By Roger Highfield

The quest for immortality probably began with the first flicker of human consciousness and was driven by the most primitive instinct of all, survival. Over the millennia, the answers have increasingly been sought not from religion but from human ingenuity, and as a result man's lifespan has grown significantly.

There have been many false starts. The book Tan chin yao chüeh (Great Secrets of Alchemy) by Sun Ssu-miao (AD 581 to sometime after 673) describes formulae to prolong life based on mercury, sulphur and arsenic. Several Chinese emperors probably died after being poisoned by these 'elixirs of life', according to the British historian of science, Joseph Needham.

Although immortality looks as remote today as it did thousands of years ago, we have made strides in prolonging life and in understanding why we age. The timing of our death is at least partly under genetic control, and insights into how we may delay it have involved the study of organisms, from fruit flies to worms and naked mole rats, to uncover the genes that influence lifespan and see how manipulating them may defer the effects of ageing.

Other research has focused on how to grow new cells and tissue to repair a decaying body. The key is to return an adult cell, say a skin cell, to its embryonic state, when it becomes a 'stem cell' with unlimited potential. A breakthrough in rejuvenating cells by genetic alterations earned Shinya Yamanaka of Kyoto University the Nobel Prize in Physiology last year. The resulting IPS cells -- induced pluripotent stem cells -- offer a practical way to create embryonic cells without having to harvest them from embryos.

Stem cells have vast potential. In theory at least, they contain the genetic recipe and the biological know-how to become any cell, any tissue or any organ in the body. With the right biochemical signals, an embryonic stem cell could be coaxed into forming muscle, which could replace tissue damaged by a heart attack, or into heart vessels to replace those clogged by deposits, or into islet cells in the pancreas to treat diabetes, or into brain cells which could be used to treat Parkinson's disease. The stem cell is so versatile and potent that it could conceivably be used to grow an unlimited supply of body parts, whether bone, blood or brain, to repair the tatters of old age.

Researchers in Japan are expected to become the first to conduct entire clinical trials using these IPS cells to seek a cure for a common affliction in the elderly. The ethics committee at the Institute for Biomedical Research and Innovation in Kobe this February approved a trial treatment for age-related macular degeneration, the most common cause of blindness in the elderly. Within the next year, trials are expected to start using IPS cells to repair a carpet of cells in the eye called the retinal pigment epithelium.

Meanwhile, we are beginning to obtain profound insights into the ageing process itself. One dates back more than 70 years, to when two British statisticians made a puzzling observation while studying mortality figures for women aged 93 and above. They expected to find that the death rate continued to rise with age, as it does throughout adult life. Instead, their data suggests that at around this age we stop getting sicker as we get older: women aged 99, they observed, were no more likely to die than those aged 93. They had discovered that ageing ceases at a certain point.

Further research showed that the cessation of ageing set in earlier among people who died before the spread of 'industrial foods', such as high-fructose corn syrup, compared to those have died more recently. This led Professor Michael Rose at the University of California, Irvine, to speculate that we could live a little longer by adopting the diet eaten by our ancestors. If we could stop human ageing at earlier ages using such ancestral diets, the health burdens of decrepitude would be reduced.

Today, quality of life in old age is seen as more important than quantity. Professor Tom Kirkwood, director of the Newcastle Initiative on Changing Age at Newcastle University, points out that the rapid increase in life expectancy seen in recent years had been forecast to level off as it bumped up against a ceiling. But research over the past few decades has shown that there is no set upper limit for the span of a human life: the biological processes of ageing are much more malleable than used to be thought and there is no 'inner clock' programming us to die by a fixed time.

Today there is increasing emphasis placed on 'healthspan'. This is part of the redefinition of ageing: it is not how long you have been alive that counts but rather how many years you have left. This mirrors the common feeling that many nearing retirement age today are as vigorous as middle-aged people were a century ago. For example, the age of a woman who was 40 in 1900 is the same as the redefined or, as some researchers put it, the 'standardized average age' of a 55-year-old today. Intriguingly, by this kind of measure the average person can get 'younger' in the sense that they can have even more years to live as time goes on.

If we do significantly prolong life, new problems will emerge. Dr John Harris, professor of bioethics at the University of Manchester, has warned that if the life advances beyond 120, some might be tempted to propose condemning the elderly to death by assisted suicide or euthanasia in 'generational cleansing' after an allotted lifespan, as they competed with the young for space and other resources. Though this dystopian vision still seems a distant prospect, it underlines how the quest for immortality has never been straightforward.

Roger Highfield is former editor of 'New Scientist' and director of external affairs at the Science Museum


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