The Cambrian Explosion

Katie Dombrauckas


The Cambrian Explosion refers to the rapid appearance of the major groups of complex multi-cellar organismsthat appeared in the fossil record dating back to the beginning of the Cambrian period. Although this "explosion" of phyla took only 5 to 10 million years, it is considered a relatively short period of time in comparison to the lengthy geological time scale. Single or undifferentiated multi-cellular organisms existed for approximately 3 billion years or 80% of the history of life until complex multi-cellular life appeared and rapid diversification of organisms began around 550 million years ago. (University of Bristol). It is believed that the complex multi-cellular organismsthat originated from this biological explosion are the ancestors that have evolved into present day life. Evidence of this rapid diversification is captured in the fossil record; however, much debate continues as to if ancestors of theorganismshad already existed and began to evolve prior to the Cambrian and, therefore, as a result of the lack of preservation of theanimals in the fossil record, the evidence of their existence is simply not available. In addition to exploring the "explosion" of organisms, a further examination of possible causes of this increase in diversification will be developed.

The fossil record is the sole source of understanding and interpreting the diversification of historical phyla; however, many animals may not have been preserved because of their soft-bodied parts or because their environment did not support fossilization. Soft-bodied animals were less likely to be preserved; however, even those animals with hard parts may not have been preserved because the surrounding environment may have dissolved their hard parts. Although some phyla may not have been preserved, it does not necessarily mean life did not flourish in the Precambrian. In fact, evidence of life before the Cambrian explosion is supported by trace fossils as soft-bodied animals left trails through Precambrian mud (PBS).

There are three groups of fossil records available that shed light onto understanding the Cambrian explosion of multi-cellular organisms: the Ediacara, Small Shelly Fossils, and Burgess Shale Type Faunas. The Ediacara was a soft-bodied fauna that exploded in population worldwide in the late Precambrian; however, before the Cambrian explosion the phyla had completely disappeared as a result of a "failed experiment" in multi-cellular design, but an opposing view believes that the Ediacara was an ancestor of future phyla, such as Arthropods (University of Bristol). The preservation of this soft-bodied organism is believed to have occurred because of the lack of scavengers or burrowing organisms.

If the Ediacara is disregarded as the first multi-cellular organism to exist at the beginning of the Cambrian explosion, then the Small Shelly Fossils can become the first explosion of hard body organisms. The organisms consist of tiny plates, cones and tubes made from various minerals; however, just as the Ediacara quickly disappeared so did the Small Shelly Fossils, leaving no lineage to future phyla (University of Bristol).

The Burgess Shale best represents the magnitude of species diversification during the Cambrian explosion. The Burgess Shale was laid down during the middle Cambrian around 530 million years ago as organisms were rapidly buried in mud or sediment creating an environment that supported exception fossil preservation of not only hard-bodied organisms, but soft-bodied organisms as well. The rapid burial of the animal created 3 dimensional impressions and reduced oxygen levels causing a lack of decay allowing for carbonization to preserve the organic soft tissue of the organism. This exceptional preservation can be seen in many localities worldwide, and this formation first introduced the brachiopods, trilobites, mollusks, and echinoderms. Although a varietyof new phyla had been uncovered, the abundance of soft-bodied organisms far out numbered the hard-bodied organisms even though preservation of soft-bodied organisms is unique occurrence, therefore, it can be implied that the Burgess Shale faunas still only represented a fraction of the living organisms that flourished during the Cambrian (University of California, Berkeley).

Evidence exists in the fossil records of a dramatic increase of complex multi-cellular life during the Cambrian period; however, there is much debate on what exactly caused this explosion of phyla. Although there is not a specific agreed upon explanation for the extreme rise in diversification, a combination of multiple triggers may allude to possible external and internal factors that resulted in this event. One possible environmental factor was the slowly increasing levels of oxygen that allowed animals to evolve larger complex body structures and create hard parts after a threshold value was crossed (University of Bristol). Another environmental factor for the creation of hard body parts could have been from chemical changes in the oceans or from the need for protection from predators. With the production of a hard external skeleton, the species best adapted could then continue to evolve.

In addition to environmental factors that have undoubtedly aided in the diversification of phyla, internal biological factors must have also contributed to the explosion of complex multi-cellular life in the Cambrian. A "genetic tool kit" evolved thatcontrolled developmental processes allowing for evolutionary experimentation (PBS). This genetic tool kit or "toolbox" assembled specific components of a given body plan allowing for rapid evolution (Morris, 1997).

Paleontology, "the study of ancient life" relies heavily on the available fossil record to shed light onto what exactly happened throughout the evolution of life, therefore, controversies are sure to arise as multiple reasonable hypotheses may contrast one another. The contrasting belief that the evolution of a variety of complex multi-cellular organisms began well before the Cambrian may less likely be proven due to the sole fact that there is minimal evidence from the fossil record of their existance. At times hypotheses can support one another by creating a broader understanding of events, such is the case that although the exact trigger of the Cambrian explosion is unknown, it is likely that the rapid diversification was a result of the combination of both biological and environmental evolutionary developments.

There is no dispute that "nearly all major animal phyla evolved during the Cambrians times…" (Starr, p. 202) or that, "Many new species have appeared since then. But nature drafted few, if any, new body plans after the Cambrian." (Levine, p. 76). It is amazing how such a huge evolutionary explosion could have occurred in "0.0001 % (one-hundredth of one percent) of the age of earth" (Starkey, p. 281). Although the cause of the explosion will continue to be debated, it is clear that 543 million years ago something was triggered that allowed organisms to evolve all the basic body plans of today’s life.









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