Theories of Aging: From Damage and Programmed Theories to Goal-Directedness

Aging is an extensive biological process characterized by morphological and functional alterations at different biological scales, resulting in a systematic decline in biological functions ultimately leading to death. Overall, two main types of theories have been proposed: damage-based theories and programmatic theories. We propose a third option, framed in the cognitive perspective on multiscale biological systems. Here, we review the different theories of aging, organize them in a conceptual hierarchy, and integrate our new model within the existing frameworks: aging as a consequence of loss of goal-directedness (in anatomical space). In this model, aging is driven by a dynamical systems-level disruption of homeostatic alignment within cellular collectives that build and repair a healthy body during development and maturation. In this model, morphogenesis is a homeostatic (goal-seeking) process; left with no goals after the developmental phase has been completed, aging can occur in the absence of damage via the functional disbanding of body components from their original aligned goal state. We suggest a roadmap with important implications for regenerative medicine and aging.