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Time of Our Lives: The Science of Human Ageing

by Tom Kirkwood Weidenfeld pounds 20

Once the noon of life is past, and death casts a lengthening shadow, the chatter about ageing that we have always heard in the background gradually begins to attract our attention. Insidiously, it starts to matter whether we are programmed to die, or whether all animals have the same number of heartbeats in their lifetime. Tom Kirkwood's book, as clear and accessible as popular science gets, is just what we need to dispose of the accumulated lumber of half- remembered half-truths that stand in the way of assessing our prospects.

Time of Our Lives is so good, in fact, that the publishers can be forgiven for the jacket's proclamation "A World Authority Shows Why Ageing Is Neither Inevitable Nor Necessary". This omits the qualification "If You Are A Sea Anemone". Some species apparently do not age; they simply die of being eaten, infected, crushed or otherwise falling victim to their environments. They falsify the idea, popular for a century, that ageing is an inevitable consequence of wear and tear on the cells. The myth of a finite number of heartbeats is a longstanding version of what Kirkwood calls the "fatalistic fallacy". But of the species identified to date as ageless, none are animals.

The belief that ageing is necessary is based on a misunderstanding of evolution which remains extremely tenacious, although its error was clearly identified 30 years ago. Death, it is widely supposed, is necessary for the good of the species, whether to prevent overcrowding or for some vaguer purpose of renewal. But nothing in evolution happens because it is for the good of the species. Natural selection acts on individuals. A population of organisms which obligingly died for the good of the group would be vulnerable to subversion by freeloading mutants. If a mutation arose which blocked the death process, its possessor would enjoy the benefits of its fellows' sacrifice, without paying the price. It would therefore leave more descendants, which would also enjoy the benefits of the mutation they inherited, and gradually the self-sacrificers would be driven out of the population.

A more prosaic reason for doubting that death from old age is necessary to the success of species is that old age is a rare phenomenon, organisms normally being eaten, fatally parasitised and so on before they get that far. However, this also means that organisms enjoy fewer benefits from natural selection as they get older. The more time an organism has to reproduce, the less effect selective forces will have upon the action of genes later in life. Harmful genes that act late are therefore likely to accumulate: Kirkwood refers to them as "genetic dust- beneath-the-cupboard", which becomes visible when the cupboard is moved by environmental changes that reduce death rates earlier in life.

He plays down the importance of this process, which in the opinion of a recent commentator on a paper co-written by Kirkwood, in the journal Nature, does indeed make ageing inevitable. His own theory goes by the catchy tag of "disposable soma". If instant karma doesn't get you, disposable soma definitely will.

By then, however, you are likely to have reproduced, which is all natural selection cares about. Agelessness carries a price: metabolic resources have to be invested in keeping the DNA free from errors. If most animals die before they reach old age, there is no adaptive percentage in creating animals that will last that long. The resources should be allocated to investment in reproduction instead. Different species strike different balances. In the nearest thing to an animal model of rock 'n' roll, the males of one species of marsupial mice stake all on a single blaze of testosterone- fuelled glory, which drives them into a frenzy of mating and fighting over mates. They are left with wounds, stomach ulcers and empty immune systems, as a result of which they almost all die within a few days. We humans are at the other end of the spectrum: built to last, but not too long.

Testosterone does seem to take its toll on us as well. Men tend to live less long than women - about five years, in this country. One exception to the general pattern is India, where the life expectancy at birth of the sexes is almost equal. This could be described as an indirect effect of testosterone: Kirkwood notes that Indian "girls are four times more likely than boys to suffer malnutrition, but 50 times less likely to be taken to a hospital when ill". The reasons why men die younger under less unequal conditions are not really clear, especially when it comes to just how testosterone shortens life. Kirkwood is surely right to depict men as more disposable, though. The marsupial mice are an extreme case of the principle that males can reproduce successfully without remaining around to look after their offspring, but females cannot. One of the distinguishing traits of human female reproduction is menopause, which may leave more than 20 years of active life after the last child is born. This may be a means of maximising the chances that a woman will raise all of her children to adulthood. Jeanne Calment raised her only child to adulthood; her daughter and her only grandchild both died at the age of 36. Mme Calment went on to live to 122, setting the current world record. It will be beaten, Kirkwood assures us, and in the wake of its successors will be a massively expanding tail of people who are remarkably old by the standards of even a generation ago. We want to be among them, but we fear what life at such an age will be like. After Professor Kirkwood's exposition of these awesome matters, it actually comes as a relief that his prescriptions are so modest. Jeanne Calment ascribed her hale longevity to olive oil and port wine. Tom Kirkwood recommends oily fish, red wine, more fruit and vegetables, perhaps a top-up of vitamins C and E, possibly a garnish of trace elements, and simply eating less.