If I'd known I was gonna live this long, I'd have taken better care of myself.

- Eubie Blake, on reaching the age of 100

In homage to Archimedes, I have always found the bath a good place to think and it was there, one February night in 1977, that it suddenly dawned on me why aging occurs.

The reason that you and I will grow old and die, I am sorry to say, is that we are disposable. And the saddest thing is that this assessment of our disposability is made by none other than our very own genes.

It is not that our genes actively destroy us - this does not make biological sense. It is that the interest that our genes have in keeping us going does not happen to coincide exactly with our own. This, then, is the bad news. The good news is that, once we realize what it is that our genes are up to, we are on track to discovering what it is that actually causes us to age. As we shall see, instead of being deterministically time-limited by some fateful hourglass, there is a plasticity to the course of our lives that we may be able to turn to our advantage.

At the time of my bath, I had been puzzling over the problem of why normal human cells grown in the test tube invariably age and die, and about what this might have to do with the aging process as a whole. I had also been grappling with the idea that cells might undergo this aging process because they were vulnerable to a hypothetical process known as "error catastrophe." Error catastrophe is a bit like the dreadful shriek you sometimes get in a public address system when the microphone picks up noise from the loudspeakers, and the sound gets amplified around and around the loop. In cells, feedback loops exist in the molecular machinery that makes DNA and proteins, because the same machinery also makes new copies of itself. A mistake at one step can create a defective unit, which in turn makes even more mistakes, and so on and on. The possibility therefore exists that mistakes can be amplified around these loops. Some mathematics had shown that error catastrophe was a theoretical possibility, but that cells could avoid it.

The question I was pondering among the soap bubbles was why cells might succumb to error catastrophe if, as my calculations showed, they did not need to. "Why?" questions were much on my mind at the time, for I had recently met John Maynard Smith, grandmaster of evolution theory, and been busy reading a series of his papers with provocative titles like "What Use Is Sex?" and "Why Be a Hermaphrodite?"

Two other concepts were also revolving around in my head. One was an idea proposed two years earlier by scientists in California and in Paris to explain how cells manage to make proteins as accurately as they do. Protein synthesis is like stringing beads, but the trick is to select the right bead each time from the twenty different kinds that are available, and to do this at speed using rather basic molecular machinery. The new idea, called "kinetic proofreading," showed that cells could in principle be as accurate as they liked, but only at a cost. The cost was the use of extra chemical energy that would be needed to fuel the high-accuracy selection process. This was important because the model of error catastrophe showed that cellular meltdown could be avoided if proteins were made very accurately.

The other concept in my mind was the much older idea of the germline and soma distinction, which was introduced into biological thinking in the late nineteenth century by the German naturalist August Weismann. The germline must be immortal if life is to continue, but no such consideration applies to the soma.

What I realized in my bath was this: a multicellular organism needs a lot of accuracy in its germline, which must transmit its genes to the next generation, but it does not need so much accuracy in its soma. Sooner or later the soma is going to die by accident. Might it not be better to save energy and make somatic cells in a more economical way, even if this results in them aging? The answer to this question hinges on how long you need the soma to last.

Let me illustrate this with a simple analogy in the form of a fairy tale. Once upon a time in the far-off land of Senescia, there lived a princess who wished to choose a husband. Three eligible princes gladly offered themselves as candidates and the princess, finding them all equally attractive, chose to set them a competition. Each of the princes was given a hammer, a chisel and a very large pile of stones. The winner would be the one who in 24 hours could build the greatest number of stone towers 1 meter square and 2 meters tall.

Prince Albert, eager and rash, cast his hammer and chisel aside and piled the stones one on top of another just as fast as he could manage. But his towers wobbled and fell, and when the 24 hours were up, poor Albert had built six towers, but all he had left were six jumbled heaps of rock.

Prince Brian, a methodical fellow, set to work at once with hammer and chisel to shape each stone into a fine rectangular block. When the time was up, Brian had a marvellous structure that would surely last a lifetime, but only half of a single tower was complete.

Prince Cedric, a canny chap, used his hammer and chisel to knock a few stones quickly into shape for the base and corners of each tower, but he filled in the walls between with rough, unhewn stones. After 24 hours, Cedric had two towers standing, albeit with slight cracks beginning to show, and a third tower half built. Cedric was the undisputed winner of the hand in marriage of the prudent Princess of Senescia. And they lived happily …

The point of this tale is that Cedric's corner stones correspond to the germline whereas the rough infill of his walls corresponds to soma. His strategy was the winning one because it optimized the deployment of his time and energy differentially between the two. It was an idea along these lines (minus the fairy tale) that occurred to me as I bathed.

Eureka!