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lifespan

lifespan Of all the land mammals, humans have the longest lifespan. The documented record holder for being the longest-lived person was Madame Jeanne Calment, a French woman who died in 1997 at the age of 122. Of the other mammals, the Bowheaded whale and the Asiatic elephant come close, but the really long-lived species tend to be cold-blooded or plants, for example the Pacific clam and Galapagos tortoise live to more than 200 years, while the Bristlecone pine is reputed to grow for up to 5,000 years and is the world's oldest organism.

All living creatures eventually die (with the possible exception of some single-celled organisms) and several factors are related to the lifespan of each animal species; longer-lived animals tend to be larger, have a bigger brain in relation to body size, and have a slower metabolic rate (this gives cold-blooded animals the advantage). Though humans are not large compared with elephants and whales our larger brain seems to offer the advantage, possibly by allowing us to adapt to change, and use reason to avoid danger and find food, so helping prehistoric humans to live longer. A large brain also means it takes longer for infants to develop, adulthood is delayed, and parents must be present to teach their offspring — so longer-lived parents were necessary for the next generation to survive. These changes have occurred over the last 1.5 million years, during which time both maximum lifespan and the numbers of neurons in the brain have almost doubled.

From earliest recorded history the ancient Egyptians suggested maximum lifespan was 110 years, and Roman funeral records describe people of over 100 years old. Although these early records may have been exaggerated it is likely that many people did at least reach their 70s and 80s in Biblical times, as noted in the Psalms:
‘The days of our years are threescores years and ten; and if by reason of strength they be fourscore years, yet is their strength labour and sorrow; for it is soon cut off, and we fly away’(Psalm 90:10, King James Version).
But for most people in history old age was not a possibility. On average the ancient Greek population (around 1100 bc) lived 35 years, and on the whole this remained the average lifespan in Europe up to the nineteenth century. Even as recently as 1901 the average life expectancy in England was only 47 years. The fast rise in life expectancy, to the present 76 years, is due almost entirely to reduction of infant and childhood deaths in the early to mid twentieth century. As Fred Astaire remarked, ‘Old age is like everything else. To make a success of it you've got to start young’, and this is as true of health, as any other part of life. Improvement has come about through better nutrition, hygiene, and housing, and medical advances in immunization and antibiotics removing the past childhood killers, tuberculosis, pneumonia, diarrhoea, and a host of infectious diseases (e.g. typhoid, scarlet fever, diphtheria, measles).

Present medical research is concentrating on improving health in later life by treating what are now the major fatal diseases—stroke, heart disease, and cancer—and the increasing risk of dementia. Because of this, the average lifespan is predicted to rise to 83 years by 2030. Some small groups of people already have a longer than average lifespan, for example the 7th Day Adventists in the US, and Japanese priests, who have a low disease rate through healthy living (no smoking, alcohol, or meat) and active lifestyle.

In spite of the obvious increases in average life expectancy, there is very little evidence to suggest that humans can increase their maximum lifespan much beyond 120 years. To do that would mean halting or slowing the complex ageing process, which involves continuous random damage to all cells and their DNA from the by-products of metabolizing oxygen and glucose — paradoxically essential for life. Lifespan is also influenced by genetics — for example, long-lived parents produce long-lived children, and women live approximately 6 years longer than men, mostly due to cardiovascular protection by female sex hormones. These factors must be accounted for before the limits on lifespan can be understood and breached.

Fashions in wonder drugs, diets, and exercise to promote immortality come and go, although so far only ‘dietary restriction’ has proved effective in mammals. This involves reducing calorie intake by half while maintaining all the other nutrients at normal levels. Using this strategy rats and mice can live up to 40% longer and have fewer diseases associated with old age. It remains to be seen if this could also apply to humans, but for now a more realistic aim is to improve health at all ages, and not to strive for immortality. As George Bernard Shaw, who lived to age 94, wrote. ‘Do not try to live forever. You will not succeed.’

Jane E. Preston

Bibliography

Smith, D. W. E. (1993). Human longevity. Oxford University Press.


See also ageing.

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lifespan

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