Evolution and aging rate
Various species of plants and animals, including humans, have different lifespans. There is an evolutionary theory of aging, and general consensus in the academic community of evolutionary theorists; however the theory doesn't work well in practice, and there are many unexplained exceptions. Evolutionary theory states that organisms that, by virtue of their defenses or lifestyle, live for long periods whilst avoiding accidents, disease, predation, etc., are likely to have genes that code for slow aging — which often translates to good cellular repair. This is theorized to be true because if predation or accidental deaths prevent most individuals from living to an old age, then there will be less natural selection to increase intrinsic life span.[57] The finding was supported in a classic study of opossums by Austad,[58] however the opposite relationship was found in an equally prominent study of guppies by Reznick.[59][60]
One prominent and very popular theory attributes aging to a tight budget for food energy called caloric restriction.[61] Caloric restriction observed in many animals (most notably mice and rats), shows a near doubling of life span due to a very limited calorific intake. Support for this theory has been bolstered by several new studies linking lower basal metabolic rate to increased life expectancy.[62][63][64] This is the key to why animals like Giant Tortoises can live so long.[65] Studies of humans with 100+ year life spans have shown a link to decreased thyroid activity, resulting in their lowered metabolic rate.[66]
In a broad survey of zoo animals, no relationship was found between the fertility of the animal and its life span.[67]
