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Both of these aging theories (pnas.org/doi/10.1073/pnas.26…) (doi.org/10.18632/aging.20643…) hold that aging is not driven by a program, but instead arises from deleterious molecular damage or system‑level collapse. Whether the aging process is programmatically controlled or results from the accumulation of random damage represents a hotly debated question dividing two schools in aging research. Researchers from both camps agree that developmental rate and the timing of sexual maturation are programmatically controlled. However, opponents of programmed aging argue that aging exerts negative effects on individuals. Since natural selection acts only upon individuals, organisms cannot evolve and maintain a program that is detrimental to themselves. Accordingly, aging must stem from the progressive accumulation of randomly occurring bodily damage. Nevertheless, when three killifish strains originating from regions with distinct rainy‑season lengths, with lifespans of 3‑month, 9‑month and 16‑month respectively, are housed in the same aquarium, their lifespan differences persist. This observation indicates that post‑sexual‑maturation aging remains programmatically controlled. Random damage accumulation cannot explain why these three congeneric, highly morphologically similar killifish exhibit such large lifespan disparities that precisely match the duration of rainy seasons in their native habitats. Critics counter that populations can evolve rapid aging under conditions of high extrinsic mortality (very short rainy seasons), and slow aging under low extrinsic‑mortality environments — a process that does not require an aging program at all. At the molecular level, nonetheless, post‑maturation aging still displays hallmarks of programmed aging. For instance, in aged hematopoietic stem cells, 1500 genes are up‑regulated and another 1500 are down‑regulated［doi: 10.1016/j.exger.2009.12.010. ］. This suggests that aging is fundamentally a programmed process: up‑regulated genes are largely enriched in stress‑response and inflammatory pathways, while down‑regulated genes mainly participate in chromatin remodelling and DNA‑repair pathways. This implies that damage accumulation is pre‑programmed rather than an insurmountable biological limitation. Simply put, organismal death originates from replicative senescence in tissue‑resident adult stem cells. Replicative senescence describes the phenomenon whereby each cell division moderately reduces the synthetic rates of ATP and proteins, accompanied by shifts in the expression of certain genes, until the cell loses replicative capacity. Given that telomeres do not shorten in some cell types, and cells engineered to express telomerase for telomere elongation still eventually cease dividing, the TRCS model of cellular senescence proposes that replicative senescence is triggered by the shortening of two countdown elements: telomeres and ribosomal DNA (rDNA) arrays. Telomere shortening should not be categorized as damage, because telomeres in certain cell types lengthen as cells age. Replicative senescence in adult stem cells means these stem cells possess finite lifespans, which aligns well with the disposable soma theory. Antagonistic pleiotropy theory states that proteins encoded by certain genes exert multiple biological functions, conferring fitness benefits early in life yet exerting harmful effects in old age. At the molecular level, antagonistic pleiotropy is also implemented via p53‑driven genetic programs activated by countdown elements. For example, the protein product of the DUX4 gene functions as a transcription factor that is highly expressed in early embryos but expressed at low levels in adult somatic cells. Mutations that re‑activate DUX4 expression in adult muscle cells cause facioscapulohumeral muscular dystrophy (FSHD), a rare genetic disorder. Telomere shortening is known to up‑regulate p53, and p53 in turn up‑regulates DUX4［ doi: 10.1038/s41588‑021‑00893‑0.］. Consequently, shorter telomeres correspond to stronger DUX4 transcriptional activity; progressive telomere shortening can boost DUX4 activity by up to ten‑fold［doi: 10.1038/nsmb.2571. ］. MHC‑I molecules present intracellular antigens on cell surfaces and are expressed by nearly all cell types. DUX4 expression is negatively correlated with MHC‑I levels. Therefore, progressive telomere shortening drives DUX4 up‑regulation and concomitant MHC‑I down‑regulation. Elevated DUX4 impairs antigen presentation mediated by MHC‑I［doi: 10.1016/j.devcel.2019.06.011. ］, which compromises immune‑system surveillance and clearance of mutated and pre‑cancerous cells. This leads to greater DNA‑mutation burden and higher tumour incidence in aged tissues. Hence, aging, age‑related diseases, age‑dependent genetic disorders, as well as DNA‑mutation rates, mutational load and tumour incidence, are all pre‑programmed outcomes, with telomere length acting as the time‑bomb for these pathologies.