rise as organic matter is buried, not respired
Devonian (416)
The "Age of Fishes"
Extensive inland seasCartilaginous and bony fishes abundant. Ammonites, nautiloids, ostracoderms, eurypteridsFerns, lycopsids, and sphenopsids
First gymnosperms
First amphibians
Silurian (443)Mild climate; inland seasFirst bony fishesFirst myriapods and chelicerates
Ordovician (485)Mild climate, inland seasTrilobites abundantFungi present
First plants (liverworts?) First insects
Cambrian (541)First vertebrates (jawless fishes). Eurypterids, crustaceans
mollusks, echinoderms, sponges, cnidarians, annelids, and tunicates present. Trilobites dominant.
No fossils of terrestrial eukaryotes, but phylogenetic trees suggest that lichens, mosses, perhaps even vascular plants were present.
Periodic glaciation
Proterozoic (2500)Ediacaran
Fossil evidence of multicellular algae, fungi, and bilaterian invertebrates
Evidence of eukaryotes
~1.8 x109 years ago
Archaean (3600)Evidence of archaea and bacteria
~3.5 x109 years ago

The Geologic and Evolutionary Record

A remarkable feature of the table above is how often evolutionary changes coincided with geologic changes on the earth. But consider that changes in geology (e.g., mountain formation or lowering of the sea level) cause changes in climate, and together these alter the habitats available for life. Two types of geologic change seem to have had especially dramatic effects on life: continental drift and the impact of asteroids

Continental Drift

A body of evidence, both geological and biological, supports the conclusion that 200 million years ago, at the start of the Mesozoic era, all the continents were attached to one another in a single land mass, which has been named Pangaea. This drawing of Pangaea (adapted from data of R. S. Dietz and J. C. Holden) is based on a computer-generated fit of the continents as they would look if the sea level were lowered by 6000 feet (~1800 meters). During the Triassic, Pangaea began to break up, first into two major land masses:

  • Laurasia in the Northern Hemisphere
  • Gondwana in the Southern Hemisphere.

The present continents separated at intervals throughout the remainder of the Mesozoic and through the Cenozoic, eventually reaching the positions they have today. Let us examine some of the evidence.

Shape of the Continents

The east coast of South America and the west coast of Africa and are strikingly complementary. This is even more dramatic when one tries to fit the continents together using the boundaries of the continental slopes, e.g., 6000 feet (~1800 meters) down, rather than the shorelines.


  • In both mineral content and age, the rocks in a region on the east coast of Brazil match precisely those found in Ghana on the west coast of Africa.
  • The low mountain ranges and rock types in New England and eastern Canada appear to be continued in parts of Great Britain, France, and Scandinavia.
  • India and the southern part of Africa both show evidence of periodic glaciation during Paleozoic times (even though both are now close to the equator). The pattern of glacial deposits in the two regions not only match each other but also glacial deposits found in South America, Australia, and Antarctica.


  • Fossil reptiles found in South Africa are also found in Brazil and Argentina.
  • Fossil amphibians and reptiles found in Antarctica are also found in South Africa, India, and China.
  • Most of the marsupials alive today are confined to South America and Australia. But if these two continents were connected by Antarctica in the Mesozoic, one might expect to find fossil marsupials there. In March 1982, this prediction was fulfilled with the discovery in Antarctica of the remains of Polydolops, a 9-ft (2.7 m) marsupial.

The Impact Hypothesis

The Cretaceous period, the last period of the Mesozoic, marked the end of the Age of Reptiles. It was followed by the Cenozoic era, the Age of Mammals. Although extinctions have occurred throughout the history of life, an extraordinary number of them occurred in a relatively brief period at the end of the Cretaceous. Why?

The Alvarez Theory

Louis Alvarez, his son Walter, and their colleagues proposed that a giant asteroid or comet striking the earth some 66 million years ago caused the massive die-off at the end of the Cretaceous. Presumably, the impact generated so much dust and gases that skies were darkened all over the earth, photosynthesis declined, and worldwide temperatures dropped. The outcome was that as many as 75% of all species — including all dinosaurs — became extinct.

The key piece of evidence for the Alvarez hypothesis was the finding of thin deposits of clay containing the element iridium at the interface between the rocks of the Cretaceous and those of the Paleogene period (called the K-Pg boundary after the German word for Cretaceous). Iridium is a rare element on earth (although often discharged from volcanoes), but occurs in certain meteorites at concentrations thousands of times greater than in the earth's crust.

After languishing for many years, the Alvarez theory gained strong support from the discovery in the 1990s of the remains of a huge (180 km in diameter) crater in the Yucatan Peninsula that dated to 65 million years ago.

The abundance of sulfate-containing rock in the region suggests that the impact generated enormous amounts of sulfur dioxide (SO2), which later returned to earth as a bath of acid rain. A smaller crater in Iowa, formed at the same time, many have contributed to the devastation. Perhaps during this period the earth passed through a swarm of asteroids or a comet and the repeated impacts made the earth uninhabitable for so many creatures of the Mesozoic.

Other Impacts

A mass extinction of non-dinosaur reptiles occurred earlier, at the end of the Triassic. It was followed by a great expansion in the diversity of dinosaurs. The recent discovery of a layer enriched in iridium in rocks formed at the boundary between the Triassic and Jurassic suggests that impact from an asteroid or comet may have been responsible then just as it was at the K-Pg boundary.

The largest extinction of all time occurred still earlier at the end of the Permian period. There is evidence off the coast of Australia that a huge impact there may have contributed to the extinctions at the Permian-Triassic (P-T) boundary.

Geological Eras

Dominance of man. Domestication of animals and agriculture. Modern genera and species evolved. Last ice age 30-40 thousand years ago. Woolly mammoth extinct.

Mass extinction. Huge floods. Ice age. Many large mammals extinct. Mastodons and woolly mammoth extinct. Prehistoric man evolved. Cave paintings.

Dry climate. Oceans shrink. Mammals increase specialization. Mountains rise. First hominids appear. First orchids.

Ice age. First man-like apes. Evolution of apes, monkeys, horse, elephant. Radiation of grazing mammals. Huge grasslands. All grass subfamilies distinct.

Archaic mammals attain their maximum diversity. Creodonts (archaic carnivores) appear. First apes. Origin of grasslands.

