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What Was The Cambrian Explosion?

Updated on September 8, 2012
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Reconstruction of the Southern Hemisphere c. 550 million years ago
Reconstruction of the Southern Hemisphere c. 550 million years ago | Source

The Cambrian Period in Earth's geologic history lasted from approximately 545 to 490 million years ago, the beginning and end points defined by two periods of mass extinction. Over the 45 million years making up the Cambrian period, animal life evolved rapidly - from simple bottom-dwelling crawlers and burrowers to very complex life forms with skeletons, shells, eyes, and body plans that appear to have been the precursors to all modern animal life. This geologically sudden appearance of diverse animal life is commonly referred to as the Cambrian Explosion.

Earth During the Cambrian Period

The planet Earth as it was half a billion years ago was a far different place than the planet we know today. The climate at the beginning of the Cambrian was cool, with polar ice caps and periods of glaciation. Toward the end of the Cambrian, the average atmospheric temperature of the planet had risen considerably to a balmy 21° C (70° F), fueled by a rise in atmospheric CO2 concentration to around 4,500 parts per million - more than 10 times current levels. Sea levels were much higher as well - perhaps 90 meters above current sea level.

Earth's geography was also quite different from what we know today. The southern hemisphere supercontinent Pannotia had recently broken up into Laurasia (present day North America), Siberia, Baltica (present-day Eastern Europe), and Gondwana (much of present day Africa, South America, Antarctica, Australia, and the Arabian Peninsula). The land masses themselves were likely barren and desertlike, with life on land consisting mainly of algae, lichens, and fungi. Life in the oceans, on the other hand, was flourishing and diversifying rapidly.

Reconstruction of what marrella may have looked like in the Cambrian period.
Reconstruction of what marrella may have looked like in the Cambrian period. | Source
Ogygopsis klotzi, 500 million year old trilobite from the Burgess Shale, Canada
Ogygopsis klotzi, 500 million year old trilobite from the Burgess Shale, Canada | Source
Burgess Shale wiwaxia fossil at the Smithsonian Museum of Natural History
Burgess Shale wiwaxia fossil at the Smithsonian Museum of Natural History | Source
Reconstruction of opabinia, based on Burgess Shale fossils
Reconstruction of opabinia, based on Burgess Shale fossils | Source

Major Fossil Finds

Fossils from more than a half-billion years ago are not easy to find. In a few locations around the world, however, paleontologists have found both trace fossils - preserved burrows and tracks made by ancient animals - and fossilized organisms that hint at what the Cambrian ecosystem would have been like.

The Burgess Shale, located in Yoho National Park in the Canadian Rocky Mountains, is one of the richest deposits of fossils in the world. Discovered in 1909 by Charles Walcott, then Secretary of the Smithsonian Institution, the site has yielded more than 65,000 fossil specimens from the Cambrian period. Many of these fossils are so well-preserved that they contain not only bones and shells, but traces of soft tissue such as muscles and internal organs. The fossils found at this site have been so critical to the study of Cambrian life that it has become a benchmark for other fossil sites around the world. Paleontologists refer to fossils found at similar sites as "Burgess Shale type fauna."

During the Cambrian period, the land that is now the Canadian Rockies was a continental shelf of Laurentia, located very close to the Equator. Frequent mudslides buried unlucky members of the local wildlife under thick layers of fine, clay-like sediment, often with very little oxygen. This low-oxygen environment allowed the structures of soft tissues to be preserved, turning into thin mineral layers before they could be eaten away by microbes. Over millions of years, these sediment deposits hardened into sedimentary rock, preserving a multi-layered record of Cambrian life.

Important fossils preserved in the Burgess Shale and similar deposits include:

  • Marrella - A soft-bodied arthropod-like creature unrelated to modern-day crustaceans. Marrella is the most-common fossil found in the Burgess Shale.
  • Trilobite - A three-lobed hard-shelled bottom-dweller that lived from 520 milion years ago until the end of the Permian 200 million years ago.
  • Opabinia - A five-eyed creature living near the sea floor, opabina had a long trunk-like proboscis that it may have used for feeding.
  • Wiwaxia - A small soft-bodied invertebrate, covered in tall spikes. Wiwaxia grew to a maximum of two inches long.
  • Anomalocaris - A large swimming predator with two giant compound eyes and long mouth appendages. Analysis of fossilized fecal matter suggests that anomalocaris fed on trilobites.

