Evolving Diseases

Subject to one of those inexplicable brain itches, I have recently been ruminating about Self Interest and the astonishing range of its implications.  Self interest is a very funny thing.  Sometimes it is so funny it looks a lot like altruism.

A few years back, Michael Crichton popularized the rule that an intelligent parasite does not destroy its host.  In the long run, any bug that is too successful at working its will upon its host  finds itself killing off the very entity it needs for its own survival.  It is not in its self-interest.

One can see the impact of this Rule in the history of the great plagues diseases.  These diseases, when they originally appeared, were fantastically infectious and staggeringly fatal.  However, each successive wave of a disease’s recurrence tended to be a bit more moderate, a little less fatal.  Not doubt some of this effect was due to the acquired immunity of the human population.  But not all.  Some of it was caused by the changing nature of the diseases themselves.  Many of our more common diseases, such as measles, syphilis, et alia, made their appearances in history as fulminating scourges that caused death within days.  Now they are more moderate infections some of whom, like measles, have become relatively benign, endemic diseases.

The process occurs at its most basic in viruses.  There is an elegant simplicity to viruses.  Essentially, they have only two functions.  Unable to reproduce (or, indeed, exhibit any functions of life) on their own, they first have to invade a living cell.  Second, they have to turn the cell, in whole or in part,  into a machine to make more viruses.  Any changes in a virus’s behavior is the result of purely evolutionary forces: one mutation is simply more successful than another.

Some viruses virtually take over the entire cell’s functions and convert its whole metabolism into wild production of viruses.  The cell ceases to be useful to the host.  If the cell (or that cell type) is vital to the organism, that organism dies.  According to Crichton’s Rule, in the long run this is not a wise strategy for the virus.

Recent virological studies have shown that most, if not all species, plant and animal, have at least one “wise” virus that seems to cause virtually no harm to its host.  Frequently, the host is infected in its youth, suffering mild symptoms.  Thereafter, the virus becomes endemic, continually being produced by the host in small numbers and expelled into the environment in such a way as to infect new hosts to continue the process.

But if that virus happens to jump to another type of organism, the result can be disastrous.  An example is the HIV viruses.  They began (according to genomic studies that compare genotypes for near relationships) as simian viruses, benign viruses endemic to species of apes.  Somewhere in the process of man’s invasion of the rain forests, hunter man and infected apes crossed paths in collisions that resulted in blood being spilled.  This gave the SIV (Simian Immunological Virus) viruses the opportunity to jump species and become HIV.

In their new host, the viruses lacked the adaptive match with its host to act as a “wise” parasite.  Instead, it proved virulently fatal.  It took up residence in a vital cell type and, through its reproduction demands, caused the death of that type and thereby its host.

In time, the HIV virus may moderate its effects and become progressively less destructive.  It could even become harmlessly endemic.  Harmless, that is, within its new host.  Should it find itself in a fresh host type, or even within a different immunological milieu, it would revert to its fatal habits.

But even this might not to too “unwise,” if looked at from a different perspective.  If you take Crichton’s Rule one step further, we could say that a very wise parasite would not only not harm its host, but it would actually protect its host.  In effect, it would become symbiotic.

This is not as far fetched as it might sound.  Consider: There is a species of squirrel monkey (saimiri sciureus) that lives in the canopy of the Amazon rainforest.  Relatively defenseless, it has nevertheless carved out and held onto a nice ecological niche for itself.  Should some invading marmoset wander into their territory (sharing the same taste in real estate), the squirrel monkeys simply breathe on the intruder.  The invader dies within a couple of days of massive, fulminating lymphatic cancer.

The trick, of course, is that the squirrel monkey carries a virus (herpesvirus saimri) which is harmless to the squirrel monkey but invariably fatal to invaders.  The result is that the harmless squirrel monkey is able to hold onto its highly desirable real estate in a very competitive market.  The virus has, in effect, protected the ecological niche of its host, thereby benefitting both.

What is most fascinating about this little scenario is how narrowly targeted it is.  A virus cannot, as a general rule, jump too far away from its biological host.  The squirrel monkey’s virus is not likely to jump to some bird that might want to share the forest canopy.  Their genotypes are too far apart.  From the monkey’s point of view, that’s fine, because the bird is also too far away in its habits to constitute real competition.

One can take this same model into some unexpected places and arrive at some surprising insights.  Take, for instance, the matter of the Europeans “discovery” of the New World.  Wonderful, prime real estate, simply bursting with opportunity.  Problem was, it was already occupied by millions of beings with exactly the same tastes (i.e. Amer-Indians) who had no desire to give it up.

No problem.  The Europeans carried such a bushel basket of endemic diseases such as measles, small pox and the like, that the new real estate was effectively cleared in next to no time (it is arguable that something between 80 and 90 per cent of the native population died of the new diseases before they ever saw a white man).

In return, the Amer-Indians, who had lived too long in a low competition environment, appear to have been able to retaliate with, at best, some new venereal disease strains.  In viral terms, they had been out-gunned.

One can argue that the disease bugs had acted in good symbiotic fashion, clearing the land for the greater health and expansion of their hosts.  The hosts, in turn, massively reproduced, thereby guaranteeing the continued existence and expansion of the bugs.

All life on this planet is based upon molecules that have a unique and peculiar property: They can self-replicate.  Exactly where, when and how these molecules first appeared on the planet is a matter of some debate.  Many have argued and continue to argue that, except in wildly improbable circumstances, such molecules could never spontaneously appear in the primordial chemical stew.  They argue that the chances of such an event occurring here on earth in the time allowed is vanishingly small.

If, however, it had occurred somewhere else in the galaxy once upon a time, those molecules (or their resultant organisms) could have been scattered by some cataclysmic event to act as seeds to begin the process here on earth.

In other words, maybe life on earth was triggered by extra-terrestrial invaders who impacted the earth billions of years ago.

It is not too wildly improbable an idea.  After all much of the cosmic stuff that impacts the earth, such as comets and the like, are in the form of “dirty balls of ice,” ideal conveyors of molecules or micro-organisms.  All it would take would be for self-replicating molecular fragments to reach the surface and start the process.  Evolution could produce the rest.

However, we are still being bombarded with space debris.  Does this mean we are in danger of being infected by some charming alien disease?

Not very likely.  Our best protection is our atmosphere, which would burn up most invaders in their passage before they got to the surface.   If a few molecules or even organisms got to the surface, they would likely be so damaged by their passage as to be raw materials for  this teeming hive of hungry competitors that is our planet.

Unless, of course, we went out and retrieved some and deliberately brought them, intact, back to earth.  Say, in the form of ice from the moon.

But not to worry.

On the one hand, after all of the billions of years of evolution it is unlikely any intruder bug would be a good enough genomic “fit” to earth DNA to be a threat.  On the other hand, if one was close enough, it would be perceived as a threat by all those symbiotic viruses of the earth bugs whose ecological niche it threatened.

Okay, you say.  So much for the microscopic. But what if we find ourselves defrosting not some microbe but some alien cousin of ours standing eight foot two with the same greedy taste in real estate that we have?  What do we do then?

Well.  Even here I think we must have faith in the power of Self Interest.  My advice would be to borrow an update of the old science fiction stories:  We’ll go and breathe on him.

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