Age of the Earth


Just as a lark and as a little exercise in making HTML tables (and to make clear what one error was in that last post), I threw together this table of the geological time scale, taken from Mayr’s What Evolution Is. I come from that generation of biologists where we were required to memorize the timescale to this level of detail; I’m a bit rusty on the dates now (but these are pretty much the same as what I had to learn in the late 1970s), and I was just realizing that we don’t even mention this stuff in introductory biology anymore.

The Geological Timescale
Eon Era Period Epoch Age
Life forms
Phanerozoic Cenozoic Quaternary Holocene    
Tertiary Neogene Pliocene Earliest Homo
Paleogene Oligocene First apes
Eocene First whales
55.8 First horses
Mesozoic Cretaceous Late Extinction of dinosaurs
99.6 First placental mammals
Jurassic Late First birds
Triassic Late First mammals
228 First dinosaurs
Paleozoic Permian  
Carboniferous Pennsylvanian first mammal-like reptiles
Mississippian first reptiles
Devonian first amphibians
416 first insects
first land plants
Silurian first jawed fish
542 first shelled organisms
Proterozoic     first multicellular organisms
4000 first bacteria
Hadean   origin of life?
oldest rocks
4650 formation of the Earth
(from Mayr, What Evolution Is, 2001)
(revised to dates at Overview of Global Boundary Stratotype Sections and Points

Here’s something that I also find peculiarly interesting. One of the older textbooks I’ve got in my little office collection is an introductory biology text from 1939; take a look at the dates students had to learn then.

The Geological Timescale
circa 1939
Era Age
Cenozoic 3
Mesozoic 9
Paleozoic 18
Proterozoic 37
Archaeozoic 61
(from Walter, Biology of the Vertebrates, 1939)

There was a big change in our understanding of the age of the earth within my parents’ lifetimes. I trust it’s obvious what happened: nuclear physics and radiometric dating.


  1. says

    There was a big change in our understanding of the age of the earth within my parents’ lifetimes.

    Even in 1939 the age of the earth was thought to be greater than 6,000 years :)

  2. says

    Wow, as a younger reader (20s) I’ve never seen the age of the earth being anything but ~4.5 billion years. It’s hard to believe how quickly our understanding changes.

  3. says


    Mayr’s dates were a little off based on recent studies. You can get more details than most of you would want at, and a condensed pdf version at

    Also, the time scale used in that 1939 textbook was based on Lord Kelvin’s late 19th Century timescale, already considered monstrously wrong by many geologists by the 1930s. (Long story short: Lord Kelvin never came to grips with the significance of radioactive decay as a potential source for the heat inside the Earth, nor ever accepted radioactive dating). Already at the turn of the 20th Century geologists were beginning to demonstrate rocks that were billions of years old: far more than could be accomodated in Kelvin’s time scale.

    (That’s not to say that there weren’t folks in the scientific community who stuck with Kelvin in the early 20th Century, though).

    A good couple reviews of the story of the discovery of the age of the can be found in James Lawurence Powell’s Mysteries of Terra Firma: The Age and Evolution of the Earth (2001, Free Press) and Cherry Lewis’ The Dating Game: One Man’s Search for the Age of the Earth (Cambridge Univ. Press).

  4. says

    Oy. You realize, don’t you, that now I’m going to have to go through that big hairy table and tweak all the dates? Science is a cruel and demanding taskmistress.

  5. says

    I that chart from my high school biology text (early 80s) and having to memorize it for tests. I’m surprised they don’t teach it anymore. It struck me as sort of important at the time. Of course, I feel like my high school years happened during the Pliocene epoch …

  6. says

    Oh, and yes, it takes a while for re-evaluations of physics to work their way into biology textbooks. I’m sure there’s a long lead time where physicists and geologists have reworked ideas, and biologists are still teaching something 10 or 20 years behind the times.

  7. says

    Hey, I have to redo the dates on my webpages and handouts (esp. for Historical Geology) pretty much every year or so. If only they were stable, like phylogenetic relationships.