Forests of monocotyledons and flowering plants appear. Ancestors of horse, camel, elephant appear. First bats.

Climate warm. Vegetation abounds. Ancestors of most modern mammals appear. Insectivores abundant. First grasses, Rhododendrons, whales and rodents.

Mass extinction. 60% of tetrapod families extinct. Himalayas, Andes, Alps arise. Dinosaurs and Ammonites extinct. First monocotyledons. First marsupials and placental mammals (Pantotheres). First flowering plants. Climate cool. Angiosperms radiate.

First bird, Archaeopteryx. Dominance of dinosaurs. Earliest mammals. Dicotyledons and conifers common. Continents become high. Origin of insect pollinators.

Massextinction. 80% of tetrapod families extinct. Continental drift begins. Arid conditions. Gymnosperms dominate. First dinosaurs.Mammal-like reptiles. First teleosts, first crocodiles and first flying reptiles.

Mass extinction. 70% of tetrapod families extinct. Single land mass, Pangaea and single ocean. Continents rise. Glaciations set in. Expansion of reptiles, origin of Cotylosauria and Therapsida. Last trilobites.

Warm and humid climate. Swamps abundant. First modern soils. First reptiles. Sharks abundant. First mammal-like reptiles. Earthworms.

Forests of ferns and gymnosperms. Foraminiferans and shell-crushing sharks abound. First winged insects. Radiation of amphibians. Little seasonal variations.

Mass extinction. Arid climate. First gymnosperm forests. First amphibians (Labyrinthodonts). First spiders. Dominance of fishes. First ferns. First vascular plants. First insects.

Algae dominate. Land plants definite. Trilobites decline. First scorpions and millipedes appear. First fishes, ostracoderms and placoderms appear.

Land submerged. Warm climate. Algae abound. Plants invade land. First corals. First vertebrates. Cephalopods and snails. First Agnatha.

Mass extinction. Mild climate. Marine algae. Many invertebrates. Trilobites. Brachiopods. Sponges. Molluscs. Explosion after mass extinction.


Primitive aquatic algae and fungi. Annelid burrows. Protozoa. Oxygenation of atmosphere. Prokaryote radiation. Skeleton of sponges.

Calcareous deposits by algae. Origin of life. Fossils of cyanobacteria.

Formation of Solar system. Strong solar wind. Formation of primitive atmosphere on earth.

The Impact Hypothesis

The Cretaceous period, the last period of the Mesozoic, marked the end of the Age of Reptiles. It was followed by the Cenozoic era, the Age of Mammals. Although extinctions have occurred throughout the history of life, an extraordinary number of them occurred in a relatively brief period at the end of the Cretaceous. Why?

The Alvarez Theory

Louis Alvarez, his son Walter, and their colleagues proposed that a giant asteroid or comet striking the earth some 66 million years ago caused the massive die-off at the end of the Cretaceous. Presumably, the impact generated so much dust and gases that skies were darkened all over the earth, photosynthesis declined, and worldwide temperatures dropped. The outcome was that as many as 75% of all species — including all dinosaurs — became extinct.

The key piece of evidence for the Alvarez hypothesis was the finding of thin deposits of clay containing the element iridium at the interface between the rocks of the Cretaceous and those of the Paleogene period (called the K-Pg boundary after the German word for Cretaceous). Iridium is a rare element on earth (although often discharged from volcanoes), but occurs in certain meteorites at concentrations thousands of times greater than in the earth's crust.

After languishing for many years, the Alvarez theory gained strong support from the discovery in the 1990s of the remains of a huge (180 km in diameter) crater in the Yucatan Peninsula that dated to 65 million years ago.

The abundance of sulfate-containing rock in the region suggests that the impact generated enormous amounts of sulfur dioxide (SO2), which later returned to earth as a bath of acid rain.

A smaller crater in Iowa, formed at the same time, many have contributed to the devastation. Perhaps during this period the earth passed through a swarm of asteroids or a comet and the repeated impacts made the earth uninhabitable for so many creatures of the Mesozoic.

Other Impacts?

A mass extinction of non-dinosaur reptiles occurred earlier, at the end of the Triassic. It was followed by a great expansion in the diversity of dinosaurs. The recent discovery of a layer enriched in iridium in rocks formed at the boundary between the Triassic and Jurassic suggests that impact from an asteroid or comet may have been responsible then just as it was at the K-Pg boundary.

The largest extinction of all time occurred still earlier at the end of the Permian period. There is evidence off the coast of Australia that a huge impact there may have contributed to the extinctions at the Permian-Triassic (P-T) boundary.

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18.12: Geologic Eras - Biology

Index fossils are another tool to determine the age of rock layers.

Index fossils are fossils of organisms that existed only during specific spans of time and lived in large geographic areas.

If an index fossil is found in a layer of rock, it means that the rock layer and other fossils found in that layer are the same age as the index fossil.

The geologic time scale organizes Earth’s history.

Eras last tens to hundreds of millions of years and have two or more periods.

Periods are the most commonly used units of time on the geologic time scale, and they last tens of millions of years.

Epochs are the smallest unit of geologic time, and they last several million years.

Cenozoic Era: 65 mya - present

Quaternary perIod: 1.8 mya - present

This period continues today and includes all modern forms of life.

Tertiary perIod: 65 - 1.8 mya

Mammals, flowering plants, grasslands, insects, fish, and birds diversified.

Mesozoic Era: 248 - 65 mya

  • Dinosaur populations peaked and then went extinct.
  • Birds survived to radiate in the Tertiary period.
  • Flowering plants arose.
  • Dinosaurs diversified, as did early trees that are common today.
  • Oceans were full of fish and squid.
  • First birds arose.
  • Following the largest mass extinction to date. d
  • Dinosaurs evolved, as did plants such as ferns and cycads.
  • Mammals and flying reptiles (pterosaurs) arose.

Paleozoic Era: 544 - 248 mya

  • PermIan perIod: 286–248 mya
    • Modern pine trees first appeared.
    • Pangaea supercontinent was formed as major landmasses joined together.
    • Coal-forming sediments were laid down in vast swamps.
    • Fish continued to diversify.
    • Life forms included amphibians, winged insects, early conifers, and small reptiles.
    • Fish diversified.
    • First sharks, amphibians, and insects appeared.
    • First trees and forests arose.

    Earliest land plants arose.

    Melting of glaciers allowed seas to form.

