Here’s a really concise discussion by Stephen Meyer, author of Darwin’s Doubt, of the two aspects that together make up the mystery of the Cambrian explosion. First there’s the problem of the missing fossils — the evident fact that the Cambrian fauna, as reflected in the fossil record, spring to existence without evolutionary forebears. Second, there’s the engineering problem, the origin of the new genetic information needed to build new animal forms, that advances in molecular biology made particularly striking in the second half of the 20th century. Enjoy.