I am not a biologist (for now): Far be it from me going into the details of a field I don’t fully understand. The objective of this website is to provide an elementary introduction based on what I have learned from more knowledgeable people than me, for the benefit of who is new to the topic. For this reason, I beg you to take anything you read on this page (and any others that deal with the science of ageing and rejuvenation) with a grain of salt; should you be interested in the details, I recommend to delve into them on your own. These pages may well contain errors, which you should attribute to me, not to the theories I try to explain or to the people who came up with them. I will hardly be able to do them justice, so if you have any serious perplexity, you should talk about them to the aforementioned more knowledgeable people (later on we’ll see who they are).
Enough talk now; let’s get to the point. Just what the heck is ageing?
Definition and an important note
The operational definition of ageing we’ll use is the following:
Ageing is (biologically speaking) the result of a damage accumulation process at cellular and molecular level. This process takes place throughout the body and throughout life; the damage it causes is a side effect of the normal metabolic functions the body does to stay alive. With time, the accumulation of damage leads to age-related pathologies, and eventually, death.
If the word ‘metabolism‘ doesn’t ring any bell, you can think of it as the vast network of chemical process that keep your body alive. It’s easy to imagine that a tangle of chemical processes may have undesired effects, but you might wonder why this should be true in the case of living being. After all, they evolved for millennia, ‘perfecting’ their inner workings and eliminating all kinds of flaws they might have. A metabolism such that it inflicts damage to the very body it tries to keep alive seems an obvious enough flaw to be fixed, right? Well, yes and no.
I think this point is definitely worth spending a few extra words on. You should keep in mind that the ability to reproduce is what determines whether or not a species is successful, evolutionarily speaking. Within a given set of creatures of the same species, individuals who are better at reproducing will take over; those less good at it will slowly disappear. For example, suppose there’s a species whose reproductive age starts at three years. Further suppose that a group of individuals of this species has a tendency to become gravely sick before they reach their third year of life. Throughout centuries, members of this group will become sick and die before they had a chance to reproduce more often than the healthier members of rest of the species do. In the long run, the unlucky sickly creatures and the genes that made them such will be eradicated from the gene pool because of their very inadequacy. The leftover members of the species will therefore be those who tended to make it to their third year in good enough health to reproduce. This applies to species (animal or not), us included. Now suppose some individual in our hypothetical species develop mutations that improve their metabolism, so that it causes less damage and allows them to live longer past reproductive age. This is certainly good news for the mutants, but it is not such a big edge over the non-mutants to drive them to extinction: both mutant and non-mutants will happily reach their third year, reproduce, and go on living however long they will. Both mutants and non-mutants will spread around more or less equally, even though mutants live longer, because there’s nothing preventing non-mutants from reproducing too. The end result is that mutant and non-mutant genes mix together in the pool throughout generations, until pro-longevity mutant genes are diluted in the pool and thus don’t take over.
In short: features that do no influence the ability to reproduce of an individual, such as post-reproductive longevity, are not subject to natural selection, or are so very mildly, because there’s no evolutionary pressure to trigger it. Even more simply, we grow old because evolution had no reason to do away with ageing. Individuals of most species, including ourselves, are generally very robust (that is, young and thus healthy) until their reproductive age. After this point, they begin to slowly fall apart, because evolution had no reason to bother preventing it. Ageing is the result of evolutionary neglect, not evolutionary intent.
Simple non-biological examples of damage accumulation processes
As a first example of this kind of process, imagine a clean, tidy room. Books are on the shelves, the bed is made, the floor is clean and washed, everything smells nicely and is exactly where it is supposed to be. However nice this may be, we all know it’s not going to last very long. If you lock the room when it’s still clean and you never set foot in it, it’s probably going to stay clean for quite a while, but if you use it regularly, you will end up leaving pens and books out of place, scattering papers all over the desk, spilling the occasional drop of coffee on the floor, and so on. They’re just tiny little things that don’t seem much of a trouble if considered individually, but they are very sneaky: They happen all the time as a side-effect of using the room, and little by little they’re going to turn your room from an immaculate, spotless retreat into the circle of Hell where slobby people go when they die.
You can of course try a preventative approach: never leave anything around, never spill anything, and so on, but let’s face it: It’s not gonna work. If you keep using your room without ever cleaning or tidying it up, it will get to a point where it’s so filthy and messy you’d rather French-kiss a wild boar than set foot in it. The room will become unusable.
Another good and more fitting example (which I shamelessly stole from Aubrey de Grey‘s repertoire) is that of cars. A car is a machine with moving parts; as you use it, these parts will wear out because of friction, oxidation, mechanical stress, and what you have. The daily amount of damage the car inflicts on itself as a consequence of its use isn’t much, but it piles up day by day. Without proper, regular maintenance, your car will eventually become a useless piece of junk.
These are both a pretty good analogies for ageing: As you ‘use’ your body—and by use I mean simply ‘being alive’—you constantly cause some damage to it (or rather, your metabolism does). The damage occurring in a day is pretty much insignificant, but it piles up throughout your life. The good news is, your body is set up to tolerate a certain amount of that damage, and it possesses biological, self-repair machinery that fixes you up to an extent; the bad news is, your body can tolerate only a certain amount of damage, and past a certain point (the peak of reproductive age) even your self-repair machinery will start falling apart. In time, the damage produced by your metabolism starts piling up faster and faster, until your body goes south on you and all kind of age-related diseases and disabilities will begin manifesting themselves.
Unfortunately, this is not all. Even if your room gets insanely messy, it’s hardly going to crumble to dust because of that, and you’ll always be able to clean it up. Too bad this is not true of humans: If your body’s inner workings get too ‘messy’, you die. Similarly, without maintenance, your car will also fall apart beyond repair, too. (As a side note, I’ve heard people comparing ageing to ‘slowly walking into the sunset’. It seems to me it’s actually more like being tied to a wheelchair as you dash down a precipice overlooking a scorpion pit. With or without sunset—it doesn’t make much of a difference.)
By now you’re probably wondering what is this damage I keep talking about, and how it could be fixed; we’ll see this in the next article.