    505–440 mya Diverse marine invertebrates evolved, as did the earliest vertebrates.

    Massive glaciers formed, causing sea levels to drop and a mass extinction of marine life to occur.

    All existing animal phyla developed over a relatively short period of time known as the Cambrian Explosion.


    The next geologic eon, the Archean, began about 4 billion years ago. During this period, the cooling of the Earth's crust allowed for the formation of the first oceans and continents. Scientists are not exactly sure what these continents looked like since there is so little evidence from the period. However, some believe the first landmass on Earth was a supercontinent known as Ur. Others believe it was a supercontinent known as Vaalbara.

    Scientists believe that the first single-celled lifeforms developed during the Archean. These tiny microbes left their mark in layered rocks known as stromatolites, some of which are nearly 3.5 billion years old.

    Unlike the Hadean, the Archean eon is divided into eras: the Eoarchean, Paleoarchean, Mesoarchean, and Neoarchean. The Neoarchean, which began about 2.8 billion years ago, was the era in which oxygenic photosynthesis began. This process, performed by algae and other microorganisms, caused oxygen molecules in water to be released into the atmosphere. Prior to oxygenic photosynthesis, Earth's atmosphere had no free oxygen, a huge impediment to the evolution of life.

    18.12: Geologic Eras - Biology

    Teaching the geologic time scale has always posed a bit of a problem for me in my Biology classes. My students don't need the depth of knowledge that they might get in an Earth Science class. On the other hand, the concept of geologic time and the appearance and evolution of life on Earth is VERY important to my class. One of my most favorite sayings, "Nothing in Biology makes sense except in the light of evolution" ( Theodosius Dobzhansky , American Biology Teacher , 1973.) is a mantra in my classes. We can't teach cellular respiration without the concept of endosymbiosis, and we can't teach endosymbiosis until our students know the differences between prokaryotic and eukaryotic cells, etc, etc, etc. The history of life on Earth is (or should be!) woven into every single lesson we teach in a Biology class.

    Then "What's the problem?" you might be asking yourself. My problem is time. Not geologic time just time to teach. It is a real struggle to cover everything in a year that needs to be covered! My solution was to find a way to quickly cover the concepts of geologic time and the evolution of life on Earth, without taking weeks to do so.

    Here are my goals. I want my students to:

    • Know was is meant by the "geologic time scale."
    • Be able to visualize the enormity of geologic time.
    • Know when life first appeared on Earth.
    • Know the order in which various life forms appeared.
    • Know the importance of fossils, especially traditional fossils, to the study of evolution.
    • Understand how scientists are able to date fossils that are found in various rock strata.
    • Understand the relationship between mass extinctions and adaptive radiations.
    • Have a clear and concise understanding of what happens in each era of Earth's history.

    • Definition of the geologic time scale.
    • How the geologic time scale was developed by scientists.
    • Relative dating and Radioactive dating.
    • Earth’s history is divided into 4 Eras which are subdivided into smaller periods.
    • How to read the information on the geologic time scale reference table.
    • Comparing lengths of geologic time.
    • The order of events in the evolution of life on Earth.
    • Transitional fossils.
    • Estimating the age of organisms based on relative dating.
    • Rock strata.
  • Students complete a 6-page handout on the geologic time scale and complete a 2-page timeline of the history of life on Earth.
  • Students make a circle graph of the time spent in each era.
  • Students use the included Geologic Time Scale Reference Table to answer a series of 30 problem solving questions.
  • Students make a scale diagram showing the length of each era.
  • Students look at pictures to evaluate characteristics of certain organisms.
  • Students complete a relative dating cut and paste activity.
  • Students using relative dating to estimate the age of certain organisms.
  • Students complete a 2-page cut and paste timeline activity showing the evolution of life on Earth.
  • Students are guided through an exercise that allows them to compare all of Earth’s history to one calendar year.

  • You can find this activity in my TpT store by clicking this link , and here is what you can expect to find included:

    • 6-Page printable and editable student worksheet set
    • 1-Page Geologic Time Scale Reference Table
    • 2-Page Timeline Worksheet
    • 8-Page Teacher Guide and Answer Keys
    • All images needed for the "cut and paste" portions of the activity.
    • Paperless digital google apps version for use in Google Drive, Google Classroom, Microsoft OneDrive, or similar.

    I hope this article has given you something to think about, and some new ideas on how to teach geologic time to your biology or life science students. Have fun teaching!

    The Clock Of Eras And Geologic Time

    The Clock of Eras is a graphic aid to help us visualize geologic time. It is nearly impossible for the human mind to comprehend the amount of time that it has taken for the Earth to develop to its present state, yet we try to imagine each stage of its unfolding and the time that passed during each phase of development.

    Just a note here: The original Clock of Eras was developed about 100 years ago. Our understanding of geologic time has come a long way. To get to the point the only true eras on the clock now are the Paleozoic, Mesozoic, and Cenozoic. The Hadean, Archean, and Proterozoic are now termed Eons

    The Clock of Eras uses the analogy of a circular clock to represent the development of our planet in geologic time. One can see at a glance the relative time lengths of each major geologic era. So how does this Clock work? The Clock represents geologic time on the Earth since its birth to the present, from the initial events that brought about the formation up to now. Each hour represents approximately 383 million years on the first clock and 375 million years on the second.

    We can’t take credit for the idea of this Clock. It is a concept that was developed in Montessori Education. The colors as used in the Montessori clock relate to the location of the life that was present during the time. So the Paleozoic Era is blue because life was primarily in the seas, the Mesozoic is brown because life moved to the land, and the Cenozoic is green because of the fresh new life: the mammals.

    The Clock of Eras has been modified several times already and will continue to change over time as scientists learn more and more information through their research and discovery.

    The first clock is our most up to date, and now includes the Phanerozoic Eon. The scale of this clock is slightly different from the second.

    The lower clock is our previous version. It does not include the Phanerozoic Eon and some of the times listed for the Eons and Eras are just a bit out of date. The most significant change is the Hadean/Archean boundary. Scientists are finding new evidence of life farther back in time. The result is a shorter Hadean Eon and a longer Archean.

    Click on the links below the clock for a description of each time period.

    Geologic and Biological Timeline of the Earth

    Astronomical and geological evidence indicates that the Universe is approximately 13,820 million years old[42], and our solar system is about 4,567 million years old. Earth's Moon formed 4,450 million years ago, just 50 million years after the Earth's formation.

