[credit provider=”en.wikipedia.org” url=”http://en.wikipedia.org/wiki/File:Nacktmull.jpg”]
Why do humans live so long? The African naked mole rat might have the answer, reports Steve Jones.September already, and the start of term looms near. In the first lecture of my genetics course, I tell the audience to look at the person to their left, and to their right, and point out that two out of each three will die for reasons connected to the genes they carry. That gets a gasp – but to cheer them up, I say that had I been speaking in Shakespeare’s time, two out of every three would be dead already.
To the students, their demise from cancer, heart disease or diabetes (all of which have an inherited component) seems immeasurably distant; and thanks to the latest research, it may be even further than they think. For the past century or so, life expectancy has been going up at the almost unimaginable rate of six hours a day, every day. Since I emerged into the world on the day of the Great Escape from Stalag Luft III (March 24, 1944) the average age at which a citizen can expect to survive for a further decade has risen by 10 years. In 1944 it was 70, but is now close to 80. These islands have almost 12,000 centenarians, four times more than in the 1960s. At the present rate, that will rise to 100,000 at the end of the century.
It is worth reminding ourselves, too, just how long-lived we already are in comparison with our fellow warm-blooded land animals. For them, each doubling of body weight leads to about a 16 per cent increase in length of life. We stand beyond that, for in spite of our modest size we live longer than any of them (elephants included).
Something is keeping us alive, and we do not know what it is. Now, a real Methuselah of the mammals is beginning to hint at what it takes to make a century. The African naked mole rat, whose name describes its charming appearance, has a pair of fearsome front teeth. It lives in burrows in which one aggressive female prevents all the others from mating and forces them to look after her own offspring. The animal is about the same size as a mouse, but it lives eight times as long, with plenty getting to 30 or so.
Once past our own teenage years, we age faster and faster. The average chance of death in a particular period doubles every eight years. The figures are more favourable in the prime of life and are at their best at the age of 10. If that schoolboy rate of mortality were to persist throughout life, most children born in 2000 would survive until the year 3300, which gives an uncomfortable insight into the power of bodily decay.
The mole rat does much better, for it stays young, healthy and fully fertile for almost all its days (which for an elderly animal is equivalent to an 80-year-old woman having the biological make-up of someone 50 years younger). And its longevity hints at some of the fundamental causes of ageing.
There are plenty of ideas around. One is that we poison ourselves with the by-products of our own metabolism; it has often been suggested that a restricted diet will help, or eating vitamins, fruit or vegetables, but the effects for people of normal weight are marginal at best.
Now, it seems that poisons from outside are more important: that fate depends not on how much you eat, but what it is. In the mole rat’s subterranean home, the only food and water comes from the roots and bulbs of plants growing above. Most plants are packed with all kinds of poisons – snowdrops, daffodils, and crocuses have bulbs that cause vomiting and worse, while garlic depends for its flavour on the same kind of stuff – and those in the African desert are even more noxious. So the mole rat has evolved defences that allow it to cope: its cells are so tough that they can deal with huge doses of lead, cadmium and other poisons and cope well with heat and starvation. Cells from other long-lived creatures (ourselves included) are also resistant to such stresses, hinting that an ability to deal with external challenges may be the source of longevity itself.
This year marks the 500th anniversary of the explorer Ponce de León’s search for the Fountain of Youth in Florida. A pill may soon emerge from the mole rat work to fulfil his dream. In the meantime, it seems the secret of long life is to have an embittered one, which should improve the prospects of today’s students as they cut down on beer to pay their tuition fees.