Metabolic complementation between cells drives the evolution of tissues and organs

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 hypothesis for this kind of
evolutionary transition based on two features of cellular metabolism:
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 favour offloading metabolic burdens from a primary
specialized cell type onto supporting cells. We illustrate this process of
‘supra-functionalization’ using the nervous system and pancreas.