    Because the composition of the rocks retrieved from the Moon by the Apollo missions is very similar to rocks from the Earth, it is thought that the Moon formed as a result of a collision between the young Earth and a Mars-sized body, sometimes called Theia, which accreted at a Lagrangian point 60° ahead or behind the Earth. A cataclysmic meteorite bombardment (the Late Heavy Bombardment) of the Moon and the Earth 3,900 million years ago is thought to have been caused by impacts of planetesimals which were originally beyond the Earth, but whose orbits were destabilized by the migration of Jupiter and Saturn during the formation of the solar system. The Mars Reconnaissance Orbiter and the Mars Global Surveyor have found evidence that the Borealis basin in the northern hemisphere of Mars may have been created by a colossal impact with an object 2,000 kilometers in diameter at the time of the Late Heavy Bombardment.[20]

    Approximately 4,000 million years ago, the earth was cool enough for land masses to form. The supercontinent Rodinia was formed about 1100 million years ago, and it broke into several pieces that drifted apart 750 million years ago. Those pieces came back together about 600 million years ago, forming the Pan-African mountains in a new supercontinent called Pannotia. Pannotia started breaking up 550 million years ago to form Laurasia and Gondwana. Laurasia included what are now North America, Europe, Siberia, and Greenland. Gondwana included what is now India, Africa, South America, and Antarctica. Laurasia and Gondwana rejoined approximately 275 million years ago to form the supercontinent Pangea. The break up of Pangea, which still goes on today, has contributed to the formation of the Atlantic Ocean.

    (mya = million years ago)
    The times are approximate and may vary by a few million years.
    Precambrian Time
    (4567 to 542 mya)

    Hadean Eon (4567 to 4000 mya)
    - 4650 mya: Formation of chondrules in the Solar Nebula
    - 4567 mya: Formation of the Solar System
    Sun was only 70% as bright as today.
    - 4500 mya: Formation of the Earth.
    Formation of the Moon
    - 4450 mya: The Moon accretes from fragments
    of a collision between the Earth and a planetoid
    Moon's orbit is beyond 64,000 km from the Earth.[33]
    Earth day is 7 hours long[34]
    - Earth's original hydrogen and helium atmosphere
    escapes Earth's gravity.
    - 4455 mya: Tidal locking causes one side
    of the Moon to face the Earth permanently.[30]
    - 4280 mya: Water started condensing in liquid form.
    - 3900 mya: Cataclysmic meteorite bombardment.
    The Moon is 282,000 km from Earth.[34]
    Earth day is 14.4 hours long[34]
    - Earth's atmosphere becomes mostly
    carbon dioxide, water vapor,
    methane, and ammonia.
    - Formation of carbonate minerals starts
    reducing atmospheric carbon dioxide.
    - There is no geologic record for the Hadean Eon.

    Archean Eon (4000 to 2500 mya)
    Eoarchean Era (4000 to 3600 mya)

    - 4000 mya: The Earth's crust cooled and solidified.
    - Atmospheric pressure ranged from 100 to 10 bar.
    - Earth day is 15 hours long
    Paleoarchean Era (3600 to 3200 mya)
    Start of Plate Tectonics
    - 3600 mya: Formation of first supercontinent Vaalbara.
    - 3500 mya: Monocellular life started ( Prokaryotes ).
    First known oxygen-producing bacteria:

    cyanobacteria (blue-green algae) form stromatolites
    - Oldest unambiguous microfossils date from this era.
    Mesoarchean Era (3200 to 2800 mya)
    - 3000 mya: Atmosphere has 75% nitrogen,
    15% carbon dioxide.
    - Sun brightens to 80% of current level.
    - 2900 mya: Pongola glaciation occurred.
    Neoarchean Era (2800 to 2500 mya)
    - 2800 mya: Break up of supercontinent Vaalbara.
    - Oldest record of Earth's magnetic field.
    - 2700 mya: Supercontinent Kenorland formed.
    - Photosynthetic organisms proliferate.

    Proterozoic Eon (2500 to 542 mya)
    Paleoproterozoic Era (2500 to 1600 mya)
    Siderian Period (2500 to 2300 mya)
    - Stable continents first appeared.
    - 2500 mya: First free oxygen is found
    in the oceans and atmosphere.
    Banded Iron Formations
    - 2400 mya: Great Oxidation Event,
    also called the Oxygen Catastrophe.
    Oxidation precipitates dissolved iron
    creating banded iron formations.[14]
    Anaerobic organisms are poisoned by oxygen.
    - 2400 mya: Start of Huronian ice age
    Rhyacian Period (2300 to 2050 mya)
    - 2200 mya: Organisms with mitochondria
    capable of aerobic respiration appear.
    - 2100 mya: End of Huronian ice age
    Orosirian Period (2050 to 1800 mya)
    - Intensive orogeny (mountain development)
    - 2023 mya: Meteor impact, 300 km crater
    Vredefort, South Africa [9]
    - 2000 mya: Solar luminosity is 85% of current level.
    - Oxygen starts accumulating in the atmosphere
    - 1850 mya: Meteor impact, 250 km crater
    Sudbury, Ontario, Canada [9]
    Statherian Period (1800 to 1600 mya)
    - 1800 mya: Supercontinent Columbia (Nuna) formed.
    - Complex single-celled life appeared.
    - Abundant bacteria and archaeans.
    Mesoproterozoic Era (1600 to 1000 mya)
    Calymmian Period (1600 to 1400 mya)
    - Photosynthetic organisms continue to proliferate.
    - Oxygen builds up in the atmosphere above 10%.
    - Formation of ozone layer starts blocking
    ultraviolet radiation from the sun.
    - 1600 mya: Eukaryotic (nucleated) cells appear.
    Origin of ancestor of all animals, plants and fungi

    Ectasian Period (1400 to 1200 mya)
    - Green (Chlorobionta) and red (Rhodophyta) algae abound.
    Stenian Period (1200 to 1000 mya)
    - 1200 mya: Spore/gamete formation indicates
    origin of sexual reproduction.[36]
    - 1100 mya: Formation of the supercontinent Rodinia

