by Primo Levi
The opposite path, that of canceling the asymmetry produced by nature instead of imitating it, is infinitely easier; from the point of view of energy, “it goes downward.” Outside the living organism, asymmetry is fragile; prolonged heating or contact with certain substances with a catalytic effect is enough to destroy it. More or less rapidly, half of the asymmetrical compound is transformed into its mirror image; the order of asymmetry has turned into the disorder of symmetry (or counterbalanced asymmetry), just as when we shuffle a deck of cards arranged by suit or color. Extremely slowly (on a scale of millennia) this process also occurs spontaneously and at a normal temperature, so that it is used to date items that in the past were part of living organisms, such as bones, horns, wood, fibers, and the like; the more advanced the loss of asymmetry, the older the object.
In the face of life’s maniacal preference for asymmetrical molecules, Stanley Miller’s famous experiment, it seems to me, loses some of its impact. In 1953, Miller subjected a mixture of water, methane, ammonia, and hydrogen to electrical discharges for several days, trying to simulate the conditions of the primordial atmosphere jolted by lightning. He obtained several well-known amino acids, thus confirming that for their synthesis the complex and selective methods until then followed by chemists were not indispensable. The fundamental building blocks of proteins “are eager” to form; they form almost spontaneously from chaos provided they are given energy, even in a crude form. Amazingly, some complex DNA components are also formed along with them. However, Miller and his numerous followers always obtained symmetrical products—i.e., balanced mixtures of the respective mirror images. The building blocks of life are eager to form, but asymmetry isn’t.
I have not yet mentioned an odd and troubling fact. I do not know what fool first said that “the exception proves the rule.” It does not prove it at all; it weakens it and calls it into question. Now, the rule according to which the amino acids of all living beings are in an optically active form (that is, are not mixtures of mirror images) knows no exception so far. There is, however, an exception to the rule according to which all these amino acids belong to the left-handed group. Amino acids of the right-handed group have been found in a few unusual, extremely marginal niches such as the skin of some exotic batrachians, in the cuticle of certain microorganisms, possibly (if confirmed, this finding should make us ponder) in some cancerous cells. But the right-handed amino acids of batrachians are not there by accident; they are part of substances carrying out intense physiological activity and if they are replaced by their normal—i.e., left-handed—mirror images, the activity ceases. Thus, they have a specific purpose, but we don’t understand it. And why do they exist only in those tissues, and not elsewhere? Maybe “once upon a time” their presence was more widespread, maybe they are the remnants of a different biochemical era? The exception does not prove the rule; it just confuses one’s ideas.
Thus, the asymmetry we are talking about is fragile. It is, however, unfailingly present in living matter, where it may be an evolutionary necessity to prevent spatial “errors” in the construction of proteins. We still have to consider the second, much more mysterious question, again with Aristotle: that of the efficient cause. Having recognized, or at least suspected, the usefulness of asymmetry (there are other asymmetries; the one under consideration here is called chirality—how easy it is to give Greek names to things we do not understand! Afterward, we feel we understand them better), we must ask ourselves where it could have originated. Evidently, in another asymmetry—but which one? Let’s examine the different hypotheses that have been, or could be, put forward.
1.The Earth rotates, and the Sun appears to rotate around the Earth. In the Northern Hemisphere and north of the tropic (and, similarly, south of the Tropic of Capricorn) the asymmetry exists, and it is conspicuous. For those looking southward, the Sun rises on the left and sets on the right. Certainly this has an impact, for example on the direction in which vine tendrils curl, and maybe also (I would kindly ask the experts to confirm or deny this) on the torsion of the trunks of many trees. It would be interesting to verify whether that direction is, at least as a trend, the same for all trees of a certain species in the Northern Hemisphere, and the opposite for the trees of the same species in the Southern Hemisphere. In fact, the “handedness” (the chirality, that is) of all phenomena related to the rotation of the Earth is reversed when we change hemisphere: the erosion of riverbanks, the preferred rotation of vortices, the direction of trade winds. This is a major obstacle to our first hypothesis; it would lead us to admit that life, or at least the asymmetry of life, regardless of how it originated, originated in only one of the two hemispheres and subsequently spread to the other when it was already linked. This is not impossible, but it is not appealing, either. It makes you think of a one-time phenomenon, a possibility that is not pleasing to many but very pleasing to a few. I will discuss it later.
