Aging, or senescence, is not a uniform phenomenon among living organisms. Some organisms experience progressive physiological decline while others show negligible or negative senescence. This review examines diverse aging patterns across taxa, focusing on evolutionary mechanisms such as antagonistic pleiotropy and mutation accumulation. We argue that extrinsic mortality rates shape the force of natural selection across all ages, resulting in different evolutionary trajectories of senescence. Importantly, this review highlights the relevance of these evolutionary theories to human geriatric research, including potential therapeutic interventions.
