WHAT WAS THE ancestral form from which all known living beings descend? When did it appear? Where did it come from? At the time of my youth, the possibility of answering those questions was so remote that very few biologists bothered to ask them. Vitalists, of one ilk or another, felt the questions to be unanswerable by science. Even those biologists, probably a majority, who believed that life must have arisen spontaneously by purely natural phenomena, theoretically accessible to research, mostly considered these phenomena unknowable in the existing state of science and not worth investigating. I remember hearing, at the Third International Congress of Biochemistry held in Brussels in 1955, a lecture by the acknowledged pioneer of origin-of-life studies, the Soviet biochemist Alexander Oparin,whose book titled The Origin of Life on Earth was first published in 1924. Prefaced by the ritual homage to Stalin, his exposition struck me as laughable, if not suspect of sinister, Marxist connotations. It seemed to me futile to look for the origin of something of which almost nothing was understood.
Things have greatly changed. Today, hundreds of distinguished investigators devote their efforts to the origin of life. They have their own society, congresses, and journals. Some of their books are bestsellers. The domain has ceased to be ridiculous. On the contrary, it has become, thanks to many discoveries, thanks, especially, to immense advances in our understanding of life, one of the most exciting research topics of our time.
In Chapter 1, life was defined as what is common to all living beings. At first sight, one would expect this definition to apply almost unchanged to the last common ancestor of all life on Earth, from which all those shared properties presumably were inherited. Things, however, are not so simple; and the properties of the last universal common ancestor (LUCA) have become a subject of intense speculation and discussion.
The main difficulty comes from the possibility that certain genes may have arisen later in a given evolutionary line and subsequently entered the other lines by horizontal transfer, that is, transfer between different species (vertical transfer being that occurring within a species by the normal mechanisms of heredity). Present evidence suggests that this is a widespread phenomenon in the bacterial world. Some genes transferred in this way, although present in all living beings, could have been absent in the LUCA. This possibility is real but can only apply to the very early days of life, when few species existed and they occupied the same environment. A species isolated from the others could no longer receive genes from them. Its progeny would lack the genes in question. Indeed, to be common to all forms of life, properties acquired by horizontal gene transfer must have been gained by all the lineages that have left descendants until our time.
The same objection applies to the possibility that certain genes that were not present in the LUCA arose later, in separate lines, by convergent evolution. Such a phenomenon, if it occurred at all, could obviously be of significance only when very few distinct evolutionary lines still existed. The more numerous the lines, the smaller the probability of convergent evolution endowing all with the same gene.
A more likely possibility is that certain genes that were present in the LUCA were subsequently lost, during evolution, by a number of lines. Thus, the absence of a property in some present-day organisms in no way proves that this property did not exist in the LUCA. This is an important point, but it leads only to our underestimating the properties of the LUCA. Taking only those properties that are common to all known living beings, we have a pretty comprehensive picture of the LUCA. The only caveat is that the LUCA was, perhaps, not a single organism, but, as has been suggested, a collection of organisms sharing a common pool of genes that were freely exchanged by horizontal transfer.
With due regard to these uncertainties, we have enough information to sketch a portrait of the ancestor likely to resemble the actual ancestor fairly faithfully. First, it was manifestly a cellular organism, almost certainly unicellular and, for evident reasons of simplicity, prokaryotic rather than eukaryotic.1 It possessed the minimum characteristic attributes of all cells, to wit a peripheral membrane, perhaps supported by an external wall; a cytoplasm, site of metabolism; and a chromosome, vehicle of heredity.
The metabolism of the common ancestor must have involved at least several hundred distinct chemical reactions, catalyzed by protein enzymes already assisted by the main coenzymes known today. These reactions must have included some of the metabolic pathways present in the great majority of extant organisms, notably certain key anaerobic fermentation pathways, as will be mentioned later. ATP no doubt was the ancestor’s main energy vector. Its genes were made of DNA, which was replicated, transcribed, and translated according to the same complementarity rules and genetic codes as prevail today. Its proteins were synthesized on typical ribosomes, with the help of the three kinds of RNA now involved in this process. In short, the last common ancestor of all life on Earth may not have been very different from some present-day bacterium. Some important gaps, however, remain in this hypothetical picture.