2.Circularly polarized light is easily produced in a laboratory. It is less easy to explain in plain language what this light is; suffice it to say that it possesses the symmetry (or asymmetry) of the thread of the screw—that is, it’s “chiral,” and therefore can be right-handed or left-handed. This light, in suitable wavelengths, can be absorbed in different measure by one of the two mirror images of a pair, and can decompose it faster than the other; or it can act on the reaction mixture with which the chemist is trying to synthesize an asymmetrical compound. In both cases, the experiments have produced compounds that are unbalanced, and therefore optically active, although to a very small degree. Now, under certain conditions in nature the light reflected by water is circularly polarized. However, depending on the angle and the hour, there is an equal probability of its being right-handed or left-handed. We encounter here a difficulty similar to the one of the first hypothesis: Is it admissible that life exists thanks to a single, precise event; that the light reflected at a certain point in time by a certain puddle of water was instantly captured?
3.As I mentioned earlier, we find this asymmetry in nature in some inorganic crystalline structures, including, among
others, common quartz. There are right-handed and left-handed quartz crystals; in laboratory syntheses of asymmetrical compounds in the presence of quartz powder of homogeneous “handedness,” the resulting product was optically active. But, again, both right-handed quartz and left-handed quartz are equally abundant in nature. Actually, some researchers have maintained that right-handed quartz is more abundant, while others have denied it. Even in scientific research it’s easy to encounter individuals who replace what is with what they
wish for.
4.The Earth’s magnetic field, currently quite weak, possesses the required asymmetry, and possesses it at all points, without the reversals and the irregularity that weaken the previous hypotheses. Therefore it could have driven the same asymmetrical synthesis to the advantage of only one of the two opposites throughout the surface of the Earth. But here two other difficulties arise. First, as far as I know (but I have never had—nor do I have now, after so many years without practicing chemistry—any competence in this field; if anyone knows more, I will be glad to recant), there are no organic reactions sensitive to a magnetic field of reasonable intensity, except perhaps those involving iron, nickel, or cobalt atoms. Second, geologists are certain by now that the orientation of the Earth’s magnetic field reverses itself every few tens of thousands of years. Is it conceivable that this field, in remote ages, was vastly more intense, and constant for a sufficiently long time to incubate life?
5.The drama could have unfolded in different stages. A “primordial soup” like that obtained in vitro by Miller, made up in equal measure of right- and left-handed amino acids; then their aggregation into filaments, probably homogeneous, RRR . . . and LLL . . . ; the inception, according to one of the many hypotheses put forward, of life in a “two state” form, in which the two forms were unable to metabolize each other and were in competition; an extremely long Iliad, a silent contest l
asting millions of years between right-handed life and left-handed life, which are enemies and incompatible; and, finally, in the absence of a reversion, the gradual prevailing of left-handed life up until the current situation, in which the enigmatic presence of right-handed amino acids in the skin of tree frogs may be just a minuscule survival. It is really a pity that fossils (except for the most recent ones, as I mentioned earlier) bear no trace of organic tissues; otherwise one could hope to find in them evidence of that old controversy, vaguely reminiscent of Zoroaster. Or maybe Radiolaria or Diatoma skeletons exist in chiral forms? This would be a good subject for a dissertation.
