Not necessarily—there are those that argue that the cambrian explosion might have had more to do with the increase in atmospheric oxygen over geological time than evolution, and we find vague evidence of multicellular creatures (worm-track type impressions in the seafloor, some strange radially-symmetrical things buried in the sediment) up to a billion and a half years ago. Oxygen lets you have big energy-gobbling multicellular creatures easier, and blocks the destructive ultraviolet radiation that would have previously sterilized the above-water land when it turns to ozone. We also might just see an explosion because that is when hard body parts that fossilize easier appeared.
If the explosion was evolution-driven it could have been due to some kind of runaway arms race between predators and prey, or due to the final establishment of the developmental plans of the various animal phyla that could then be modularly tweaked to enable diversification and rapid evolution.
Not necessarily—there are those that argue that the cambrian explosion might have had more to do with the increase in atmospheric oxygen over geological time than evolution, and we find vague evidence of multicellular creatures (worm-track type impressions in the seafloor, some strange radially-symmetrical things buried in the sediment) up to a billion and a half years ago. Oxygen lets you have big energy-gobbling multicellular creatures easier, and blocks the destructive ultraviolet radiation that would have previously sterilized the above-water land when it turns to ozone. We also might just see an explosion because that is when hard body parts that fossilize easier appeared.
If the explosion was evolution-driven it could have been due to some kind of runaway arms race between predators and prey, or due to the final establishment of the developmental plans of the various animal phyla that could then be modularly tweaked to enable diversification and rapid evolution.