[Centurion2000]

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

Cmabrian explosion

I normally do not care for Wikipedia but it seems to be fairly accurate on the pure sciences.

[Virginia-American]

S2R: Do scientists have a theory on what spurred the Cambrian explostion?

C2k: Pre-Cambrian life was unicellular

Not all of it. Wiki article

This article has nice pictures.

[Robert A Cook PE]

From that, what is earliest accepted time of:

First uni-celluar life?

First "thing" that could be considered a "plant"? (Those are the limestone rock columns off of Australia, aren't they?)

Solidification of the earth from interstellar rubble into what we'd recognize as the original stable continents? (Canada shield is oldest rock still around, right?)

Collision that created the moon?