BIOLOGY: Why do we age ?

At the moment there seems to be only one robust method to slow down ageing in a wide
variety of animals, namely restricting caloric intake.

There have been thousands of scientific papers on this topic and many explanations have been put forward. Mostly people try to identify specific molecular mechanisms.

As a physicist I have always been quite sceptical to pin this effect down to something genuinely biological because in physics ageing is a commonly studied phenomenon (e.g. spin glasses, polymers) without being able to allude to mitochondria, hormones etc.

Therefore it is not surprising that the model I favour most at the moment stems from a physicist, Jeoffrey West, who has done quite interesting work in biology about scaling laws.

Here are the transparencies of a talk where he, among other things, explains this inofficial model of ageing which I find highly plausible:

1 2 3 4 5 6 7 8

The basic input into the model is the finding that branching networks that supply the body with nutrients and energy scale as M^0.75 (they are fractal!), where M is the body mass. (Naively one would expect M^1.0).
This way the well known one quarter power law for life expectancy T, T ~ M^0.25 is reproduced.

Interesting are also West's remarks, that lifespan per se is not given genetically. What is given genetically is the repair rate. So if one wants to manipulate lifespan, one would have to modify the repair rate and there is nothing else one could do.
(Interpreted this way, the difference in life expectancy between a dog and a human being for example is not due to their different genetic makeup but due to their mass specific energy expenditure, whereas the exceptionally long life expectancy of birds in regard to their low body mass, is due to their comparatively good repair mechanisms).