As I have already pointed out, there is evidence for Precambrian complex
animal forms, specifically the Ediacaran fauna.
This, however, brings up another point. Even if the Ediacaran fauna did not
exist, the concept of Cambrian ancestors in the Precambrian is predicted by
everything we currently know about life, including the idea that complex
lifeforms do not simply appear out of nowhere like magic. Rather they
derive from other complex lifeforms by some mechanism, whether that
mechanism be reproduction or evolution.
By suggesting that the Cambrian fauna have no Precambrian ancestors you are
in fact challenging that paradigm. It is therefore your responsibilty to
prove the paradigm wrong by offering evidence or theoretical reasons why the
Cambrian fauna cannot have ancestors. Pointing out the deficiencies in the
paradigm is not a bad start, but it puts you an weak ground, especially if
those deficiencies can be eliminated or effectively explained. You would be
better off explaining why the Cambrian fauna cannot have ancestors, and how
they therefore originated.
Kevin L. O'Brien