The Mitochondrial Free Radical Theory of Aging - Supernova: Pliki

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The Mitochondrial Free Radical Theory of Aging
Unfortunately, though this correlation certainly exists, it is not a fantastically good
correlation. Primates, birds and bats are probably the most extreme classes of homeotherm
that live a great deal longer than would be predicted by their sizes and specific metabolic
rates. The search has thus continued for other variables which, when added into the equation,
give a more universally accurate prediction of species lifespan.
6.5.2. How Does Lifespan Evolve?
The search for such factors has been greatly aided by evolutionary biology, so I shall
digress for this section into a summary of current theory regarding the evolution of lifespan.
I mentioned in Section 5.7.1 that lifespan seems to be rather easily adjustable by evolution;
since civilisation has found it so very hard to adjust by medical intervention, we may
reasonably ask “why?”
Well, first of all: why not? Evolution, of course, works by selecting between slightly
different sequences of DNA based on their phenotype, which largely derives from the
three-dimensional structure of the proteins that the DNA encodes. The mechanism(s)
whereby genetic makeup determines longevity is no different, in this respect, from any other
aspect of our physiology: it involves chemical reactions, and those reactions involve
macromolecules—mainly proteins. Thus, small changes to those proteins’ three-dimensional
structure, due to changes in their sequence, will affect the rates of the reactions in which
they participate.
So, here is the current evolutionary explanation for both the general “rate of living”
correlation and the major exceptions to it.* Fundamental Darwinian logic tells us that all
species gravitate, by natural selection, to a lifespan which maximises their chance of producing
offspring and raising them to maturity, and that this maximum is a balance between the
avoidance of predators and the optimising of the environment in which the offspring will
mature. An animal which is at a severe risk of death by predation will be best advised to live
its life fast, ensuring that it has some offspring before it succumbs. One whose risk of predation
is very low will, conversely, be best advised to wait until the environment is particularly
favourable for procreation—for example, until there is a particularly plentiful food supply.
And in order to wait, it must invest in better self-maintenance so as to avoid dying of old
age. Now, the simplest way of avoiding predation is to be large: this explains why, in general,
large animals live long. But that is not the only way to avoid predation: one of the other
good ways is to be able to fly, so this also explains why bats and birds live so long. Finally, an
even better way to live a long time is to be highly intelligent; that is what gives primates (and,
among them, humans) their advantage. This general idea, that the main determinant of
lifespan is one’s position in the food chain, is the ecological description of current thinking
on the evolution of lifespan. 27 It has also been strikingly confirmed by intraspecies
comparisons. 28 Another way to say essentially the same thing is to note that, while the cells
of an organism's germ line must be maintained in a pristine state in order to give rise to
viable offspring, the rest of us (our somatic cells) need only be maintained well enough to
stay just about working throughout the optimal lifespan that our ecological niche determines.
This is, naturally, called the disposable soma theory. 29
* This theory is the culmination of a century of analysis of the evolution of lifespan. It and its predecessors,
due primarily to Weissmann, Haldane, Medawar and Williams, are discussed in detail in, for example, ref.
43.