These and many other bizarre creatures appear in the fossil record for the first time during the Cambrian. Some of these display an evolutionary progression from the early fossil layers to later ones. Trilobites, for example, diversified greatly over time, with different species developing different shell thicknesses, eye size and placement, and number of body segments.

While the trilobites, wiwaxias, and other unusual creatures that existed during the Cambrian are no longer around today, this was an important era in the history of animal evolution. Of the phyla - major branches of the animal kingdom - that comprise all animals today, the vast majority make their first appearance in the fossil record during the Cambrian explosion. Moreover, it was during this period that the main body plans of modern animals - the relative placement of heads, limbs, and other parts - solidified into the standard configurations we see today.

To the question why we do not find rich fossiliferous deposits belonging to these assumed earliest periods prior to the Cambrian system, I can give no satisfactory answer. ... The case at present must remain inexplicable; and may be truly urged as a valid argument against the views here entertained.

Charles Darwin, On The Origin of Species

Cambrian Fossil Locations in North America

show route and directions
A markerBurgess Shale -
Yoho National Park, British Columbia, Canada
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B markerHouse Range -
Utah, USA
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C markerMarble Mountains -
California, USA
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D markerWhite-Inyo Mountains -
Death Valley National Park, California, USA
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Does The Cambrian Explosion Disprove Evolution?

Creationists have seized on the relatively rapid appearance of such a wide variety of animal phyla in the Cambrian fossil record as evidence against evolution by natural selection.While some have cited it as evidence for an intelligent designer, others have used it to argue for a literal Genesis interpretation involving rapid creation of all life on a 6,000 year-old Earth.

Even Charles Darwin himself - in a quote often mined by the creationism industry - felt that the abundance of Cambrian fossils, contrasted with the the dearth of pre-Cambrian fossils, presented an easily-exploitable gap in his theory. More recent scientific findings have filled this gap quite well, however, and provided a number of explanations for the rapid appearance of animals during the Cambrian explosion.

The first major flaw in the creationists' argument is that the time span, while rapid in geologic terms, is actually quite long in biological ones. Looking back from the present day to 45 million years in the past, we see in the fossil record the beginnings of the cetacean order that eventually became whales and dolphins, as well as the simian branch of primates that eventually became us, Quite a lot of evolution can happen in 45 million years.

Secondly, there have been animal fossils found since Darwin's time that pre-date the Cambrian. Though many of these are trace fossils - preserved burrows in the seafloor or animal tracks through fossilized microbial mats, for example - some fossilized creatures have been found that demonstrate animal diversification before the Cambrian. This physical evidence is also backed up by genetic evidence, and "molecular clock" studies of the genomes of current animal species have estimated that the split between invertebrates and vertebrates occurred more than 600 million years ago. If this is the case, the animal kingdom was already beginning to explode well before the Cambrian period began.

Finally, the rapid appearance of fossils during the Cambrian is primarily due to the way in which fossils are formed. Fossilization is a rare occurrence, only happening under specific geologic conditions that allow an expired creature to be preserved in layers of sedimentary rock. Preservation of soft tissue is even more rare - usually bones, shells, and exoskeletons are the only parts of an ancient animal that remain. Since the Cambrian period saw the development of these hard parts simultaneously in many different animal phyla, it stands to reason that the number of fossilized animals will increase as more animals evolve these fossilizable structures.

There have been a number of hypotheses proposed to explain the Cambrian explosion. These range from an excess of oxygen due to geologic processes to opening of an evolutionary bottleneck following the previous extinction period to an evolutionary "arms race" between predator and prey species. Whether the cause was one or more of these reasons, they are all well within the predictions of the Theory of Evolution.

The Cambrian explosion, far from being "Darwin's dilemma," is one of the best examples we have in the fossil record of evolution at work.

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    • somethgblue profile image

      somethgblue 5 years ago from Shelbyville, Tennessee

      Your map of the globe doesn't seem to match the current map of the globe . . . could a Polar Shift have occurred in our past?

      Oh that's right its called Continental Drift, must have been a strong current if you get my drift.

      How exactly would one prove these creatures lived over 500 million years ago, or is that just a guess?

    • scottcgruber profile image
      Author

      scottcgruber 5 years ago from USA

      Good questions, Blue!

