(2) The embryonic constraints explanation
Body plans impose so many constraints on embryonic development that any novelty would disrupt too many characters and would bring development to an end.
This seems a more reasonable hypothesis, at first sight, but surely it cannot apply to Cambrian animals. They too had embryonic developments and body plans, and the Cambrian explosion means precisely that those ancestral plans were modified. If the constraints on embryonic development did not prevent the modification of body plans “before” the explosion, why should have prevented it “after” the explosion?
(3) The laws of form explanation
Body plans are the expression of “laws of form” which organisms cannot change, just as minerals cannot change their crystallization rules.
According to this classical explanation – which today has been reproposed with the name of biological structuralism – body plans are immutable either because they embody mathematical laws, or because are shaped by physical forces that organisms cannot change, just as they cannot change gravity and chemical bonds. Even this explanation, however, collapses before the historical fact that body plans did change in the Cambrian, which means that there is nothing immutable about them.
As we can see, none of the hypotheses which have been proposed so far is satisfactory, and this is probably due to the fact that the conservation of body plans and the origin of body plans are treated as if they were two disjoined problems. In reality, what we need to explain is not the conservation of body plans per se, but from a certain point onwards. More precisely, the problem consists in understanding why the body plans were modifiable “before” but not “after” the Cambrian explosion.

The Cambrian singularity

Paleontology has shown that the history of life has been full of adaptive radiations, processes where an ancestral taxon gave origin to descendant taxa which diverged by adapting to different environmental conditions.