It was likely important that none of the worst disasters we have suffered occurred early in the Cambrian, just as complex life was beginning to emerge and was at its most fragile. As it was, many, perhaps a majority, of the phyla found in Cambrian fossils went extinct during and with the extinction event ending the Cambrian. The phyla during this time were represented by very few species each, so the removal of an entire body plan from the stage was not an unlikely event. The authors do not speculate on the necessity of the chordate body plan surviving, an event which others have suggested was necessary for technical life. The do point out that since the Cambrian no phylum has gone extinct, apparently because each phylum diversified into enough species that the phylum was able to survive even if a majority of its species died out.
Diversity is encouraged along north-south coastlines by the cold poles and warm equator.
Asteroid impact seems to have been responsible for two of the major extinction events. Other outside events, such as a supernova within thirty light years, could have led to major extinctions or surface sterilization, but do seem to have happened. The frequency of the major extinctions may be yet another balancing act. If the rate were too high then life would constantly struggling to recover from the latest disaster. Furthermore, with frequent major extinctions, a sterilization event must be considered more likely.