12. Aging Is Both Stochastic and Directional

The individual microlevel molecular events that build the deleteriome are stochastic, but the overall macrolevel process is directional and constrained, yielding reproducible statistical patterns (e.g., age-associated signatures and mortality acceleration). Moreover, rather than producing arbitrary damage, living produces damage according to the biological functions that operate. For example, a particular human enzyme would produce a particular set and quantity of by-products, an orthologous mouse enzyme would contribute slightly differently to the deleteriome, and a different enzyme would provide a very distinct contribution. The process is stochastic at the level of individual events, structured at the level of damage types, and directional at the level of aging trajectory (2–5, 22–24). Heterogeneity in the starting state (biological age at ground zero), genotype, developmental processes, exposures (e.g., lifestyles, diets), and chance produce diverse trajectories, but the commonality is the net rise of damage burden over time.