Why do humans age?

Aging, or, “growing old” is a very interesting biological mechanism.  It fits the definition of the term “disease” in almost every sense.  Yet we, as well as the majority of species on this planet, are born with it and it would appear that our bodies are specifically designed to age and expire.

Take, for example, the telemeir.  A telemier is a strand of non-coding DNA that exists on the tails of our coding DNA strands.  Each time the cell divides, the telemier becomes shortened.  When the telemier is gone, the cell will no longer reproduce.  This is the principal cause of skin aging.  Your skin cells are constantly dying and reproducing.  Every time you cut yourself, the nearby skin cells must reproduce to produce new skin cells to fill in the gap.  But eventually there aren’t enough cells left with long enough telemiers.  They die and can’t be replaced.  So as we age, our skin becomes thinner, and appears “wrinkled” as it more closely hugs the bits it’s hiding – bones, tendons, veins, and so on.

Telemiers are a feature of DNA.  They evolved.  Our bodies evolved a self-destruct mechanism.  But why?  That doesn’t make any sense!

It’s not just us, either.  All mammals, all birds, and most reptiles have telemier strands on their DNA.  Their length in effect determines the life span of an animal.  I have a pet rat.  She is grey and elderly after only 2 years.  Why wouldn’t a rat who lives in total comfort and protection from predators not live to the same age as a human?

There are two primary reasons, both of which are entirely motivated by a cause much higher than individual survival – species survival.

The first reason that animals have a built-in expiration date is to encourage evolution.  In a species that allows its members to potentially live forever, the same individuals would be able to reproduce significantly more often.  This would have a net negative effect on diversity.  Each new generation has the potential to mutate and change.  If old generations are allowed to continue to reproduce with newer generations then it is more likely that any mutation would be bred out of a population since the “old blood” could mate with a mutated younger generation and possibly undo or dilute that mutation in their offspring.

It’s easy to see this principle in action.  One of the few families that we know that does not seem to have a fixed life expectancy is the crocodilians.  We also know that crocodiles are much the same as they were many millions of years ago.  They have evolved very little.  Why?  Because crocodiles are as close to perfect for their particular niche – river predators – as they possibly could be.  They don’t need to mutate further to enhance their survival so they don’t.  This also means that it wouldn’t matter much if a 200 year old crocodile mated with a 2 year old crocodile.  Their genome hasn’t varied much in millions of years.

The other, more pressing reason that creatures like us age for the benefit of the species is demonstrated by one of the two major human causes of death: cancer.  A cancer is essentially a mutation in a cell’s DNA that typically results in abnormal cellular reproduction.  In many cases the cell’s normal reproduction is supplanted by runaway reproduction which results in a tumor.  These cancerous cells begin to interfere with the functioning of the organism or at the very least begin to starve it of resources as these large, useless cell masses are still sucking oxygen and nutrients.   The result is usually death.

You might think, then, that we ought just let cancer solve the aging problem for us.  Since it is virutally inevitable that our species will at some point develop cancer on a long enough timeline, why do we also need to become frail and age as we naturally do?  Wouldn’t death by cancer correct the issues associated with the decreased diversity that non-aging would produce?

Possibly, but there’s another issue.  Cancer is a mutation caused by contamination of DNA, and in humans, it often happens after fifty years or so, due to the fact that we are constantly being bombarded with things that can damage DNA.  Sunlight, for example, causes melanoma.  Ultraviolet radiation can corrupt skin DNA, and eventually, that radiation might corrupt the DNA of a skin cell in such a way that it becomes a malignant tumor.  But it’s not just the sun.  It’s cosmic rays from space not all of which are blocked by Earth’s magnetic field.  It’s also trace toxins that we can’t help but be exposed to no matter how careful we are.  In the food we eat, the water we drink, everything we come in contact with.

Normally, said mutations result in cancer which kills the person.  But imagine a world in which we did not age naturally.  Although we humans in our present state are very susceptible to cancer, not everybody gets it.  If we didn’t age and die, over time, the people who appeared to be largely immune to cancer – or lucky enough to avoid it, through chance or behavioral practices – would be the ones who kept living.  They’d keep reproducing, too.  Look at crocodiles and sharks.  They don’t appear to die from cancer very often.  Whatever it is they do, they don’t seem to be as affected by DNA mutations as we are.

The key here, though, is that it is virtually guaranteed that even in these people who appear to be “immune” to cancer will experience DNA mutations.  DNA is simply not that resilient.  These people simply have not contracted fatal cancer, because either their body can defend against it (which our body does, for a large variety of mini-cancers that spring up in our bodies every day but we don’t notice because our immune systems handle it silently).  In a population filled with people who don’t age and die, over time, the DNA in their ovaries or testicles would become mutated.  And thus, they would pass down their damaged DNA to their offspring.  And now the species itself has begun to go down the path of corruption due to accumulated lifetimes of damage.

Although evolution is really nothing more than a series of mutations, remember that mutations go both ways.  Sometimes they help the individual survive better, but the vast majority of the time, the mutation in fact harms the invidual.  Evolution is also a very slow process.  Baby steps.  Hyper-mutation has a highly negative impact on species evolution as a whole, for the same reason that a scientist attempts to isolate one variable to test in every experiment.  When one is trying to tinker with a system to change its behavior, the smart one changes one thing at a time and observes the result.  If the result is positive, one keeps the change in place and then makes another change.  If the result is negative, he undoes it and tries something else.  I’ve just described evolution.  The dumb one, on the other hand, changes every aspect of the system at the same time and they tests.  Even when one notices an improvement using this technique, it’s not possible to isolate which action caused the positive outcome and therefore he has no idea which direction to continue in.  I’ve just described what happens when a large number of mutations occur in a single generation of a species.

If human beings are ever going to cure aging, we’ll first need to find a way to combat these two problems.  We are mostly immune to significant evolution – we’re terminal, like the crocodile – so the primary problem we must solve is how to keep our DNA intact.  Coincidentally, this is also the primary problemt that keeps us stranded on Earth.  Even a modest six month voyage to Mars would very likely kill the astronauts with cancer before they got there since we have no way of artificially shielding them from the enormous amounts of cosmic radiation they’ll receive along the way.  Their DNA is going to be bombarded with high doses of damaging radiation.

If we can’t shield them, then we have to repair them.  There is no way to prevent our DNA from becoming contaminated by our environments even here on earth, although we’re protected by our atmosphere and it takes a lot longer.  As a programmer, I can tell you the algorithm: for each cell in the human body, inspect its DNA.  If its DNA does not match the genome recorded at the time of your birth, replace it or repair it with a copy of your original DNA.  Simple enough.  But simple is not easy.  The means to check and repair the DNA of every cell in the human body – of which their are trillions – are still science fiction at this point.  The two most promising techniques would be some type of nano-machine or a retrovirus (custom made for each invidiual, of course – another very tall order).  If we had the ability to make body-wide DNA repairs, we would theoritically be able to restore our telemiers as well, thereby circumventing natural aging.

Will we see it in our lifetimes?  It is certainly plausible.  I suppose it depends on how old you are.

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