    Oh, wait. Yeah. :-)

  8. says

    When I started learning geology, the Cambrian began 600 my ago (that was the age given in my freshman historical geology text, copyright 1969). Lots of the ages have been adjusted in the past few decades, and the wingnuts point to that as evidence against evolution. But in those decades, an awful lot has been learned about the Earth, other planets, biology, evolution and much else; what have the Creationists learned in that same period?

  9. HP says

    Science is a cruel and demanding taskmistress.

    Obligatory Simpsons reference:

    “Ah, there’s nothing more exciting than science. You get all the fun of sitting still, being quiet, writing down numbers, paying attention…. Science has it all.” [cite]

  10. says

    “Science is a cruel and demanding taskmistress.”

    The laws of science be a harsh mistress.
    — Bender Unit 22

  11. David Haasl says

    Something odd about that 1939 timescale. I would have thought that in 1939 – the ages would have been older than those you’ve listed. I know that Charles Lyell (19th cent) had an estimate for the base of Cenozoic that wasn’t too far off from our present estimate.

  12. Flounder says

    I have wondered if creationist deny that nuclear weapons exist. The same nuclear physics that allow radiometric dating allow for the bomb. In his excellent and easy reading book: ‘Age of the Earth’ Dr. Dalrymple notes that in radiometric dating, errors like Pb contamination from another source always give you younger dates than what is possible and older dates are impossible. The creationists bring up the Argon dating of Mt. St. Helens as an example that this dating doesn’t work. Well the argon dating of Mt. St Helens gives you a date of 2 Ma or so (sorry, writing this from memory). Well you had rocks of today that travelled through some that are 10 Ma old or so. You acquire some argon contamination from the older rock and you end with an age somewhere between the oldest and the youngest rock, not some weird date like 200 Ma. Of course it is easily explained but it doesn’t stop the crazies from lying about it in their rants.

  13. Phobos says

    what have the Creationists learned in that same period?

    New ways to present the same arguments in order to appeal to a broader audience and to test the courts.

  14. quork says

    A year or two ago I read a book dating from the 1890s, and by then it was clear that the age of the Earth was known to be unBiblically ancient, even before the introduction of radioisotope dating. I recall mention of 500,000 years, and this was based on how long it would take to lay down certain known alluvial deposits or some such.

  15. Scott Hatfield says

    Dawkins, in his book ‘The Ancestor’s Tale’ (2004), implies the terms ‘Tertiary’ and ‘Quaternary’ are outdated (footnote, pg. 17), but this hasn’t penetrated my high school biology text and I haven’t seen this anywhere else. Can anyone here tell me more, or shoot me a source that could tell me more?

    (PS….PZ, MY high school students still have to learn divisions of the Paleozoic and Mesozoic, even if they aren’t in the precious state standards (sigh). “Cows On Seesaws Don’t Change Places”…!


  16. says

    What fortuitous timing on this entry. A few weeks ago, I took my nephew and burgeoning science nerd to The Field Museusm, specifically to the Evolving Planet exhibit. Yesterday, I received a polite thank you note from him. Sort of:

    Hi Aunt [CMD]!

    I’ve wrote this letter for two reasons. One, I wanted to thank you for taking me to the museum of natural history! It was very fun! Two I wanted to argue against [two scratch outs] Paeleontologists (sic). I watched a video made in 2003. It said that big Theropods never lived at the same time as big Sauropods. for the exeption (sic) of theropod Giganotosauras and sauropod Argentinosuarus. But, Allosaurus is a big Theropod right? It’s almost as big as T-rex. And Allosaurus lived long massive sauropods such as Apatosaurus, Brachiosaurus, and [scratch out, but please note that he embraces the serial comma, bless his buttons] Barosauras!

    P.S. the grey numbers on the timeline [he includes a hand-drawn, color-coded timeline showing the ranges during which Allosauras, Brachiosauras, Apatosauras, and Barosauras lived] mean million years ago.

    Quibbling about geological time at the tender age of 10. I couldn’t be prouder.

