It stands to reason that before any mass evolution spurt such as the Cambrian explosion there would have to be a mass extinction event such as a monster meteor impact to "clear the way" so to speak.
Do scientists have a theory on what spurred the Cambrian explostion?
Pre-Cambrian life was unicellular. There was no need for an extinction event as the ecological niches were unfilled.
Here's a link you might like Eons of Earth Geological History
I normally do not care for Wikipedia but it seems to be fairly accurate on the pure sciences.