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Comparison of species lifespan provides convincing evidence that longevity is genetically influenced. An elephant lives about 10−20 times longer than a mouse, yet both animals have roughly the same number of lifetime heartbeats -- the
elephant at 30 per minute and the mouse at 300 per minute. Both species take about 200 million breaths in a lifetime. And both species have a
metabolic potential (total kilocalories used per gram of body weight per lifetime) of about 200 kcal. This figure is much the same for other mammals, but humans are exceptional with a metabolic potential of 800 kcal. Brains use
more energy than any other human organ. (
Basal metabolic rate for humans is about 80 watts = 70 Calories per hour.) Birds have a metabolic potential of 1,000 to 1,500 kcal.
Gerontologists who compare the longevity of species "explain" this discrepancy by saying that while body weight correlates well with longevity, there is a better correlation with brain weight for primates. And flight, like brain
weight, also confers a longevity advantage. Finches & robins live about 3 times as long as rodents the same size. Flying squirrels live twice as long as their close relatives the chipmunks. Parrots have a maximum lifespan in excess of 90
years. The Andean condor may be the most long-lived of any bird, but its maximum lifespan has not been confirmed. Porcupines are the longest-lived rodents -- all spined mammals have exceptionally long lifespans. All adaptations that afford
protection from predators and other hazards justify greater developmental resources to build a more durable animal with a longer maximum lifespan.
The relatively long lifespan of humans is partially explained in molecular terms by the low level of free radical production, the low level of fatty acid unsaturation and the high level of DNA repair enzymes in human cells. But an
evolutionary explanation would make reference to the fact that the survival advantages conferred by a large brain & large body size justify the metabolic investment in building the more lasting cellular & tissue structures and
protecting them with antioxidant enzymes, DNA-repair enzymes and a powerful immune system. The contribution of nurturing to offspring survival justifies human parents living well beyond the age of procreation.
Russell Wallace, who with Charles Darwin discovered natural selection, speculated that longevity much beyond the age of procreation would be a disadvantage for a species. Parents would threaten their children by competition for resources.
This would imply an evolutionary advantage to genetically programmed aging -- or to genes that have beneficial features in the young, but detrimental features in the old.
If aging were the product of evolutionary forces, aging could reasonably be expected to result from programming. But since most animals in the wild die of accident, attack or disease -- and since there are no forces for selection after
reproduction & parenting -- it seems unlikely that evolutionary forces determine aging. Robins in the wild, for example, have an estimated 12-year maximum lifespan and a 40% chance of surviving any given year. With a (0.4)12
-- or 1 in 60,000 -- chance that a robin can avoid accident, attack or disease for 12 years, there is little opportunity for natural selection to play a role in the evolution of senescence.
An alternative to the view that senescence is the product of evolution compares genetic programming to the engineering of a fly-by satellite designed to gather data about a planet. The engineering is focused on ensuring that the satellite
reaches its destination and performs its data gathering/transmission when passing the planet. Beyond the planet it is a matter of indifference to the engineers how long the satellite continues to function -- random decay occurs. Applying the
analogy, the satellite passing the planet is like an organism passing its reproductive period. World War II soldiers on ships headed for the beaches of Normandy were not inclined to worry about the health hazards of smoking -- longevity
concerns are a waste of time & resources when hazards are high. Once the objectives of reproduction & parenting have been achieved the organism decays by random malfunction.
A short-lived organism would waste metabolic energy by over-investing in anti-oxidant or DNA-repair enzymes when the energy could be spent on rapid growth and reproduction. For species with special survival traits like brainyness or the
ability to fly, a greater investment in metabolic energy could be justified. Adaptations that reduce the threat of predators & other hazards lead to conditions in which competition between members of the species for mates & resources
becomes of primary importance -- leading to longer development time devoted to the creation of more hardy animals. It is conceivable that an animal with well-engineered cells could live thousands of years. Human germ cells have arguably lived
for millions of years through an investment in DNA-repair enzymes, antioxidant enzymes and telomerase.
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