    Neoproterozoic Era (1000 to 542 mya)
    Tonian Period (1000 to 850 mya)
    - 1000 mya: Multicellular organisms appear.
    - 950 mya: Start of Stuartian-Varangian ice age
    - 900 mya: Earth day is 18 hours long.
    The Moon is 350,000 km from Earth.[31]
    Cryogenian Period (850 to 630 mya)

    - 750 mya: Breakup of Rodinia
    - 650 mya: * Mass extinction of 70% of dominant sea plants
    due to global glaciation ("Snowball Earth" hypothesis).
    The Moon is 357,000 km from Earth.[31]
    Ediacaran (Vendian) Period (630 to 542 mya)
    - 600 mya: Formation of the supercontinent Pannotia
    Earth day is 20.7 hours long.[35]
    - 590 mya: Meteor impact, 90 km crater
    Acraman, South Australia

    - 580 mya: Soft-bodied organisms developed:
    Jellyfish, Tribrachidium, and Dickinsonia appeared.
    - 570 mya: End of Stuartian-Varangian ice age
    Shelled invertebrates appeared
    - 550 mya: Pannotia fragmented into Laurasia and Gondwana

    Phanerozoic Eon
    (542 mya to present)

    Paleozoic Era (542 to 251 mya)
    Cambrian Period (542 to 488.3 mya)
    - Abundance of multicellular life.
    - Most of the major groups of animals first appear
    Tommotian Stage (534 to 530 mya)
    - 510 mya: Vertebrates appeared in the ocean.
    Solar brightness was 6% less than today.
    Ordovician Period (488.3 to 443.7 mya)

    - diverse marine invertebrates, such as trilobites,
    became common
    - First green plants and fungi on land.
    - Fall in atmospheric carbon dioxide.
    - 450 mya: Start of Andean-Saharan ice age.
    - 443 mya: Glaciation of Gondwana.
    * Mass extinction of many marine invertebrates.
    Second largest mass extinction event.
    49% of genera of fauna disappeared.
    Silurian Period (443.7 to 416 mya)

    - 420 mya: End of Andean-Saharan ice age.
    - Stabilization of the earth's climate
    - Land plants and coral reefs appeared
    - First fish with jaws - sharks
    - Insects (spiders, centipedes), and plants appear on land
    Devonian Period (416 to 359.2 mya)

    - Ferns and seed-bearing plants (gymnosperms) appeared
    - Formation of the first forests
    - Earth day is

    21.8 hours long.
    - 400 mya: Land animals appeared, wingless insects
    - 376 mya: Viluy Traps Large Igneous Province volcanic eruption
    - 375 mya: Vertebrates with legs, such as Tiktaalik appeared.
    - Atmospheric oxygen level is about 16%

    - First amphibians appear.
    - 374 mya: * Mass extinction of 70% of marine species.
    This was a prolonged series of extinctions
    occurring over 20 million years.
    Evidence of anoxia in oceanic bottom waters,
    and global cooling. Surface temperatures dropped
    from about 93°F (34°C) to about 78°F (26°C)
    - 370 mya: First trees appeared
    - 359 mya: Meteor impact, 40 km crater
    Woodleigh, Australia
    Carboniferous Period (359.2 to 299 mya)
    Mississippian Epoch (359.2 to 318.1 mya)
    (Lower Carboniferous)
    - 350 mya: Beginning of Karoo ice age.

    - Large primitive trees develop
    - First winged insects
    - Forests consist of ferns, club mosses, horsetails, and gymnosperms.
    - Oxygen levels increase
    - 324 mya: Synapsid vertebrates, the ancestors of mammals,
    appear on land.
    - Seas covered parts of the continents
    - Animals laying amniote eggs appear (318 mya)
    Pennsylvanian Epoch (318.1 to 299 mya)
    (Upper Carboniferous)
    - 300 mya: First reptiles (diapsids) appeared.
    They were ancestors of crocodiles, dinosaurs and pterosaurs.
    - Atmospheric oxygen levels reach over 30%
    - Earth day is

    22.4 hours long.
    The Moon is 375,000 km from Earth.[31]

    - Giant arthropods populate the land
    - Transgression and regression of the seas
    caused by glaciation
    - Deposits of coal form in Europe, Asia,
    and North America
    Permian Period (299 to 251 mya)

    - 275 mya: Formation of the supercontinent Pangea

    - Conifers and cycads first appear
    - Earth is cold and dry

    - 167 mya: Meteor impact, 80 km crater
    Puchezh-Katunki, Russia [9]
    - 166 mya: Evolutionary split of monotremes from primitive mammals

    - 150 mya: First birds like Archaeopteryx appear
    - 148 mya: Evolutionary split between
    marsupial and eutherian mammals
    - 145 mya: Meteor impact, 70 km crater
    Morokweng, South Africa [9]
    Cretaceous Period (145.5 to 65.5 mya)
    - Period of Active Crust Plate Movements
    - 133 mya: Meteor impact, 55 km crater
    Tookoonooka, Australia [9]
    - 125 mya: Africa and India separate from Antarctica

    - Flowering plants (angiosperms) appeared
    Montsechia vidalli was one of the first angiosperms.
    - 120 mya: Global warming event starts
    Carbon dioxide levels were 550 to 590 ppm [27]
    - Proliferation of single-cell organisms (diatoms, dinoflagellates,
    and calcareous nannoplankton) changed ocean composition.
    - 116 mya: First birds with beaks without teeth.
    - 110 mya: Crocodiles appeared
    - Snakes evolved during the mid-Cretaceous
    - 105 mya: South America breaks away from Africa
    - Formation of the Atlantic Ocean

    Tertiary Period (65.5 to 2.58 mya)

    Paleocene Epoch (65.5 to 55.8 mya)
    - 63 mya: End of Deccan Traps volcanic eruptions in India
    - Flowering plants become widespread.
    - 70% of new bird lineages evolved within five million years
    after the extinction of the dinosaurs.
    - Social insects achieve ecological dominance.

