The Geological Timescale Chart

|    Era      |    Period      |   Epoch1    | Millions of years ago |
|             |                |  Holocene   |                   0.01|
|             |  Quaternary    |-------------|-----------------------|
|             |                |Pleistocene  |                    1.8|
|             |----------------|-------------|-----------------------|
|             |                |  Pilocene   |                    5.3|
|             |                |-------------|-----------------------|
| Cenozoic    |                |  Miocene    |                   23.7|
|             |                |-------------|-----------------------|
|             |   Tertiary     | Oligocene   |                   36.6|
|             |                |-------------|-----------------------|
|             |                |  Eocene     |                   57.8|
|             |                |-------------|-----------------------|
|             |                | Paleocene   |                   66.4|
|             |         Cretaceous           |                    144|
|             |------------------------------|-----------------------|
|   Mesozoic  |          Jurassic            |                    208|
|             |------------------------------|-----------------------|
|             |          Triassic            |                    245|
|             |          Permian             |                    286|
|             |------------------------------|-----------------------|
|             |       Pennsylvanian/         |                       |
|             |     Upper Carboniferous2     |                    320|
|             |------------------------------|-----------------------|
|             |       Mississippian/         |                       |
|             |     Lower Carboniferous      |                    360|
|  Paleozoic  |------------------------------|-----------------------|
|             |           Devonian           |                    417|
|             |------------------------------|-----------------------|
|             |           Silurian           |                    443|
|             |------------------------------|-----------------------|
|             |          Ordovician          |                    495|
|             |------------------------------|-----------------------|
|             |           Cambrian           |                    544|
| Precambrian3|              N/A             |                   4600|


1: It's true that all periods are divided into epochs. However, with the exception of the Cenozoic, these are all referred to as "Early", "Middle", and "Late" sections of the period. I don't really see a need to subdivide them this way for our purposes here.
2: The period from 360 to 286 million years ago is called the Carboniferous in Europe and elsewhere (named after the coal beds found in that rock layer). American geologists instead split it into two periods, the Mississippian and the Pennsylvanian.
3: Technically speaking, the Precambrian (or Cryptozoic) is not an era, but an aeon. It's often treated as an era because, quite simply, it's easier considering how relatively little happened during that time (except, you know, the emergence of life as we know it). I've put it this way on the chart because making another column for aeons would needlessly consume space.

Brief Overview of Periods

Some of the events I'm listing here are also listed in the various small writeups written for each of the periods. For the sake of simplicity, I've consolidated them into this writeup.


  • Formation of the Earth (duh)
  • First lifeforms emerge around 3.5 billion1 years ago - unicellular, anaerobic, heterotropic prokaryotes.
  • Modern atmosphere and ocean form in mid- to late Precambrian
  • Two major glaciation periods (or "Ice Ages"). First is the Gowgonda glaciation in the Paleoproterozoic (c. 2500 m.y.a.), second is the Varangian in the Neoproterozoic (c. 600 m.y.a.). The Varangian Ice Age was indirectly responsible in part for the Cambrian Explosion (see below).
  • First multicellular lifeforms appear near the end of the Precambrian.

    1: Note that I am using the American definition of billion (109), not the British definition, which is equal to the American trillion (1012).



  • Rhodinia, the original supercontinent, breaks up.
  • The Cambrian Explosion. In a matter of 10 to 15 million years (very short in geological time - or evolutionary time for that matter), known lifeforms grow from about 15 families to 100 or so. The Varangian Ice Age may have contributed to this by causing the extinction of archaeocyathids (small plants which attached themselves to rocks or other lifeforms), opening up niches for new species. Other contributing factors: Increase of oxygen in the atmosphere (essential to multicellular lifeforms); near-extinction of stromatolites (another form of primitive plant life); possible rise in nutrient levels (there is currently little evidence for this hypothesis). Trilobites are arguably the dominant lifeform at this point.
  • The Ross Orogeny begins, creating the Antarctic mountain ranges.


  • Further adaptive radiations; by the Ordovician there are 400 families of fauna.
  • Brachiopods (which first appeared in the Cambrian) flourish. Appearance of crinoids, echinoids, bryozoans, others.
  • First signs of primitive vertebrate fish.
  • Ross Orogeny ends.
  • The end of the Ordovician marks the second-worst mass extinction in Earth's history (in terms of numbers of species). It is theorized that this came about due to mass glaciation.