6.The hypothesis of the single event, of the una tantum, is not appealing and does not take us far, but it can’t be ruled out. We saw it peeping out as a premise of some of the hypotheses I outlined. A germ (a DNA molecule, a spore, a protein fragment) containing the foundation of asymmetry and of life could have fallen from space. This time-honored proposal was recently revived by no less than Francis Crick, who discovered the genetic code. However, it merely shifts the problem to a place and a time that are not accessible to us. We are left with the option of a single terrestrial event, unique, spontaneous, and random: not impossible but extremely improbable. You can’t build science upon single events, and so the discussion ends quickly, in an act of faith (or doubt). This option is made less irksome by a phenomenon studied by Giulio Natta (the 1963 Nobel Prize laureate) and described in his beautiful book Stereochemistry. If we build long chains of molecules—i.e., polymers—with no special precautions, we obviously obtain symmetrical and inactive products. On the other hand, if the polymerization is carried out in the presence of small quantities of an inert but highly asymmetrical substance, then the resulting polymer is also asymmetrical for its whole length. The inert substance acts as a sort of mold; from an asymmetrical mold we can obtain asymmetrical pieces in virtually unlimited quantities. Let’s resort to another comparison: by pressing dough on a perforated plate we obtain straight, very long spaghetti; but if the hole in the plate is crooked we obtain spaghetti that is equally long but curled, that is asymmetrical, leftward or rightward according to the shape of the hole. In other words, there exists, or we can imagine, a multiplying mechanism that could have magnified a local asymmetry, originating in one of the above-mentioned causes or in an infinitesimal fluctuation, and led it to the conquest of the world. Besides, hasn’t the recent discovery of isotropic and fossil radiation forced the majority of scientists to swallow, along with the Big Bang, the bitter philosophy of the unique event?
7.Chirality could have universal roots. I will not try to pretend, I will not expect to make you understand what I have not understood and what can’t be understood in the customary sense of the word, namely by resorting to visual models. Chirality might reside in the subatomic domain, where no other language is valid but the language of mathematics, where intuition does not reach and metaphors fail. One of the forces that link particles to each other, the weak interaction, is not symmetrical. The electrons emitted by certain radioactive disintegrations are unavoidably left-handed, without compensation. Therefore, all matter, even below the sensitivity of our measuring instruments, is optically active. The mirror images are never true mirror images; one of them, always the same, the left-handed one, is a bit more stable than its brother. The inactive mixtures obtained by the chemist are never exactly fifty-fifty; there is always an imbalance, in the range of one in a billion billion, but unfailing. It is small, but so is the key to a safe holding a ton of diamonds. If this is how things are (the debate is very recent, still hot) the entire universe would be pervaded by a slight chirality, and the deviations from parity would be only apparent; the “true” mirror image of the right-handed lactic acid, or of my right hand, would be not the left-handed acid or my earthly left hand but those in the distant realm of anti-matter. Later, through the eons, the magnifying mechanisms we spoke of would have acted on this infinitesimal inclination, on this tendentious whim. All right? For the moment, this is the answer we must settle for.
Maybe I should apologize; it’s difficult to be clear about things that are not clear to us, to make oneself understandable to the layman without boring or shocking the experts. Moreover, I know I have strayed into a field that is not (any longer) mine. It was, however, the subject of my dissertation. I have returned to it with reverence, some regret, and the fear of having made mistakes; you pay a price for years of retirement.
I have tried to bring up to date a problem that remains current in spite of the innovative hypothesis that I attempted to describe last, with the reverence of the outsider, one who stops at the threshold of the temple. Maybe it is a “pointless” problem, even if it is not always pointless. If, for instance, pharmacologists had paid more attention to the question of optical isomerism, the tragedy of thalidomide could have been avoided. This product has an asymmetrical molecule; it was originally put on the market as a “raceme,” that is, a balanced mixture of the two mirror images as obtained from synthesis. Subsequent research—such as by Blaschke et al., Arzneimittel-Forschung (1979)—on rats showed that only the left-handed mirror image was teratogenic; the right-handed one had a normal tranquilizer action. Had the two mirror images been separated and examined separately, nothing would have happened.