    - Appearance of placental mammals
    (marsupials, insectivores, lemuroids, creodonts)
    - 60 mya: Earliest known ungulate (hoofed mammal)
    - The Laramide orogeny starts forming the Rocky Mountains
    and draining the Western Interior Seaway.
    - First appearance of grasses.[43]
    - 55.8 mya: Major global warming episode (PETM)[39]
    North Pole temperature averaged 23°C (73.4°F),
    CO2 concentration was 2000 ppm.
    Eocene Epoch (55.8 to 33.9 mya)
    - 50 mya: India meets Asia forming the Himalayas
    - 45 mya: Earth day is 24 hours long.
    The Moon is 378,000 km from Earth.[32]
    - Modern mammals appear
    rhinoceros, camels, early horses and
    lemur-like primates appear
    - 35.6 mya: Meteor impacts, 90 and 100 km craters
    Chesapeake Bay, Virginia, USA, and
    Popigai, Russia [9,10]
    - Global temperature dropped 10°C during the Eocene.
    - 34 mya: Global cooling creates
    permanent Antarctic ice sheet [21]
    Oligocene Epoch (33.9 to 23.03 mya)
    - 30 mya: Australia and South America separate from Antarctica

    - First elephants with trunks
    - 27.8 mya: La Garita, Colorado supervolcanic eruption

    Miocene Epoch (23.03 to 5.3 mya)
    - African-Arabian plate joined to Asia
    - 21 to 14 mya: Miocene warming period
    - 14 mya: Circum-polar ocean circulation builds up Antarctic ice cap.
    - 14 to 6 mya: Global temperature drops by 4°C.
    - First raccoons appear.
    - Drying of continental interiors
    - Forests give way to grasslands
    - 6 mya: Upright walking (bipedal) hominins appear
    Pliocene Epoch (5.3 to 2.58 mya)
    - 4.4 mya: Appearance of Ardipithecus, an early hominin genus.
    - 4 mya: North and South America join at the Isthmus of Panama.
    Animals and plants cross the new land bridge.
    Ocean currents change in the newly isolated Atlantic Ocean.

    - 3.9 mya: Appearance of Australopithecus, genus of hominids.
    - 3.7 mya: Australopithecus hominids inhabit Eastern and Northern Africa.
    - 3 mya: Formation of Arctic ice cap.
    - Accumulation of ice at the poles
    - Climate became cooler and drier.
    - Spread of grasslands and savannas
    - Rise of long-legged grazing animals
    Quaternary Period (2.58 mya to today)
    Pleistocene Epoch (2.58 mya to 11,400 yrs ago)
    - Several major episodes of global cooling, or glaciations

    - 2.4 mya: Homo habilis appeared
    - 2.1 mya: Yellowstone supervolcanic eruption
    - 2 mya: Tool-making humanoids emerge.
    Beginning of the Stone Age.
    - 1.7 mya: Homo erectus first moves out of Africa
    - 1.3 mya: Yellowstone supervolcanic eruption
    - 1.3 mya to 820,000 yrs ago: Sherwin Glaciation
    - Presence of large land mammals and birds
    - 790,000 yrs ago: First use of fire by hominds[40]
    - 700,000 yrs ago: Human and Neanderthal lineages start to diverge genetically.
    - 680,000 to 620,000 yrs ago: Günz/Nebraskan glacial period
    - 640,000 yrs ago: Yellowstone supervolcanic eruption
    - 530,000 yrs ago: Development of speech in Homo Heidelbergensis[15]
    - 455,000 to 300,000 yrs ago: Mindel/Kansan glacial period
    - 400,000 yrs ago: Hominids hunt with wooden spears
    and use stone cutting tools.
    - 370,000 yrs ago: Human ancestors and Neanderthals
    are fully separate populations.

    - 300,000 yrs ago: Hominids routinely use controlled fires
    - 230,000 yrs ago: Neanderthal man spreads through Europe
    - 200,000 to 130,000 yrs ago: Riss/Illinoian glacial period
    - 160,000 yrs ago: Homo sapiens appeared.
    Origin of human female lineage.[3]
    - 125,000 yrs ago: Eemian stage or Riss/Würm interglacial period.
    Hardwood forests grew above the Arctic Circle.
    Melting ice sheets increased sea level by 6 meters (20 feet)
    - 110,000 yrs ago: Start of Würm/Wisconsin glacial period
    - 105,000 yrs ago: Stone age humans forage for grass seeds such as sorghum.
    - 80,000 yrs ago: Non-African humans interbreed with Neanderthals[28]
    - 74,000 yrs ago: Toba volcanic eruption
    releases large volume of sulfur dioxide
    - 71,000 yrs ago: Invention of the bow and arrow.[41]
    - 70,000 yrs ago: Tahoe glacial maximum
    glaciers cover Canada and northern US.
    - 55,000 yrs ago: Humans colonize Australia
    - 46,000 yrs ago: Australia becomes arid,
    bush fires destroy habitat, and megafauna die off.

    - 40,000 yrs ago: Cro-Magnon man appeared in Europe.
    - 28,000 yrs ago: Neanderthals disappear from fossil record.[29]
    - 26,500 yrs ago: Taupo supervolcanic eruption in New Zealand.
    - 22,000 yrs ago: Tioga glacial maximum
    sea level was 130 meters lower than today.
    - 20,000 yrs ago: Invention of fired ceramic pottery.
    - 19,000 yrs ago: Antarctic sea ice starts melting.[22]
    - 15,000 yrs ago: Bering land bridge between Alaska and Siberia
    allows human migration to America.

    - 12,900 yrs ago: Comet impact by Great Lakes [23, 24, 25]
    Extinction of American megafauna such as the mammoth
    and sabretooth cat (Smilodon), as well as the end of Clovis culture
    - 12,000 yrs ago: Construction of Göbekli Tepe in Turkey.
    - 11,400 yrs ago: End of Würm/Wisconsin glacial period.
    Sea level rises by 91 meters (300 ft)
    Holocene Epoch (11,400 years ago to today)
    - Development of agriculture
    - Domestication of animals.
    - 9,000 yrs ago: Metal smelting started
    - 5,500 yrs ago: Invention of the wheel
    - 5,300 yrs ago: The Bronze Age
    - 5,000 yrs ago: Development of writing

    - 4,500 yrs ago: Pyramids of Giza
    - 3,300 yrs ago: The Iron Age
    - 2,240 yrs ago: Archimedes develops formula
    for the volume of a sphere.
    - yrs ago: Start of the Common Era (CE)
    - 1781 CE: James Watt patented a steam engine
    that powered the Industrial Revolution.
    - 1880 CE: Beginning of commercial use of electricity.
    - 1945 CE: Atomic age - First nuclear explosion
    - 1957 CE: Space age - Launch of Sputnik
    Man-made satellite orbits the Earth.

