The Cambrian "explosion"

By Branden Holmes (Surroundx) on November 28, 2010

“If modern zoology admits of anything approaching a full-blown origin myth, it is the Cambrian Explosion. The Cambrian is the first period of the Phanerozoic Eon, the last 545 million years, during which animal and plant life as we know it suddenly became manifest in fossils. Before the Cambrian, fossils were either tiny traces or enigmatic mysteries. From the Cambrian onwards, there has been a clamorous menagerie of multicultural life, more or less plausibly presaging our own. It is the suddenness with which multicultural fossils appear at the base of the Cambrian that prompts the metaphor of explosion.” (Richard Dawkins, The Ancestor’s Tale, pg.449) There were three great flourishings of life. The first was the origin of life itself. The second was the evolution of the eukaryotic cell. The third, the one for which we have palaeontological evidence, was the “Cambrian Explosion”. Within a geologically short period of 15 or 20 million years most of the current Phyla appear abruptly. Nobody knows exactly why this is, though of course many have taken guesses; some more educated than others. If you read the fossil record literally then it was in every sense an explosion. With no identified ancestors, the Cambrian fauna seems in every sense to have been an explosion of life forms.

However, it only seems to be an explosion because of a lack of identifiable ancestors. Though of course, they must have had them. We now know that spontaneous generation is impossible. Complexity must be built up, accumulated. Knowing that the Cambrian fauna must have had ancestors, the next question that arises is, why don’t we find them in the fossil record? As I alluded to earlier, there are a number of explanations being thrown around to explain this. These range from the sudden evolution of eyes, to the evolution of hard shells, through to the evolution of multicellularity. (I list these three because they are the best candidates. Though many other explanations have been proposed, most have no evidence to backup the claims of the authors of the hypotheses.). (paragraph). What makes the task even harder is the fact that there are only about 10 Cambrian period fossil sites in the world, including: Sirius Passet in Greenland, the Burgess Shale of British Columbia (immortalized by Stephen Jay Gould’s Book Wonderful Life, which prompted Simon Conway Morris, one of the three major re-studiers of the Burgess Shale fossils, to give an alternative view, which culminated in The Crucible of Creation), Chengjiang, China, and Orsten in Sweden. (paragraph). If the Cambrian explosion really coincided with the evolution of multicellularity, which allowed adaptive radiation on a scale never seen again, though it wouldn’t be conclusive, we should not find multi-celled organisms of any kind in the fossil record. Yet, we have sufficient fossils of multicelled life, and over such a great period of time (as we have just seen), that this notion is immediately disposed of. The only way in which this would not be fatal to the hypothesis is if multicellularity evolved multiple times. However, for this to be true, there had to be at least two separated unions of bacteria which independently evolved in eukaryotic cells. It is hard to quantify exactly how likely this is, and when it occurred. The latter of these variables is important in the context of divergence dates of the major groups of organisms from each other. But unfortunately for this hypothesis, the molecular evidence suggests a single origin for all multi-celled organisms.

Related articles:
Cambrian explosion, Evolution, phyla

About Branden Holmes
I am an amateur evolutionist interested in the theoretical side of the subject.

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