Several theories have sought to explain aging, here precisely defined as “increasing mortality with increasing chronological age in populations in the wild”. They all fall within one of two opposite and incompatible paradigms. For the first (“old paradigm”), aging is the result of degenerative phenomena that natural selection cannot counteract completely, due to insufficient strength or opposing selective pressures. For the second (“new paradigm”), aging is favoured by natural selection in terms of supra-individual selection: it belongs to a broader category of phenomena, on the whole defined as “phenoptosis”, which are explicable only in terms of supra-individual selection. For the new paradigm, aging is a specific function that is genetically determined and regulated, with its own physiology, pathology and phylogeny. This paper describes the theoretical arguments and the empirical evidence that support or are in contrast with each of the two paradigms. Subsequently, on the basis of an imposing and authoritative amount of research, aging mechanisms at the cellular and organismal levels are described. The clear existence of such mechanisms is indispensable proof to support the new paradigm and is in complete and unsolvable contrast with the old paradigm.