    - 1969 CE: Humans walk on the surface of the Moon.
    - 1969 CE: Start of the Internet global information network.

    Extinction Events
    There have been five major mass extinctions events: the terminal Ordovician (443 mya), Late Devonian (374 mya), terminal Permian called the "Great Dying" (251 mya), terminal Triassic (201), and terminal Cretaceous called the K/T event (65.5 mya). The Ordovician extinction was caused by global cooling and glaciation. The other four are associated with prolonged volcanism in large igneous provinces, although a meteorite impact also contributed to the K/T event.

    Humans as agents of environmental change
    Some scientists have tried to correlate the migration of humans to America with the extinction of the megafauna of the Pleistocene Epoch while others feel that weather changes brought about by the explosion of an asteroid or comet over North America might have been responsible.[25] There is no doubt that human activities can have a substantial impact on the environment and native species. The dodo, a flightless bird indigenous to Mauritius, became extinct in the late 17th century from massive hunting and the introduction of animals such as dogs, pigs, and cats. The Passenger Pigeon went from being the most common bird in North America to extinction by the end of the 19th century due to hunting and loss of habitat by deforestation. Overfishing the costal waters of California in 1945 produced 235,000 tons of fish, but in 1948 only 15,000 tons of fish were caught which led to the collapse of Cannery Row. The Dust Bowl was a man-made ecological disaster caused by deep plowing of the top soil of the Great Plains which destroyed native grasses whose roots had protected the soil from erosion. Drought and wind created a period of severe dust storms between 1930 and 1936. Soils that had been fertile became incapable of growing crops after the top soil was blown away. The contemporary destruction of tropical forests by the logging industry and the large-scale clearing of forests to plant commercial crops is already having harmful ecological effects that are likely to become worse if non-sustainable practices continue to be used.

    The Earth's near-term future
    Human industrial activity that relies on burning fossil fuels, such as coal and petroleum products, has been generating the greenhouse gases carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O), in large quantities since about 1750. The chart below shows the levels of atmospheric carbon dioxide during the last millennium and its sharp rise during the last century.[2] Atmospheric models predict that elevated greenhouse gases will cause global warming and influence weather patterns that will melt polar ice and destroy the habitat of animals such as the polar bear. The increase of global temperatures will also reduce the amount of snow deposited on mountains thus decreasing the flow of water in rivers which are now used for navigation, irrigation, and as sources of potable water. Carbon dioxide will also increase the acidity of sea water and threaten coral reefs and shell-building oceanic life forms.

    As of February 2016, the monthly average level of carbon dioxide was 404.02 ppm at the National Oceanic & Atmospheric Administration (NOAA) laboratory in Mauna Loa, Hawaii, and the level continues to increase steadily. In the following image, the dashed red line represents the monthly mean values of CO2 with the points centered on the middle of each month. The black line represents the same, after correction for the average seasonal cycle.

    Analysis of core sediments in the Arctic Circle indicate that 55 million years ago, the carbon dioxide concentration was 2,000 ppm and the North Pole's temperature averaged 23°C (73.4°F) compared to a mean annual temperature of -20°C today.[4] Satellite images by NASA show approximately a 20% reduction in the Earth's minimum ice cover between 1979 and 2003.[5] Arctic perennial sea ice has been decreasing at a rate of 9% every ten years. At this rate, the summertime Arctic Ocean will be ice-free before the year 2100.

    There is a large amount of water stored as ice over the landmasses of Greenland and Antarctica. If the ice sheets melt, the resulting rise in global sea level will flood many coastal areas around the world. The Greenland ice sheet contains enough water to increase the global sea level by 24 feet (7.3 meters), the West Antarctic ice sheet could raise sea level by 19 feet (5.8 meters), and the East Antarctic ice sheet could raise the sea level globally by 170 feet (51.8 meters).[12] The combined effect of melting all the ice on Greenland and Antarctica would result in a sea level rise of 213 feet (65 meters).

    Using computer models, scientists at the University of Arizona Department of Geosciences have created maps that show areas susceptible to rises in sea level (in red). The following map shows that a 6-meter (20-foot) rise would flood Miami, Fort Lauderdale, Tampa, and the entire Florida coastline, as well as parts of Orlando and other inland areas. Most of the city of New Orleans, Louisiana will disappear under water if the sea rises six meters. Some scientists have warned that by the year 2200, at the current rate of greenhouse gas emissions from human activities, the atmospheric levels of carbon dioxide, methane, and nitrous oxide will be at the same levels associated with mass-extinction events in the Earth's past.[8]

    The Earth's long-term future
    The future of the Earth is linked to the fate of the Sun. The Sun is halfway through its life cycle and will exhaust its supply of hydrogen fuel in around 4,000 million years. As the Sun cools, its core will collapse and its atmosphere will expand transforming the Sun into a red giant star. The swelling Sun will engulf the planets closest to it, and the Earth will be completely vaporized. The Sun will die in several stages. When its core crashes inwards, it will start fusing helium atoms into carbon. When the helium supply runs out, the center will collapse again and form a white dwarf star that will become dimmer until its light finally fades. The final collapse of stars which are a few times larger than the Sun ends in a massive supernova explosion that leaves behind a rapidly spinning neutron star.

    Long before the Sun becomes a white dwarf, 2,000 million years from now, our Milky Way Galaxy is predicted to collide with the Andromeda Galaxy.[13] The collision will take place for several million years and result in one combined super galaxy named Milkomeda. The sun may become part of the Andromeda system during the collision and could eventually end up far away from the new merged galactic center. The Earth may also eventually lose its Moon. Scientists using the laser ranging retroreflector positioned on the Moon in 1969 by the Apollo 11 astronauts have determined that the Moon is receding from Earth at a rate of about 3.8 centimeters per year.

    (my = millions of years)
    Earth's Long-Term Future
    +200 years: Possible global warming event caused by anthropogenic carbon dioxide (CO2)[8]
    +1500 my: Sun is about 6000 million years old and 15% brighter than today.
    +2000 my: Milky Way Galaxy starts colliding with Andromeda Galaxy.[13]
    +3000 my: Solar system becomes part of the new Milkomeda Galaxy.
    +4000 my: Sun is about twice as bright as today and its radius is 40% greater.
    Sun starts to exhaust its supply of hydrogen.
    +5000 my: Sun starts changing into a red giant star, 3 times its present size.[18]
    Earth is engulfed by the red giant Sun.
    +10000 my: Red giant Sun collapses and becomes a white dwarf.
    +20000 my: White dwarf Sun becomes a black dwarf.