At any rate, this is a fine and fertile problem. Unlike philosophical problems, this one, I think, will not remain unresolved forever; even minor discoveries may help. Are the amino acids apparently found in meteorites optically active? Has anyone tried to synthesize in a magnetic field an asymmetrical molecule containing iron? To me, the discovery of the chirality of the universe, or just of our galaxy, seemed overwhelming—dramatic and enigmatic at the same time. Does it have a meaning? And if so, what meaning? How far does it take us? Is it not a “game of dice,” the same that Einstein refused to attribute to God?
Prometeo 2, no. 7 (September 1984)
Foreword to Diary of a Jewish Boy During
the Second World War by Marek Herman
The diary of a European Jew that covers the years of the Second World War is dramatic by definition. The fact that the author survived to write it is in itself evidence of either an extreme determination to live or extraordinary luck. In this diary by Marek Herman, however, there is little room for luck. Marek, “born in L’viv—which was in Poland—on October 15, 1927,” owes his survival, and the well-deserved peace he enjoys today, much more to his own virtues than to good fortune. His virtues, which he never boasts of but which shine through every line of this unadorned account, are intelligence, courage, perseverance, and an unbelievable strength of mind in an adolescent.
This strong, enterprising man displays an unusual modesty. He describes the innumerable trials to which fate subjected him without ever raising his voice either in complaint or in hatred. At the young age of fifteen he had already lost everything a man can lose: family, country, home, language. Yet, stubbornly, systematically, Marek clings to that atavistic hope which has sustained and unified Israel through the millennia. From this standpoint we have here a truly exemplary text. Marek never yields to desperation or sorrow, he never stops to weep over the ruins, he never doubts that life is worth living. Paradoxically, in his painful journey, there is much more room for gratitude and love than for bitterness. Evil is present, it pervades everything, disrupts everything around him, but Marek does not allow it to corrupt him. He always sees a way forward for himself; yet he is not a believer, he has no political North Star, and he comes late even to the Zionist creed, when the great tragedy of European Judaism is coming to an end.
Page after page, we follow him, starting in childhood—a childhood that, already before the war, unfolds in an atmosphere of Dickensian poverty, where bread (literally!) is won in a daily struggle against exhaustion, disease, and entrenched Polish anti-Semitism. There is a home, albeit limited to a single damp, crowded, and dreary room-workshop, and in this home there is the sweetness of family love. The war comes, the German invasion, and ev
erything is swept away. One by one, family members disappear, and Marek learns precociously to live by his wits, until he is told that there is a barracks of Italians in L’viv. These soldiers are very different from the Germans, their formal allies. They are kindhearted and aren’t particular about military discipline, permissions, and prohibitions. In their barracks they shelter a dozen other young orphans, Jews and Christians. The Italians make no differentiation, and this surprises Marek; prudently, however, he conceals his Jewish identity and secures false “Aryan” papers.
When the Italians are demobilized and return to Italy, Marek follows them, and here begins his great adventure. Compared with Poland, the tragic Italy of 1943 appears to him a great country, rich and generous; everybody helps him and nobody betrays him. A peasant family in the Canavese region takes him in and treats him like a son. They even enroll him in a Salesian school where he learns how to serve at Mass—and he is “no worse than the others” at it. But shortly thereafter he comes into contact with Czech partisans and has no hesitation: he knows where right and wrong lie, he has an account to settle, and he becomes a partisan, even if he is “just a few centimeters taller than his rifle.” Within a short time, helped by his intelligence, eagerness, and knowledge of languages, he makes himself indispensable. He fights in the Orco, Lanzo, and Susa valleys, and is dazzled by the beauty of the mountains. He lives in an exciting new world filled with experiences that stir him and allow him to develop: the splendor of Creation, freedom, and faith in his fellow fighters. In the last few months, the American secret service entrusts Marek with a radio transmitter; on April 25 he is in Turin.