Treatment of MortalityA lot of what we are turns out to come down to us from our parents. No surprise, really. Without our parents, we wouldn't be here to wonder about it.
Medewar treatment of mortality works like this. First, bear in mind that everything in our genomes was passed down in this way ever since the first cell, and has developed its particular combination of traits due to selection. Selection just means differential success in passing on genetic information to the next generation. Our particular genes (and therefore we ourselves) are the outcome of that selection over many, many, many generations.Now, since selection means nothing more than differential reproduction, it simply doesn't work at all on traits expressed after the age of reproduction has past. So, there has never been selection against forms of the genes which code for products that just wear out after reproduction is over. And there is no selection for products that last any longer. We could be built of longer-lasting parts, but that would not pay off in the game of selection.
In fact, longer-lasting parts would probably be costly (in terms of energy) to fabricate and that would diminish the energy available for reproduction, so there is probably selection against building organisms out of unnecessarily durable parts. [This interpretation is charmingly called the "disposable soma" theory.] Consequently, of course our parts all wear out after reproductive age. And they all do, at various rates, as you and I notice.
AHA, you say, there is a logical circularity in this argument. Why is there the decline in reproductive ability with age? Doesn't this phenomenon itself smuggle in the concept of ageing? But natural selection explains this too. You see, in the wild scarcely any animal lives much beyond the years of peak reproductive activity. Most animals die of starvation, predation, or infectious disease in what we would call youth or early middle age. In nature, death scarcely ever occurs because of old age. So, there has never been selection to maintain the machinery of reproduction, much less the machinery of survival and good health, beyond the earlier ages at which virtually all animals die anyhow.
Hence, the very existence of survival into old age is an artifact of civilization. It has been enjoyed by ourselves (and our pets and zoo animals) for only that last few seconds of evolutionary time --- far too little time to affect the structure of our genes.
So there it is: old age itself is an artifact of civilization, like literature, antibiotics, Heathrow Airport, the World Wide Web, and patio furniture. Darwin wrote of natural selection that "there is grandeur in this view of life." Speaking now that I am an artifact myself, like my lawn chair, it comes to me that there is a sense of humour, of sorts, in this view of life as well.
The evolutionary view of mortality has clear medical implications, but they are none too cheery. Since our somatic machinery is all built of shoddy parts, like a Ford Taurus, all our parts start wearing out at about the same time. Fixing this one part or that other part will not save us from mortality.
The way the organism as a whole ages is hard-wired, so to speak, into the way all the parts interact with one another, the organism's entire physiology. The total physiology of some animals does permit longer function than others, but this is no help to us, because we are constructed the way we are. In other words, the only way to live as long as a carp is to be a carp.