Cohesive Membranes and the Emergence of Multicellular Basin Identity: When Many Compartments Become One Organism

Multicellularity is commonly explained in terms of genetic coordination, cooperation, or evolutionary advantage. Such accounts, however, do not provide a principled criterion for when multiple compartments constitute a single biological individual. This paper proposes a regulation-first definition of multicellularity based on basin identity rather than cell number, genetic similarity, or signalling complexity. We define multicellular organisms as systems in which viability is determined primarily by a shared, cohesive boundary, such that identity resides at the collective level rather than within individual compartments. Under this condition, internal compartments function as replaceable sub-basins, and failure becomes global only upon disruption of the shared boundary. This criterion is formalised using a basin-sensitivity inequality and shown to be sufficient and necessary for organismal unity. The framework applies across biological scales and resolves longstanding ambiguities surrounding embryos, syncytia, chimaeras, and biofilms. It further demonstrates that multicellular identity can, in principle, arise prior to genes or differentiated cell types, provided cohesive boundary control is established. By relocating multicellularity from cooperation narratives to boundary-level regulation, this work provides a physically grounded definition of organismal identity that integrates naturally with regulation-first theories of life and sets the stage for understanding development, differentiation, and sexual reproduction. This paper is Companion Paper 4 in a series developing a regulation-first, boundary-centric framework for biological identity. Foundational Paper: Life as a Phase Transition: https://doi.org/10.5281/zenodo.18176157 Companion Paper 2: The Basin of Identity: microRNA as a Kinetic Anchor in Multicellular Dynamics https://doi.org/10.5281/zenodo.18181118 Companion Paper 3: Boundary Shedding as Basin Maintenance A Control-Theoretic Extension of Life as a Phase Transition https://doi.org/10.5281/zenodo.18215113 Companion Paper 4: Cohesive Membranes and the Emergence of Multicellular Basin Identity When Many Compartments Become One Organism https://doi.org/10.5281/zenodo.18215283 Companion Paper 5: From Control to Replay: Archives, Development, and Sexual Reproduction in Boundary-Defined Life https://doi.org/10.5281/zenodo.18215367