Cameron Park - June 4, 2001
In the wake of the latest report on preparing for samples from Mars, the old arguments for and against have again taken center stage in this perennial debate.
The basic argument against returning Mars samples is that the chances that "extant" (that is, still-living) microbes still exist on Mars are higher than NASA is making out, and that there is a genuine and serious chance that such microbes might prove harmful to Earth's biosphere -- and perhaps to human beings themselves. How accurate is this?
This argument is largely based on two themes: that Dr. Gilbert Levin -- the creator of the Viking Landers' "labeled release experiment for detecting Martian microbes -- still thinks, unlike most scientists, that his instrument did detect viable Martian microbes growing inside it; and that the chances that such microbes might be dangerous to Earth are really quite high.
In his book "Mars The Living Planet" Barry DiGregorio writes: "One of the most common misconceptions about bringing back living microorganisms from Mars is that because 'Martian bugs' would have evolved independently of 'Earth bugs', they will be harmless to life on Earth."
This theory assumes that Earth life has had millions of years of competitive [evolutionary] struggle... [which] is what makes microbes effective enough to cause harm, and that a Martian microbe would not 'know where to begin' and that any feeble attempts at 'learning' how life on Earth 'operates' in order to take advantage of it, would be 'fended off' by immune systems with millions of years' worth of head start...
"However, the alarming truth is that Martian microbial life would not have to 'learn' how Earth life 'operates' at all. All that is required is that Martian microbes find Earth life to be a good source of food, and then have the capacity to harvest it... All higher life on this planet has evolved in the context of the microbial life that exists here.
That is, it evolved to cope with the latter's constant incursions... It is unlikely that terrestrial animals and plants could withstand attacks of alien microbial life which it has not evolved to cope with.
"All any Martian microbe has to do is perceive Earth life as a source of food, and then have the capacity to 'harvest' it. For example, if Martian microbes had amino acids similar to terrestrial life and liked some of the same compounds -- that would be good enough to make us attractive to them.
Earth microbes might in turn find the Martian microbes a good food source, but by no means does that imply they would wipe it out. When the entire biosphere hangs in the balance, it is adventuristic in the extreme to bring Martian microbes directly to the surface of the Earth as is now planned."
The Terra of Fightback
Without this ability, after all, we would literally be rotting alive, as our bodies were constantly invaded by unfamiliar species of Earth bacteria, which we regularly encounter on a new basis -- all of whom are eager to "harvest us as a source of food", and promptly do just that as soon as we die and our immune systems shut down.
(Indeed, I wonder whether this may be one of the reasons why complex multicellular organisms evolved in the first place -- they can develop more complex immune systems, in which a single immune cell which discovers a successful antibody against an invading germ can quickly spread that ability to the rest of the body.)
The simple and dramatic fact is that your body is the site of a constant and dramatic war against new types of invading enemies -- and that war is almost always entirely successful, or we wouldn't be around.
Our immune system is a kind of chemical military computer capable of adapting quickly to attacks by a huge range of enemies. The few species of bacteria that can successfully slip around it and establish themselves in large numbers in our bodies are those which have themselves stumbled across, by evolutionary chance, some complex stratagem for doing so -- a stratagem that is specifically tailored to deal with the operating details of the human immune system (and often involves a specialized attack on some aspect of the immune system itself).
As for viruses, they have evolved in an even more specialized way to commandeer part of a particular cell's genetic sequence in order to reproduce themselves -- most of them can infect only one species of living thing, and virtually none of them infect more than a few.
Any Martian microbes would be biological "rubes" on Earth, almost certain to be obliterated by Earth organisms' immune systems with which they would be unprepared to cope.
But the risk of a Martian disease harmful to humans -- while incredibly small -- is not quite zero, because a small number of Earth diseases (such as the botulism germ) secrete, by sheer chance, substances so toxic to humans that even though the germs are swiftly killed off by our immune systems, they can still do fatal damage to the body during the brief time before that occurs. (This is also what Michael Crichton's fictional "Andromeda Strain" did.)
And what about the risk that an invading Martian microbe might find some niche to its liking in Earth's incredibly varied ecosystem and establish itself as a serious ecological pest?
Again, the chance that it would be a disease bacterium specifically adapted to thrive in the body of any species of Earth organism is virtually impossible; but it still might establish itself as a new species capable of competing with Earth organisms for some food source and thus crowding out some native Earth species -- as has certainly happened often enough on Earth itself, when foreign newcomers found some new country greatly to their taste and crowded out native species to the point of extermination.
Microbes, unlike more complex organisms, are so unspecialized in their food sources that it is extremely unlikely that any Earth organisms could be starved into extinction by competition from some invading Martian germ -- but the possibility does exist, and it is distinctly more likely than the chance of a harmful Martian disease.
(And, again, what if -- by sheer long-shot bad luck -- some Martian microbe that happens to secrete a poison to which Earth organisms have not yet evolved a defense should thrive on Earth?)
In short, while I am convinced that DiGregorio has tremendously exaggerated the risks that alien organisms might present to Earth, I also think that some other writers -- including Robert Zubrin in his book "The Case for Mars" -- are much too blithely sanguine about the risk.
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