  • Little change in animal life, though some new families and genera appear.
  • Plants begin to grow on dry land.


  • Fish become the dominant lifeforms. Many species of fish evolve.
  • First land vertebrates appear - primitive amphibians.
  • Insects and spiders also make their first appearance here.
  • There is relatively little orogenic activity during this period.


  • Amphibians and insects continue to develop. Primitive reptiles appear.
  • Coral, brachiopods, crinoids, and other aquatic invertebrates flourish.
  • Seed ferns and other primitive trees proliferate, creating huge forests which would later form the major coal beds.


  • The Appalachian/Hercynian orogeny takes full effect; Laurentia (essentially North America) and Baltica (essentially Western Europe) hit Gondwanaland (essentially everything else), forming the supercontinent of Pangaea.
  • First turtles appear.
  • First "mammal-like reptiles" (Synapsids), e.g. Dimetrodon.
  • Sea reptiles appear: Icthyosaurs and Plesiosaurs.
  • Primitive conifers become the dominant plant life.
  • The end of the Permian marks the worst mass extinction in Earth's history. As much as 95% of marine life died out, including trilobites, bryozoans, and many species of fish. Land animals suffered as well, especially amphibians. The causes of the Permian extinction are not fully understood, but it's generally believed that a major eustatic sealevel change (in this case, a major regression of the ocean) caused sudden climate changes that many species could not adapt to.



  • The beginning of the Age of Reptiles. Emergence of first dinosaurs: coelosaurs, prosauropods.
  • First appearance of crocodiles and alligators.
  • First pterosaurs.
  • Ammonites flourish as the dominant marine invertebrates.
  • Appearance of earliest known bird, Protoavis (note: whether Protoavis was an early bird, and even whether it was a real species, is still a matter of controversy).
  • Pangaea breaks apart into Laurasia and a new Gondwanaland.


  • Dinosaurs continue to develop: Sauropods, stegosaurs, carnosaurs, etc.
  • The largest known land animal in Earth's history appears: the sauropod Ultrasaurus. Based on fossil remains, this dinosaur is believed to have been as much as 60 feet tall, 100 feet long, and weighed 80 tons.
  • First mammals appear, in the form of early rodents.
  • The much better-established bird ancestor, Archaeopteryx, appears in this period.
  • Antarctica, Australia and the Indian subcontinent break away from Gondwanaland.


  • Even more new dinosaurs: Tyrannosaurs, anklyosaurs, ceratopsians, etc.
  • The first flowering plants (angiosperms) appear.
  • First "true" birds appear.,
  • K-T extinction occurs. Most reptile species die out, as do ammonites, most plankton, and many other species. The most common hypothesis concerning this extinction is that a meteorite or asteroid hit the Earth, wreaking havoc with the climate.
  • South America and Africa break apart; Africa collides with Laurasia.



  • Beginning of the Age of Mammals. With many of the ecological niches filled by the various dinosaurs and other reptiles gone, the mammals underwent a series of adaptive radiations producing a wide variety of new mammalian families. The two main groups were carnivores and hooved herbivores.
  • In the Eocene, many new mammals appear: rhinoceroses, horses, mastodons, etc.
  • Birds become more prolific.
  • In the Pilocene, one of our earliest hominid ancestors appears: Australopithecus.
  • Continental drift continues, until the continents look roughly as they do today.


  • Appearance of modern humans, homo sapiens sapiens.
  • The latest Ice Age covers much of the northern regions of the world with glacial ice until a few thousand years ago.
  • Most of the large mammalian species, such as the wooly mammoth and the giant sloth, die out after several thousand years.
  • Humans invent nuclear weapons, the internal combustion engine, and television, and are now well on their way to out-doing the Permian extinction. (I kid, I kid.)
    Please note: In the above summary, I'm trying to limit the number of events I list. I suppose I could write down every orogeny and the names of all the animal families that existed in a given period (at least until we get into Carboniferous or so), but I don't think that would be very effective.

    That said, I *am*, after all, working mostly off of my notes from Geology 102 and 104, so if you think I'm missing an important event (or that some of my above information is incorrect), please /msg me about it. Unless, of course, you're some wacky Young-Earther or something, in which case I direct you to read Hutton's Theory of the Earth. Granted, it's somewhat out-of-date, being from the early 18th century; but then, so are you. (It's also about a thousand pages long, so it'll keep you busy for awhile.)

    Thanks to Tiefling for pointing out my (now fixed) error in the British definition of 'billion'