Abstract
Major evolutionary transitions in individuality (METI) represent profound reorganizations in biological and sociocultural systems, yet the organizational mechanisms that render these transitions directional and irreversible remain incompletely understood. In this review, a mechanistic framework for fraternal evolutionary transitions was proposed, focusing on general system-level properties rather than substrate-specific genetic or cultural mechanisms. The model emphasized sustained growth, energy availability, information expansion, and functional specialization as interacting drivers that push cooperative systems toward new organizational levels. The analysis showed that, as growing systems increase in size and interaction density, informational demands for coordination, regulation, and control rise nonlinearly. When these demands exceed the processing capacity of individual components, functional specialization emerges as a necessary structural response. It was also identified that, while specialization enhances efficiency and enables further growth, it is accompanied by functional simplification, loss of plasticity, and increasing interdependence among components, thereby introducing irreversibility into the system. In addition, communication and transport infrastructures were proposed as indispensable for maintaining integration as specialization deepens. By integrating energetic, informational, and organizational processes into a unified mechanistic model, this framework explains why fraternal evolutionary transitions---such as the emergence of multicellularity and complex human societies---exhibit consistent directionality, irreversibility, and competitive advantages under conditions of intergroup competition.
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