The disposable soma theory revisited: time as a resource in the theories of aging.

All life processes are subject to time constraints. At the cellular level, damage repair and cell cycle arrest are interrelated, allowing sufficient time for repair prior to cell cycle progression. Organisms have evolved so that developmental timing is linked to environmental conditions, such as nutrient availability and predation. Recent results in mammals regarding species-specific differences in cell cycle arrest and DNA damage suggest that a stable cell cycle arrest is a feature of longer-lived species. The implication of these results is that longer-lived species delay cell cycle progression to a greater degree than shorter-lived species, allowing for higher fidelity repair. We suggest that the ability to devote longer periods of time to repair and maintenance is a key feature of longer-lived species, and that evolutionary pressure to complete repair and resume cell division is a determinant of species lifespan. Thus, time is a resource that must be managed by the organism to attempt to maximize the fidelity of repair while completing development and reproduction in the limited window of opportunity afforded by environmental pressures. This viewpoint on time as a resource has implications for theories regarding the aging process and the development of species lifespan.