The Cambrian explosion (~538.8–521 Ma) produced virtually all animalphyla represented in the fossil record within approximately twenty mil-lion years. Existing theoretical accounts address the onset of this diversi-fication but leave its termination—and, critically, its irreversibility acrossfive subsequent mass extinctions—without adequate mechanistic expla-nation. We propose a reframing: the Cambrian diversification windowconstitutes a stable attractor basin, maintained by a self-reinforcing feed-back loop between body-plan innovation and ecological niche expansion,rather than a transient transition between two equilibria. Within thisframework, we identify symbiogenesis—organismal merger in the tradi-tion of Margulis—as the primary mechanism of phylum-level morpholog-ical discontinuity, and immune permissiveness as its enabling condition.Early Cambrian metazoans possessed innate-only immune systems withloosely defined self/nonself boundaries, providing the wide negotiationwindow that symbiogenetic integration requires. The exit from the at-tractor basin was two-staged: ecological saturation attenuated the posi-tive feedback loop as niche space filled; the subsequent convergent evolu-tion of adaptive immune specificity in vertebrate lineages (~450–500 Ma)permanently closed the negotiation window, rendering the mechanism ofbody-plan innovation structurally unavailable. This immunological lock-in explains why mass extinctions, which reopened ecological space butnot the negotiation window, produced no new animal phyla. The frame-Immune Permissiveness and the Cambrian Explosion 2work unifies dynamical systems theory, symbiogenetic evolutionary the-ory, and evolutionary immunology, generates four classes of testable em-pirical predictions, and identifies the immune system as a regulator ofevolutionary scope—a role not previously theorized.