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. Life as a Phase Transition — Series Overview Foundational framework A. An Architectural Origin of Organismal UnityFoundational Paperhttps://doi.org/10.5281/zenodo.18305134 B. Life as a Phase TransitionFoundational Paperhttps://doi.org/10.5281/zenodo.18176157→ Defines life as a maintained, non-equilibrium basin governed by regulation and boundary constraints. Heredity constraint and theory closure C. The Analogue Heredity Ceiling and the Basin Digitisation TransitionHeredity constraint frameworkhttps://doi.org/10.5281/zenodo.18250555→ Formalises the limit of compositional (analogue) heredity and explains why replayable archives (digitisation) become unavoidable within the life-as-a-phase-transition framework. Companion papers — Mechanisms and consequences 2. The Basin of Identity: microRNA as a Kinetic Anchor in Multicellular Dynamics https://doi.org/10.5281/zenodo.18181118 → Explores how kinetic anchoring mechanisms stabilise basin identity in multicellular systems, now interpretable as local management of identity dimensionality and heritability stress. 3.Boundary Shedding as Basin MaintenanceA Control-Theoretic Extension of Life as a Phase Transitionhttps://doi.org/10.5281/zenodo.18215113→ Describes boundary shedding as a regulatory strategy for maintaining basin coherence under stress. 4.Cohesive Membranes and the Emergence of Multicellular Basin IdentityWhen Many Compartments Become One Organismhttps://doi.org/10.5281/zenodo.18215283→ Examines how multicellular organisation emerges from coupled boundary systems and shared basin identity. 5. From Control to ReplayArchives, Development, and Sexual Reproduction in Boundary-Defined Lifehttps://doi.org/10.5281/zenodo.18215367→ Investigates replay, development, and sexual reproduction as stabilising mechanisms, now unified by the Basin Digitisation Transition. 6. Boundary Architecture and Failure in Living SystemsA regulation-first classification of biological formhttps://doi.org/10.5281/zenodo.18226987→ Classifies biological form and failure modes through boundary architecture and regulatory breakdown. 7.The Boundary Renewal Principle:An Architectural Account of Heredity and Evolutionhttps://doi.org/10.5281/zenodo.18335521