Stemness as a Systems-Level State: From Ancestral Plasticity to Systemic Control in Regeneration, Aging, and Cancer.
Multicellularity necessitated the evolution of cellular diversity and specialization, yet across organisms, the retention of cellular plasticity within defined physiological contexts is a recurring principle. Here, we examine early-diverging metazoans to reevaluate the evolutionary logic of stemness. Rather than viewing stem cells as exceptional, we argue that cellular plasticity represents a deeply conserved attribute of early life. The ability of cells to remain responsive, multipotent, and regenerative under ecological or physiological contexts challenges the notion of cellular identity. We integrate evidence across three layers: evolutionary origins of cellular plasticity, systemic physiological axes that govern stem cell behavior, and metabolic and epigenetic mechanisms that execute fate decisions. This synthesis reveals that stemness is not a default cellular state but a licensed state, permitted when organism-level physiological signals align with local tissue demands. Within this framework, regeneration, age-associated decline, and cancer emerge as evidence of how effectively systemic governance regulates cellular plasticity across multicellular life.