First, did the primitive ancestor manufacture its own foodstuffs or did it derive them from outside? In technical terms, was it autotrophic or heterotrophic? At first sight, one would expect it to be self-sufficient. But this is not necessarily so. We shall see that the primitive Earth may have been abundantly supplied with organic substances of nonliving origin, from which the first forms of life are suspected to have arisen. Therefore, these forms may well have been heterotrophic, feeding on those substances or, alternatively, on coexisting autotrophic forms that subsequently disappeared without leaving descendants. However, such situations, if they ever existed, can only have been temporary. Autotrophy must necessarily have developed in a stable form before available food supplies were exhausted. Otherwise, life would have become extinct. The question is whether the primitive ancestor already was self-sufficient, or whether autotrophy arose later in some of its descendants. Scientists remain divided on this issue.
Another question concerns the energy source exploited by the primitive ancestor. One can rule out oxidation, whether of organic or mineral substances, as all the available evidence points to the atmosphere of the primitive Earth as containing little or no oxygen, which is a by-product of biological photosynthesis. The primitive ancestor, therefore, was adapted to life without oxygen—it was anaerobic—and it most likely depended on the kind of fermentations, such as the conversion of sugar to alcohol or lactic acid, that sustain anaerobic life today. This hypothesis is all the more plausible because fermentation systems exist in the great majority of living beings and involve energy-retrieval mechanisms that are simpler than the oxidative processes.
A heterotrophic organism could have subsisted on such reactions. But we have seen that the primitive ancestor may well have been autotrophic. Here we have a choice between two known forms of autotrophy: photosynthesis, which derives its energy from light, and chemotrophy, which depends on mineral chemical reactions. The latter are mainly oxidations, but some exist that do not require oxygen, for example, the formation of methane from carbon dioxide and hydrogen. Several hydrogen- generating processes are known that could have taken place in the oceans or in the soil of our young planet and could have supported such a metabolism.
This leads to a last question, closely related to the preceding one: what type of environment was occupied by the last common ancestor? The organism most likely lived in water. But at what temperature? At the surface or deep down? If it was photosynthetic, its habitat must perforce have been on the surface and, hence, temperate. However, there has been considerable interest lately in the possibility that life may have originated in deep, very hot waters, such as are found in volcanic geysers and, especially, in those deep-sea hydrothermal vents (black smokers) that spew high-pressure jets of overheated water, laden with mineral elements, through cracks opening at the bottom of oceans. In the last few years, these sites have been found to harbor a number of strange bacteria adapted to very high temperatures, sometimes exceeding 100° C. According to molecular sequencing studies, these organisms are among the most ancient known. We shall see later the possible significance of these findings (see Chapter 8).
A striking feature of our reconstructed portrait of the primitive ancestor is its modern character. Should this organism be encountered today, it might well not betray its immense antiquity, except by its DNA sequences. It must necessarily have been preceded by more rudimentary forms, intermediate stages in the genesis of the elaborate structural, metabolic, energetic, and genetic systems shared by all present-day living beings. Unfortunately, these forms have left no similarly primitive descendants that would allow their characterization. This lack greatly complicates the problem of the origin of life.2
The Earth was born about 4.55 billion years ago. It condensed, together with the other planets of the solar system, within a disk of gas and dust whirling around a young star that was to become our Sun. Phenomena of extreme violence, incompatible with the maintenance of any sort of life, surrounded this birth. For at least a half billion years, comets and asteroids battered the forming Earth, rendering it incapable of harboring life during all that time. Some impacts may even have been sufficiently violent to cause the loss of all terrestrial water by vaporization, following which the oceans would have been replenished with water brought down by comets. According to this version of events, present oceans would date back to the last wave of intense cometary bombardment, which experts believe took place some four billion years ago. There are signs that life was present on Earth soon after these cataclysms came to an end.
