| Summary This third  and final paper in this series considers ageing mechanisms across species, with  emphasis on conserved metabolic pathways that relate to disease. The growth  hormone (GH)-insulin-like growth factor (IGF-1)-insulin axis continues as an  example of how critical pathways might relate to longevity and senescence.  Aligning theory, research outcomes and clinical investigations at the levels of  the cell, organism and population, is suggested as a means by which to consider  the many complexities of the ageing process in an orderly fashion. A  contentious debate revolves around whether ageing is purely a combined effect  of stochastic events on residual programming relating to reproductive  robustness, or whether ageing itself is programmed by natural selection.  Emerging data indicate that the influence of genetic programming on specific  late-life diseases, and even individual tissue pathologies, will probably need  to be reconsidered in the light of newer theoretical possibilities. In  particular, the evidence that late life and its diseases are objects of  considerable investment of energy challenges theory that couples longevity with  reproduction. Furthermore, the author suggests that ageing may have evolved at  least partly as a means of niche preservation for contemporaries and for  progeny.
 KeywordsAgeing,  Chaperone, Disease, Germ-soma conflict, IGF-1, Insulin-like growth factor,  Natural selection, Portuguese water dog, Telomere.
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