The “Cambrian explosion” refers to the initial appearance of the fossils of many major animal groups (“phyla”) within a period of about 10 million years, about half a billion years ago in the Cambrian period. If you consider only some of the relevant facts, this can appear as a challenge to the theory of evolution. If you consider more of the facts (e.g. known patterns of fossil preservation and discoveries of fossils immediately before the Cambrian explosion), this challenge fades considerably.
Stephen Meyer published Darwin’s Doubt in June, a heavily-researched and well-written book that powerfully presents part of the truth. Meyer argues that currently-understood natural causes cannot account for the Cambrian explosion, so we need to invoke the intervention of an Intelligent Agent.
This volume has called forth both attackers and defenders on the internet. I wrote up a description of this verbal jousting, and added a tutorial on fossils and the Cambrian period. This piece, “Cambrian Contention: Disputing Darwin’s Doubt,” got kind of long to put here as a regular post, so I placed it up at the top of the blog window along with other longer essays — it shows there as Darwins_Doubt. I hope some folks find it educational.