Fossilized remnants of typical bacteria (microfossils) and, even, of complex bacterial colonies, called stromatolites, astonishingly similar to extant living formations, have been found in a number of ancient rocks, including some Australian cherts estimated to be almost 3.5 billion years old. According to their discoverer, the American microfossil expert William Schopf, the Australian traces originate from highly evolved bacteria, closely related to present-day cyanobacteria, that is, bacteria that carry out a sophisticated kind of oxygen-generating photosynthesis. This claim, which, as we shall see in Chapter 8, raises some difficulties, has recently been seriously questioned.3 There remains, however, a distinct possibility—many would say a strong likelihood—that some forms of life were already present on Earth 3.5 billion years ago, perhaps even earlier. This is indicated by the finding, in certain ancient carbon deposits, of what is generally interpreted as an atomic signature of biological activity, that is, an excess of the light carbon isotope, 12C, over its heavy isotope, 13C4. This clue has been detected, in Greenland, in rocks that are 3.85 billion years old (and also in the Australian traces referred to previously). Life could even be more ancient. We would be unable to know, as any trace it might have left could not have been preserved until our days.
Some investigators believe that the time elapsed between the moment when Earth became livable and when life appeared was too short for something as complex as a living cell to emerge. Hence the hypothesis that life came from elsewhere. What are we to make of it?
The notion that life is of extraterrestrial origin has had illustrious proponents. Among them, the Swedish chemist Svante Arrhenius, winner of the 1903 Nobel prize in chemistry and remembered today for a prophetic view of the greenhouse effect,5 coined the term “panspermia” for his theory that germs of life exist everywhere in the cosmos and continually fall on Earth. More recently, a celebrated British astronomer, Sir Fred Hoyle, who died in 2001, has claimed, together with a Sri-Lankan colleague, Chandra Wickramasinghe, to have detected spectroscopic proof of the presence of living organisms on comets. We shall see later what this evidence is. Francis Crick, codiscoverer, with James Watson, of the double-helical structure of DNA, has even proposed, with another scientist of British origin, Leslie Orgel, that the first living organisms may have reached Earth on board a spaceship sent out by some “distant civilization.” He has given the name “directed panspermia” to this hypothesis.
Leaving aside the spaceship, of which no sign has been found so far, an extraterrestrial origin of life is perfectly plausible. The often-voiced objection that living organisms could not withstand the physical conditions that prevail in space, especially the intense ultraviolet radiation, does not hold, as it is readily conceived that comets or meteorites may offer protection to the organisms. Destruction by heat upon entry into the terrestrial atmosphere could similarly be prevented. Moreover, the possibility that life may be a widespread phenomenon, existing in many sites of the universe, is increasingly being entertained. I shall examine this question in Chapter 17. Thus, the eventuality of living organisms travelling through space on various “flying objects” is far from implausible. But what about the evidence?
The argument that there was not enough time for life to arise locally on Earth rests on a purely subjective and arbitrary estimate, supported by no objective element. There is no proof that the emergence of life must have required hundreds of millions of years, as has been maintained. On the contrary, as I shall point out later, the essentially chemical and deterministic vision one must have of this phenomenon rather leads to the belief that life arose relatively fast, in a time span probably to be counted in millennia rather than in millions of years. In this view, the window of some 100 million years allowed by present data leaves more than ample time for life to have been born on Earth. It is even possible that life arose and disappeared many times before establishing itself.
There remain the many observations, clearly undeniable, showing that the elementary constituents of life exist on comets and other celestial objects. But are these substances products of life, as is believed by the defenders of panspermia? Or are they, on the contrary, the fruits of spontaneous chemical reactions? We shall see that the second explanation is considered the more probable of the two.
For millennia, all that humans have learned of the Universe around them has been provided by the “pale light falling from the stars,”6 from which, since Galileo, the growing power of telescopes has been extracting increasingly detailed information. But our eyes, even helped by the best optical instruments, perceive only a minute fraction of the radiation that comes to us. They see only radiations of wavelengths comprised between 400-(violet) and 800-(red) millionths of a millimeter. This narrow band is inserted within a huge span of invisible radiation, which, on the side of shorter wavelengths (higher energies) ranges from the ultraviolet to X rays, γ rays, and cosmic rays, with wavelengths reaching below one- billionth of a millimeter, and, on the side of longer wavelengths (lower energies), extends from the infrared to kilometric Hertz waves. Today, the new discipline of radioastronomy sweeps a good part of this span by means of instruments of ever increasing sensitivity. The information gathered in this way is immensely richer than that provided by visible light alone.
