Mrs Hughes was as old as could be : The days of our lives are five score and fifteen maximum: Steve Connor explains

MODERN medicine may have done wonders for life expectancy, but it has proved useless at giving us the secrets of immortality. Even with the best possible care and attention, men and women cannot live much beyond the ripe old age of 115, the point at which Charlotte Hughes, the oldest person in Britain, passed away on Wednesday in her sleep. She lived as long as a human being can.

By 115 or so, the human body, any human body, has reached its ultimate limit. Humans, even Godless ones, have a form of immortality; the life of our genes, which live on down the generations. In geneticists' terms our bodies are merely disposable vehicles, carrying our genes from one generation to the next. The surprising thing about human beings is not that their life has a restricted term but that they live so long. Few animals outlive humans, many simply reproduce and die. Slow ageing and gradual decline does not occur in the animal world, only in ourselves.

The 'disposable soma' theory, developed over the past decade by Tom Kirkwood, a theoretical biologist at the National Institute of Medical Research, near London, is now accepted as the most promising solution to a problem that has dogged biology for ages: why do we die when we do? The theory is complex, but it states that our genes will not build a body which will live for centuries because this strategy will not work in evolutionary terms. The human organism has adopted a strategy that makes a major investment in repairing the damage the body takes as the decades go by. But this investment will not go to the extremes of repairing a body for centuries. Mr Kirkwood's theory postulates that there comes a point at which repairing damage simply becomes too costly and could only be done at the expense of risking what is ultimately the raison d'etre of life: passing on genes.

This theory sets a limit to what has been an extraordinary trend. A hundred years ago, a typical newborn baby in Britian could expect to live about 42 years. Now average life expectancy is 76.5 years for females and 71.5 years for males. The improvement is entirely due to environmental factors: better diet, better housing, better medicine. But there is a point at which these environmental factors come up against the genetic barrier - the ultimate facts of life.

Physically and mentally we peak early, at about the time we are sexually most active, and from then on it's a downhill journey. We begin to feel the effects of our age: tiredness, lack of agility and so on. Our hearts, lungs, muscles and kidneys all gradually lose their function. This occurs because of what happens at the molecular level. The molecules of our cells are constantly being damaged. Repairing this wear and tear is costly. Our bodies spend a lot of energy doing it and this means there is less energy for that other vital ingredient of life: sex. Humans as a species have followed the strategy of repair at the expense of quick reproduction, which is why we live much longer than other comparable animals.

Two hundred years ago men lived longer than women because life was much harder for them than for men. The difference we see today, putting women ahead in the life expectancy stakes, is not because one is exposed to more wear and tear than the other. One of the geneticists' theories suggests that Homo sapiens lived in polygamous societies many hundreds of thousands of years ago; males mated with several females and therefore were following a mini-strategy of reproduction at the expense of repairing the wear and tear of the passing years. The fact that they passed their genes on so easily and fecundly left them with little reason for investing in the repair of their own bodies, unlike females. And so when the genes of males and females get transferred to a modern setting - where there are roughly equal environmental influences on both men and women - the result is a difference in life expectancy of roughly five years.

Recent research on fruit flies has lent experimental weight to the disposable soma theory. In fact, scientists who have painstakingly monitored the lives of a million flies living in a laboratory believe that if humans can survive the first 70 or so years of life, with all the dangers it contains, they are well on the way to living a good deal longer because they have proved they are blessed with the genes that could take them past the century.

But this does not mean that we can cheat on death itself. Mrs Hughes was luckier than most of us will be. The rest of us will have to be content with three score years and ten, plus a little more if we are lucky, depending on which sex we are.

(Photograph omitted)