  17. thwaite says

    Wikipedia has good tables (in color with pics!) of time scales used in
    geology and evolution . Dunno how these compare to’s, which sound definitive.

  18. says


    In fact, giant theropods (in particular carcharodontosaurids) and giant sauropods actually DO co-occur, particularly in the mid-Cretaceous. Mapusaurus and Argentinosaurus; Carcharodontosaurus and Paralititan; Acrocanthosaurus and Sauroposeidon. For that matter, Giganotosaurus itself is a bit too old to have attacked Argentinosaurus: reports that both were found together were based on specimens now called Mapusaurus. Even if it it didn’t live among the 70+ ton giants (so far as we know), Giganotosaurus still had a number of 10-40 ton sauropod species to eat.

    As for Allosaurus: it was big, but not quite Tyrannosaurus‘s class. Very large specimens of Allosaurus (sometimes considered a distinct dinosaur called Saurophaganax) are known, and do reach up to 3-4 tons, but typical Allosaurus specimens are in the 1-2 ton range.

  19. says

    Scott Hadfield,

    Yes, the Tertiary has indeed been formally abandoned by the International Commission of Stratigraphy, and the Quaternary was (but is back from the dead in new form: see below). In fact, non-US geologists have typically employed either a two period scheme (Paleogene for Paleocene – Oligocene; Neogene for Miocene – Holocene) or a three period scheme (Paleogene as above; Neogene for Miocene & Pliocene; Quaternary for Pleistocene and Holocene) for much of the 20th Century.

    The US Geological Survey, on the other hand, retained formal use of the Tertiary and Quaternary. (Incidentally, that’s where the “T” in “K/T” boundary comes from. Just as in chemical elements and astromomical charts, there is a formal symbolism for Periods on geologic maps. Cretaceous is formally “K” [damn Carboniferous go the “C”…], and so the boundary between the Cretaceous and Tertiary is the “K/T” boundary).

    The Quaternary story: the ICS recently reapproved the use of the Quaternary as a Sub-Era. However, breaking with a century and a half of tradition, they’ve recognized a time unit that doesn’t fall within a nested hiearchy. Specifically, the “new improved” Quaternary includes all the Holocene, all the Pleistocene, and the last Stage (but only the last stage!) of the Pliocene!

    (To translate this into a comparable case for the biological sciences: this is equivalent to creating a Linnean Order that contains two complete Families and one genus, but only one genus, from a third Family. While at the same time leaving that last genus within it’s third Family.)

  20. says

    “Cows On Seesaws Don’t Change Places”…!

    I’d love to know how that ends. When I was studying Geology, it was “Cows Often Sit Down Carefully; Perhaps Their Joints Creak. Persistant Early Oiling May Prevent Premature Rheumatism”. And it seems to have taken.

  21. says

    Hey…I teach a section of introductory biology at Southern Illinois University, and I make my undergraduates learn some of this! Well, I only require them to know the eras of the Phanerozoic and a couple of the major periods (Cambrian, Permian and Cretaceous), but they also have to know some of the important dates.

    I do, however, require my graduate students to learn all the periods and the dates but again, only for the Phanerozoic (I’m a zoologist…what do you expect?).

    Keep up the good work, BTW. How do you have time to keep up on all this stuff?

  22. NJ says

    For the Paleozoic and the Mesozoic periods:

    Can Oscar See Down My Pants Papa? Tom and Jerry Can

    For the Cenozoic epochs:

    Please Empty Our Mailbox Postman Please Hurry

  23. Bachalon says

    Prof. Myers, didn’t you post a few links that had pages that illustrated how little of this planet’s lifepsan humans have occupied?

  24. says

    I couldn’t believe the ages in that 1939 table, so I cracked my ancient copy of Longwell Knopf and Flint’s “Textbook of Geology” (Wiley 1932). They’ve got the table of eras and epochs, but none of these have dates assigned. Above the table they say, “Total time represented, 1,600,000,000+ years. Time since beginning of the Cambrian, 500,000,000 years. Time since beginning of Pleistocene epoch, 1,000,000 years.” That’s closer to what I expected. Today we say 4550 million, 542 million and 1.8 million years respectively. Goes to show that biology students of the thirties were behind the times–but at least everything was in the right order. That’s the important thing. Only quite recently was it first possible to calibrate evolution against the time scale well enough to ask good questions.