The most important data are chemical. This is because substances betray their nature by the radiation they emit or absorb. Sodium, we know, emits yellow light; neon, red light. If the light emitted by a sodium lamp is decomposed with a prism, only two yellow bands are seen, instead of the usual rainbow. If, on the other hand, a ray of white light that has passed through sodium vapor is likewise decomposed, the same two bands are seen, but now in the form of black bands in the yellow region of the spectrum. Emission bands have become absorption bands. It is by this kind of analysis that hydrogen has been identified as a component of the Sun. Helium, as its name recalls (hêlios means sun in Greek), was even discovered first in the Sun, before being found on Earth.
What is true for visible light is true also of the radiations that escape the eye. Such radiations can be similarly decomposed by appropriate “prisms” and the spectra thus produced can be recorded and analyzed for the signature of certain atoms or molecules. The wavelength region around one centimeter is particularly rich in this respect. Microwave ovens function with this type of radiation. The waves that come to us from space could never power the tiniest of ovens, but their feeble messages can nevertheless be amplified and decoded in a detailed manner with the instruments now available. A complication in this kind of analysis comes from the atmosphere, which blurs the signals and adds its own. For example, there is so much water in the atmosphere that detecting traces of this substance elsewhere is impossible. But there are ways of getting around this; the simplest is to put the instruments above the atmosphere, on satellites or spaceships, as is increasingly done.
Spectral analysis at a distance is only one means. Robot instruments carried by spaceships have performed a number of direct measurements on comets. And, especially, it has been possible to apply all the resources of modern technology to meteorites that have fallen on the Earth. These various explorations have revealed the surprising fact that organic chemistry is the most banal and abundant chemistry in the whole universe.
Two centuries ago, the founders of chemistry designated as organic the chemistry of substances made by living organisms with the help, many believed, of a special vital force. This notion was first contradicted in 1828, when the German chemist Friedrich Wöhler synthesized urea; and it was definitively disproved in 1897, when another German chemist, Eduard Büchner, discovered that yeast juice devoid of living cells could convert sugar into alcohol. In the opinion of Pasteur, who unfortunately died two years before Büchner’s results became known, this fermentation required “Life.”
Since then, laboratory organic chemistry has produced spectacular developments that have fertilized industry and given us the entire gamut of modern plastic materials and synthetic fibers, an abundance of drugs, and many other so-called synthetic substances. It has become evident that organic chemistry is none other than carbon chemistry and that it owes its exceptional richness to the particular associative properties of the carbon atom.
Some sort of residual vitalistic aura has, nevertheless, persisted around organic chemistry, perceived almost subliminally as a kind of chemistry practiced only by living beings, including organic chemists. Space chemistry has shattered this last refuge of vitalism by showing that organic substances are spread throughout the cosmos, where they make up an important fraction of cold matter. Small radicals and molecules, made of only a few atoms of carbon, hydrogen, oxygen, sometimes nitrogen or sulfur, are present on minute dust particles that make up extremely tenuous clouds—more rarefied than the best vacuum we are capable of producing on Earth—but immensely extended, filling vast regions of space with what is known as interstellar dust. When such particles get together, the small molecules they contain interact to generate larger entities, of which many have now been identified on comets and other celestial bodies, especially meteorites, which have lent themselves to detailed analyses.
The results of these analyses are nothing less than flabbergasting. Not only have they revealed the existence of numerous organic molecules of manifestly extraterrestrial origin, but these molecules turned out to comprise many characteristic constituents of life, such as, for example, amino acids. Astonishingly, these findings have made little impact in the scientific world, even less so in the world in general. Yet, the message they broadcast is supremely important. The chemical germs of life are banal products of space chemistry. There is “vital dust” everywhere in the universe.
Before this conclusion can be accepted, it must be ascertained that the molecules do not come from some terrestrial contamination. Especially, the possibility that they have been manufactured by extraterrestrial living organisms must be ruled out. As far as contamination is concerned, detailed examinations have allowed this explanation to be categorically excluded in a number of cases. As to a biological origin of the substances, this, obviously, is the interpretation favored by the partisans of panspermia. The majority opinion, however, is that the molecules are of non- biological origin. A good reason for adopting this view is that the same molecules are readily obtained in the laboratory under conditions that could have prevailed on Earth four billion years ago.
