The Cambrian Explosion

The Cambrian Explosion is a fitting term to describe the actual event, which was an enormous boom in the fossil record of complex organisms, even if it was originally dubbed this for the wrong reason. It was originally thought that this was a massive speciation event because there was no fossil record dating back before the Cambrian at Darwin’s time (Ridley 2004). We know now that the Cambrian Explosion marks the first appearance of animals that possessed hard body parts capable of preservation (Jones 2000). Prior to this adaptation animal remains were soft and fragile, thus decaying soon after the organism’s death. There are several hypotheses that attempt to explain this major step in evolution. Life biologists offer the emergence of predators, visually hunting predators especially, to explain the sudden appearance of hard parts as a means of defense (Ridley 2004). This is supported by direct evidence of predators’ gut contents, bite markings and jaw characteristics (M. 2009). Physical scientists look to an increase in oxygen levels or the glaciation of the Earth at that time to explain an increase in body size (S. 2009). It would seem that the combination of all the above could offer a more complete explanation and a lot of current research in biology, paleontology and molecular studies is focused on this period (Ridley 2004). No matter what caused the emergence of fossils, it is important to remember that the Cambrian Explosion was exactly that – an emergence of fossils, not specific animals.

The image is a drawing of Hallacigenia sparsa, which was an inhabitant of the Burgess Shale in British Columbia, Canada. The fossils were originally interpreted to have the animal walk on its spikes and later it was realized it was drawn upside down. This type of artistic interpretation, or misinterpretation, of the fossil record contributed to the pre-1940 belief of radical speciation in the Cambrian Explosion. With such bizarre looking animals people continued to ask how these characters could have evolved and could selection have been a plausible mechanism for their appearance (Jones 2000).

Literature Cited

Jones S. 2000. Darwin’s ghost: the origin of the species updated. London: Ballatine Publishing.

M. SC. 2009. Burgess shale. In: Ruse M, Travis J, editors. 2009. Evolution: The first four billion years. Cambridge: The Belknap Press of Harvard University Press.

Ridley M. 2004. Evolution, third edition. Malden: Blackwell Publishing.

S. JW. 2009. Organismic evolution and radiation before the Cambrian. In: Ruse M, Travis J, editors. 2009. Evolution: The first four billion years. Cambridge: The Belknap Press of Harvard University Press.