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Although evolutionary transitions of individuality have been extensively theorized, little attention has been paid to the origin of levels of organization within organisms. How and why do specialized cells become organized into specialized tissues or organs? What spurs a transition in organizational level in cases where the function is already present in constituent cell types? We propose a model for this kind of evolutionary transition in terms of two key features of cellular self-maintenance: metabolic constraints on functional performance and the capacity for metabolic complementation between parenchymal and supporting cells. These features suggest a scenario whereby pre-existing specialized cell types are integrated into tissues when changes to the internal or external environment favor offloading metabolic burdens from a primary specialized cell type onto supporting cells. We illustrate this process of supra-functionalization using the nervous system and the pancreas as examples. The model reveals distinctive concerns for explaining this kind of evolutionary transitioninstead of the suppression of cheating by components, the issue is how a tissue comes under modular genetic control as a distinct body partand also points to a novel form of evolutionary complexity-increasing ratchet.
Pavličev et al. (Mon,) studied this question.