The body plan is the foremost feature to appear in Cambrian animals. Both individual animals and phyletic lineages developed very rapidly at the start. Early development in the embryo is extremely rapid. In human beings all systems and morphological features are in place in slightly more than three months after conception. The formation of body plans in the Cambrian occurred with great rapidity, geologically speaking, in a matter of 5-10 million years.
Body plans are extremely stable.
Early stages of individual development are remarkably resistant to evolutionary change, either because they are resistant to mutations or have extremely efficient repair mechanisms. Only later appearing, superficial features of the animal, such as coloration, are subject to adaptive change. Evolution thus does not change the body plan and other early-appearing features of the embryo. Phyletic body plans have not changed in more than 500 million years, with the possible exception of some reformulation of sea urchins' body plans. Development proceeds in characteristic time sequences and directions.