The range of dates I've seen for the Cambrian explosion from 5 million to 30 million years, which is hugely problematic for the theory of evolution. So, in literally no time at all geologically, all of the phyla of the world appear literally at once. Their fossil precursors were all microbes.
BTW, the term "explosion" has no meretricious appeal for me. I find that "explosions" are used by theorists to get themselves out of the corners they've painted themselves into.
Tens of millions of years isn't no time at all biologically. I don't think that is quite true that all phyla originate in the Cambrian, I'd have to check, but I agree many phyla do originate in the cambrian. Don't get over hung-up on the phyla thing anyway. The importance of phyla is retrospective. At the time the difference between one phylum and another would have been very small, just like modern speciations, its just that subsequently they ended up going different ways that formed different branches of the phylogenetic tree.