      There is no evidence that a physical shift in Earth's poles has occurred in the past, other than the long-term precessional wobble and regular changes in axial tilt caused by interactions between the gravity of the sun and moon. The "Polar Shift" scenario you keep pushing in your hubs is unsupported by evidence.

      Continental drift has been demonstrated by geologic evidence - matching strata from multiple continents, for example, shows that separate land masses were once together. We have observed continental drift resulting from earthquakes today, with large earthquakes displacing islands and even parts of continents.

      As for proving that these creatures lived 500 million years ago, this was accomplished by radiometric dating of radioactive inclusions in igneous rock above and below strata that contain Cambrian fossils. This dating provides an upper and lower limit for when the creatures had to have lived.

    • scottcgruber profile image
      Author

      scottcgruber 5 years ago from USA

      After doing a bit more research, I will withdraw part of the first paragraph in my response. There is, in fact, some evidence that changes in the distribution of continents due to plate tectonics may also cause changes in the Earth's tilt by as much as 20 degrees, possibly more. These are very slow - about 1 degree per million years.

      One hypothesis that has been proposed is that such a shift occurred during the Cambrian, and that this shift is partially responsible for the flourishing of life during the period. So far paleogeomagnetic reconstructions from the Cambrian have not been conclusive in showing a drastic global shift in orientation - just changing orientations of continents and supercontinents.

    • cryptid profile image

      cryptid 5 years ago from Earth

      Great Hub! A lot of people don't realize how old the earth is, and think of the dinosaurs as the measuring stick of prehistory. There were millions of years before that, when all kinds of interesting things lived. Voted up and awesome!

    • scottcgruber profile image
      Author

      scottcgruber 5 years ago from USA

      cryptid: Very true. We also tend to forget just how long the dinosaurs reigned - 135 million years. By the time T. rex roamed the Earth, stegosaurs and other early dinosaurs were already fossils.

    • Teresa Coppens profile image

      Teresa Coppens 5 years ago from Ontario, Canada

      Well researched and written Scott. Despite this controversy seems to follow you and your hubs. You handle it with grace and a cool head. Extremely interesting as always!

    • nicomp profile image

      nicomp really 2 years ago from Ohio, USA

      "we see in the fossil record the beginnings of the cetacean order that eventually became whales and dolphins,"

      No, we don't.

    • somethgblue profile image

      somethgblue 2 years ago from Shelbyville, Tennessee

      Too elaborate on nicomp's comment, what I find so fascinating about the so-called 'Cambrian Explosion' is that for basically 4 Billion years (that's a lot of zero's) we only have simple celled bacteria, then suddenly 530 million years ago BOOM, baby every single phyla (body design) that exist today is created overnight ( in evolutionary terms).

      Hardly what I would call gradualism a term which Darwinist banter about like it's the gospel and since the Cambrian Explosion 530,000,000 years ago, NO NEW PHYLA, what happened to gradualism?

      Here is the thing Nicomp, I think was trying to say . . . where is the fossil record showing the gradualism between simple cell bacteria and the twenty-six new and different complex celled animals of the Cambrian Explosion? Could it be that they don't exist? Are missing links really just fairy tales that Darwinist and others like to tell their children?

      You'd have an easier time convincing me a mother-ship came down and dumped a bunch of pre-made animals on Gaia than explaining the missing link between simple cell bacteria and fully formed animals of the Cambrian Explosion.

    • Insane Mundane profile image

      Insane Mundane 2 years ago from Earth

      @Somethgblue: Yeah, I must have mentioned something about a divine substrate or something. Why people want to confuse adaptation and acclimation with magical speciation, sure beats me. Gaia is a freaking substrate in which the chemical and biological factors were already created to produce an ongoing diversity of life. Why people want to confuse it with Evolution theories simply relates to the fact that they can't admit that the substrate is divine to begin with. I asked a flying squirrel the other day what he thought about it and the poor freaky fellow said he had no idea who in the hell the tree and ground squirrels were related to nor why they were so handicapped for so many generations and still couldn't glide or fly. LOL! I never did meet that walking whale thingy, so the interview is still open for further questions...

    • nicomp profile image

      nicomp really 24 months ago from Ohio, USA

      "The Cambrian Period in Earth's geologic history lasted from approximately 545 to 490 million years ago, the beginning and end points defined by two periods of mass extinction. Over the 45 million ..."

      Isn't that 55 MY? 545 to 490 is 55.

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