That's a very nice illustration to put it in perspective. Thanks!
I have briefly seen videos on youtube about the subject of the cambrian explosion, And I had a question burning in my head. Why was the cambrian explosion a threat to the darwinian theory?
Maybe, the cambrian explosion is just a metaphor for the evolution and advancement of new forms with complex body parts that simply are modified ontogenetically through natural selection and mutation.
The reason why the Cambrian event may be problematic for Darwinian Evolution is that within canonical Darwinian theory is the assumption that phylotypic change occurs gradually, and is selected at each change. For the changes that occurred around the Cambrian Event, hundreds of millions of years would be required if later evolutionary changes scale linearly in time. Niles Eldridge et al proposed punctuated evolution which allows for rapid phylotypic change spaced with phylotypic stasis, one of the causes is hyppothesized to be geographical isolation which is often associated with an extinction event.
The systematics plot above is a bit misleading in that the large blue branch is all bacteria. I don't think that it would come as a shock to anyone that there are many more species of bacteria than metazoa. The Cambrian event affected the multi-cell body plans, and in a very significant way. Its effect on bacteria is not preserved in the fossil record. Therefore, to indicate that the Cambrain even affected only the branch indicated is not positively supported in the fossil record.