There is an ongoing debate about the causes of the Cambrian Explosion. Scientists come to very different conclusions. This section of the Royal Ontario site discusses possible explanations. And another here: What Sparked he Cambrian Explosion? From the magazine Nature here is another article about What Caused the Cambrian Explosion.
The period before the Cambrian is called the Ediacaran. Here is an article about the Ediacaran. The animals in the Cambrian must have ancestors, but what were they? There are fossils in the Ediacaran that some believe are primitive animals; here is one possible first animal: Before the Cambrian Explosion. Scientist think that the ecosystem before the Cambrian was more complex than previously thought.