    Age - An age is a unit of geological time shorter than an epoch, usually lasting several million years.

    Anthropocene - A proposed era denoting the time when human activity started having a global impact on the Earth's surface, atmosphere and hydrosphere.

    Archean, Archaean - An eon of geologic time extending from about 4000 to 2500 million years ago. Derived from the Greek archaios meaning "ancient". The Archean eon is divided into four eras: Eoarchean, Paleoarchean, Mesoarchean, and Neoarchean.

    Cambrian - The first period of the Paleozoic Era, during which most modern animal phyla developed. The name derives from Medieval Latin Cambria "Wales".

    Cenozoic, Caenozoic, Cainozoic - The current geologic era, which began 65.5 million years ago and continues to the present. The word comes from the Greek kainos "new" + zoe "life".

    Cretaceous - A Period from 145 to 65.5 million years ago divided into two epochs:
    The Early Cretaceous Epoch had six Ages: Cenomanian, Turonian, Coniacian, Santonian, Campanian, and Maastrichtian.
    The Late Cretaceous Epoch had six Ages: Berriasian, Valanginian, Hauterivian, Barremian, Aptian, and Albian.

    Eocene Epoch - An epoch from 54.8 to 33.9 million years ago with four Ages: Ypresian, Lutetian, Bartonian, and Priabonian.

    Eon - A primary division of geologic time lasting over 500 million years, four of which have been defined: Hadean, Archean, Proterozoic, and Phanerozoic. Eons are divided into Eras, which are in turn divided into Periods, Epochs and Ages.

    Epoch - A division of geologic time lasting tens of millions of years. Epochs are subdivisions of geologic periods.

    Era - A division of geologic time of several hundred million years in duration. An era is smaller than an eon and longer than a period.

    Geologic Time Scale - A categorization of geological events based on successively smaller time spans: eons, eras, periods, epochs, and ages.

    Hadean - The earliest eon in the history of the Earth from the first accretion of planetary material until the date of the oldest known rocks. The name "Hadean" derives from the Greek Hades "Hell".

    Holocene - An epoch starting 11,400 years ago to today. From holo- "whole" + Greek kainos "new".

    Jurassic - A Period from 200 to 145 million years ago divided into three epochs:
    The Early Jurassic Epoch has four Ages: Hettangian, Sinemurian, Pliensbachian, and Toarcian.
    The Middle Jurassic Epoch has four Ages: Aalenian, Bajocian, Bathonian, and Callovian.
    The Late Jurassic Epoch has three Ages: Oxfordian, Kimmeridgian, and Tithonian.

    Mesoproterozoic - an era with three periods: Calymmian, Ectasian, and Stenian.

    Mesozoic - An era of time during the Phanerozoic eon lasting from 251 million years ago to 65.5 million ago. Derived from the Greek mesos "middle" + zoe "life".

    Miocene Epoch - An epoch from 23.03 to 5.3 million years ago with six Ages: Aquitanian, Burgidalian, Langhian, Serravalian, Tortonian, and Messinaian. The name is derived from Greek meiōn "less" + kainos "new".

    Neogene - A period from 23.03 to today. This is the new name given to the time starting from the Miocene Epoch to today.

    Neoproterozoic - An era with three periods: Tonian, Cryogenian, and Ediacaran.

    Oligocene Epoch - An epoch from 33.9 to 23.03 million years ago with two Ages: Rupelian and Chattian. Derived from oligo- "few" + Greek kainos "new".

    Paleocene, Palaeocene Epoch - An epoch from 65.5 to 54.8 million years ago with three Ages: Danian, Selandian, and Thanetian.

    Paleogene - A period from 65.5 to 23.03 million years ago. This is the new name given to the first portion of the Tertiary Period.

    Paleoproterozoic - an era with four periods: Siderian, Rhyacian, Orosirian, and Statherian.

    Paleozoic, Palaeozoic - An era of geologic time lasting from 542 to 248 million years ago. Derived from the Greek palai "long ago, far back" + zoe "life".

    Period - A division of geologic time lasting tens of millions of years which shorter than an era and longer than an epoch.

    Phanerozoic - The most recent eon of geologic time beginning 542 million years ago and continuing to the present. Derived from the Greek phaneros "visible" + zoe "life".

    Pleistocene - An epoch from 2.58 mya to 11,400 years ago. Derived from Greek pleistos "most" + kainos "new".

    Pliocene - An epoch from 5.3 to 2.58 million years ago with two Ages: Zanclean and Piacenzian. Derived from Greek pleiōn "more" + kainos "new".

    Precambrian - Geologic time from the beginning of the earth to the beginning of the Cambrian Period of the Paleozoic Era.

    Proterozoic - The geologic eon lying between the Archean and Phanerozoic eons, beginning about 2500 and ending 542 million years ago. Derived from the Greek proteros "earlier" + zoe "life". The Proterozoic eon is divided into the Paleoproterozoic era, Mesoproterozoic era, and Neoproterozoic era.

    Quaternary - An informal sub-era from 2.58 or 1.8 mya to today. The Quaternary is traditionally associated with the Holocene and Pleistocene, but an alternative definition sets its start during the cycle of glacials and interglacials around 2.6 mya.

    Stage - A succession of rock strata laid down in a single age on the geologic timescale.

    Tertiary - An informal sub-era from 65.5 to 2.58 or 1.8 million years ago, depending on how the Quaternary is defined. The Tertiary overlaps with the Neogene Period and is divided into five epochs:
    Paleocene, Eocene, Oligocene, Miocene, and Pliocene.

    Triassic - A Period from 251 to 200 million years ago divided into three epochs:
    The Early Triassic Epoch has two Ages: Induan and Olenekian.
    The Middle Triassic Epoch has two Ages: Anisian and Ladinian.
    The Late Triassic Epoch has three Ages: Carnian, Norian, and Rhaetian.


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    Frequent misspellings of geologic terms and Evolutionary periods of the Earth:
    creataceous, cretaceus, cretacous, jurassique, jurasik, jurasic, jurossic, myscene, myocene, myoscene, phanaerozoic, triasic



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