  25. Henry Holland says

    As a gay man, it’s nice to see that the first of my kind appeared about 1.8 million years ago. :-)

  26. llewelly says

    we don’t even mention this stuff in introductory biology anymore.

    Why not?

    Related question: Assume you were not taught the basic geological
    timescale until much later. How would that have affected your
    understanding of biology?

  27. bernarda says

    I haven’t read his biology textbooks, but doesn’t Ken Miller give the chronology in them? Apparently his textbooks are the most widely used in schools.

  28. says

    PZ is not giving biologists enough credit for not slavishly following ancient errors by physicists. At least, that’s how I inerpret “Oh, and yes, it takes a while for re-evaluations of physics to work their way into biology textbooks. I’m sure there’s a long lead time where physicists and geologists have reworked ideas, and biologists are still teaching something 10 or 20 years behind the times.”

    Biologists and geologists attacked Kelvin’s notion of the Earth’s age long before there was any radioactive dating or any notion of how (for instance) the Sun could burn for a very long time, because they preferred to believe the evidence of their own fields rather than the dicta of Lord Kelvin. As scientists so often must, they nursed a Micawberish hope that Something Would Turn Up.

    The 1939 date of that wrong timescale looked funny to me, because I learned the outline of the timescale maybe 15 years after that, and I don’t remember it as being hugely different from the modern one. So, out comes Man and the Vertebrates by A. S. Romer, copyright 1933 and 1937 (a first-year college text already getting old when I was in school). Time since the beginning of a few selected periods, in millions:
    Tertiary, 55
    Cretaceous, 120
    Triassic, 190
    Carboniferous, 300
    Cambrian, 550

    Not bad, really. I don’t know how they did it. But that circa-1939 text seems to have been badly, maybe perversely, out of date.

  29. says

    I hate to add another burden to the maintenance of the elegant HTML table, but nobody seems to have noticed that since 2004 we have a new geological period.

    The Ediacaran period covers some amount of time before the Cambrian, starting with the end of the Marinoan glaciation, whatever that is. It may not be easy to assign a length to it in the table; at least in ’04 the starting date was ill defined. See

    BTW I never thought of geological periods as having type specimens, just like species. But there it is in the Science article, complete with a photo, though in geo-talk it’s a Global Stratotype Section and Point (GSSP).

  30. Michael Hopkins says

    To support what Porlock Junior and others have said, I own a 1941 edition of Romers Man and the Vertebrates. Here is what is given, in millions of years, since the beginning of the…

    Quarternary – 1
    Tertiary – 55
    Cretaceous – 120
    Jurassic – 155
    Triassic – 190
    Permian – 215
    Carboniferous – 300
    Devonian – 350
    Silurian – 390
    Ordovician – 480
    Cambrian – 550
    Proterozoic – 925
    Archeozoic – 1,500?

    Other than the last date which is clearly marked with a question mark as uncertain, these dates are not that different from what are accepted today. Romer puts the K/T at 55 million years go instead of 65.5 million years in modern table. The Cambrian for Romer was 550 million years go while the modern table that PZ provides us has 542 million years. That might be a bit of luck since the Permian moved from 215 to 299. Romer says that his figures are based on a combination of thickness of rocks and the decomposition of radioactive minerals in the rocks.

  31. says


    Yep, there are indeed “type localities” for units of geologic time (or, more strictly speaking, for the chronostratigraphic units upon which the geochronologic units hang).

    Also, the Ediacaran is the most recently fully approved one, but there are a whole passle of new Periods in the works. From oldest to youngest, they are: (for the Paleoproterozoic Era) Siderian, Rhyacian, Orosirian, Statherian; (for the Mesoproterozoic Era) Calymmian, Ectasian, Stenian; (for Neoproterozoic Era) Tonian, Cryogenian, Ediacaran.

    Hope this helps,