First off, thank you so much for putting together a forum such as this.  I've never been to college or even taken a biology class, but am absolutely fascinated by science and devour science media in my spare time (I'm a self-taught physics geek).  Recently I've found myself trying to explain the process of evolution to my evangelical girlfriend and her father, but in the process have realized there are quite a few things I'm not entirely sure of myself.  Understand that I am firmly in support of the Scientific Method vs any religious dogma, as one is based on required falsifiabilty and the other infalability.  And I've already burned through the common misconceptions creationists have about evolution that are most often based on ignorance to begin with.  But one area that I stumbled upon that I feel I may need to explain someday is something that I'm not entirely sure about myself and that's the Cambrian Explosion.

Where do Biologists (or maybe Paleontologists) stand on this issue?  There seem to be a wide variety of theories.  From what I gather the fossil record shows an abrupt spike in the rise of very diverse life roughly 500 million years ago and logic would lead me to believe that this can be explained by the a severe change in the Earth's environment that created a rich habitat for life to propogate.  From what I've been reading, Creationists use this as some kind of evidence of instantaneous divine creation, regardless of the "age" issue.

Hi Brando,

I hope that several answers will come your way, and that you can see how scientists debate with one another, until a consensus based on evidence can be reached.
The Cambrian explosion is a snapshot in the fossil record showing an amazing diversification in animal form and evolution.
Fossil embryos of animals  appear in rocks just older than the Cambrian (by as much as 40 Million years), but no adults have been been found that are definitely animals, but the embryos are very 'animal-like', and probably represent the first attempts for animals to evolve.
So, in answer to your question the Cambrian Explosion does seem to represent the appearance of all the animal groups (Phyla: arthropods, worms, molluscs, chordates -our group, echinoderms- sea urchins etc.)  we see today.  What could this mean?  Well maybe we just don't have the adults preserved in older rocks.  The mode of 'soft-body' preservation is only seen from just before the Cambrian and into the Ordovician (approxiamately from 580-450 Million years ago).  The beautiful fossils of the Cambrian showing worms, and other soft bodied forms, may be the result of an appearance of the animals in an 'explosion', but the explosion is something, which if true, occurred over several million years, so an explosion 'geologically' speaking, but  a very long 'bang' biologically.  Another possibility is that animals have a longer history, but they are not preserved, either, because they live in the water column, and so don't end up being covered by sediment after death, or, if they are covered the adults may not have had a body that is likely to be fossilised.  As I said, we only have a 'window' from the late Pre-Cambrian to the Ordovician when soft bodied fossils are commonly preserved.  So if you die before or after this window you probably won't preserve as a soft body fossil.
A further possibility is that the animals seen in the Cambrian are the first representatives of animals that have evolved skeletons.  We see a huge number of arthropods in the Cambrian rocks, and maybe the fact that we see so many of these types of animals is due to decay resistance, of their exoskeleton, and so the bodies remain long enough to begin the fossilisation process.  The evolution of the skeleton has been suggested as a means by which animals become large.  This was linked to the rise in atmospheric oxygen, meaning animals could become bigger, requiring a skeleton.  This leads to a fourth, and for me a final possibility ( for my answer at least, but by no means the final from other scientists), which I will give below.
So,  the possible explanations for the Cambrian Explosion are:

1. all animal phyla appear in the Cambrian
2. the Cambrian represents a time when taphonomy (the decay process) and palaeoecology/geology 'selects' for preservation of soft bodied organisms, not seen in older or younger rocks.
3. the evolution of skeletons, which resist decay, means that organisms are preserved as body fossils.
4. the increase in size of animals meant that they were no longer microscopic, and so 'seen' in the fossil record.