What Was The Cambrian Explosion?
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.
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
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.
Sources and Further Information
- Fossil Museum: Cambrian Explosion
Most major animal groups appear for the first time in the fossil record some 545 million years ago on the geological time scale in a relatively short period of time known as the Cambrian explosion.
- Peripatus: Cambrian Explosion
This page describes the phenomenon popularly known as the Cambrian Explosion. Apparent discrepancies between genetic calibration techniques and the fossil record are discussed.
- Mechanism for Burgess Shale-type preservation
Exceptionally preserved fossil biotas of the Burgess Shale and a handful of other similar Cambrian deposits provide rare but critical insights into the early diversification of animals.
- The Cambrian Period
The Cambrian Period marks an important point in the history of life on Earth; it is the time when most of the major groups of animals first appear in the fossil record.
- Cambrian Explosion: New timeline for appearances of skeletal animals in fossil record
Geologists at UC Santa Barbara and a team of co-authors have proposed a rethinking of the timeline of these early animal appearances.