Aging, or senescence, varies markedly among living organisms. Some 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-death caused by environmental factors such as predation and accidents rates, shapes the force of natural selection across all ages, resulting in different evolutionary trajectories of senescence. Fitness, which is defined as an individual’s ability to survive and reproduce, declines with age in most species. Negative senescence, referring to improved fertility or survival with age, is however, observed in a minority of taxa. Importantly, this review highlights the relevance of these evolutionary theories to human geriatric research, including potential therapeutic interventions.
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