The story starts in Chicago in 1953—the year of the double helix!—in the laboratory of Harold Urey, an American physicist world-renowned for the discovery of heavy hydrogen, or deuterium. Later in his career, Urey had become interested in the origin of the planets. He had put forward the hypothesis that the atmosphere of the young Earth was very different from what it is today. It was, he believed, devoid of oxygen and rich in hydrogen and hydrogen-containing substances, such as methane (CH4), ammonia (NH3), and water vapor (H20). There is agreement among experts on the absence of oxygen, almost certainly a product of life, but the abundance of hydrogen is disputed by many. Be that as it may, a young student working in Urey’s laboratory was sufficiently impressed by Urey’s theory to ask the question how repeated lightning might have affected the atmosphere postulated by his mentor. Against the advice of the latter, who found the project too iffy for a doctoral thesis, the student built a glass enclosure within which the gas mixture postulated by Urey was subjected to a succession of electric discharges. The results exceeded the student’s wildest dreams. In a few days’ time, almost 20 percent of the methane carbon had been converted into amino acids and other typical biological constituents.
This historic experiment almost instantaneously propelled the name of the student—Stanley Miller—into the firmament of celebrities. It also inaugurated a new discipline, abiotic (without life), or prebiotic (before life), chemistry, which aims at synthesizing biological compounds under conditions that might have prevailed on Earth before the appearance of life. Many elementary constituents of life have thereby been obtained under plausible prebiotic conditions. The products of this new chemistry show remarkable similarities, both qualitative and quantitative, with substances detected in meteorites. What is reproduced in the laboratory seems close to what occurs spontaneously in space.7
These discoveries have returned to the foreground the possibility that life arose naturally, a possibility long discredited by the celebrated experiment done by Pasteur, which crushed poor Félix-Archimède Pouchet, a defender of spontaneous generation, in front of the entire Académie des Sciences assembled in solemn gathering.8
For a large part of the general public, life arose through direct action by a Creator. Not only strict creationists, who rest on a literal interpretation of the Bible, subscribe to this opinion. So do many members of more open-minded religious groups. Even outside any religious creed, the origin of life is often viewed as an insoluble mystery, within the context of some unconscious latent vitalism. Rare are those who, being cultured but devoid of scientific grounding, picture life as having spontaneously arisen through the play of the same physical and chemical laws as rule other natural phenomena, such as the formation of planets, the shifts of the Earth’s crust, tidal movements, or the erosion of mountains. Pasteur’s triumph, one of the rare scientific events to have earned a place in popular history books, is perhaps not foreign to this attitude.
Yet, all that we have seen so far supports a naturalistic explanation of the origin of life. There is first the fact, related in the preceding chapters, that life has proved entirely explainable in physical-chemical terms. What is true of life now is very likely to be true also of its origin. If life functions without the help of a vital principle, as we know it does, we are entitled to assume that its birth likewise took place without the intervention of such an entity. Another encouraging fact is the discovery, just recalled, of the vast cosmic chemistry that abundantly produces amino acids and other organic substances entering into the composition of living beings. If, as seems reasonable to suppose, those substances represent the chemical seeds from which life developed, it may be said that at least the first step in the birth of life was the outcome of natural processes.
But this is only a first step in what must have been a very long succession of steps. As will be seen in the next chapters, we are mostly left with speculative hypotheses to explain the manner in which the basic building blocks provided by cosmic chemistry might have combined into larger molecules, such as proteins and, especially, nucleic acids, not counting the more complex assemblages from which the first biological structures arose. One may well wonder, therefore, whether we will ever succeed in explaining the origin of life naturally or, even, whether this phenomenon is naturally explainable.
In the view of most scientists interested in the problem, one can but answer the last question affirmatively, at least as a working hypothesis. No scientist could think otherwise, as this hypothesis represents the fundamental postulate of any scientific investigation. To assume the opposite amounts to denying the possibility of finding an explanation for the phenomenon one studies and thus declaring one’s research futile. Independently of any preconceived idea, science must proceed on the assumption that the problems it approaches are soluble. There will always be time to call on “something else” after all attempts at finding a natural explanation have failed. In the case of the origin of life, this is still far from being the case.
The fact remains that, as long as the problem is not solved, the tendency to invoke “something else” will subsist. It is the attitude even of a small minority of scientists, very few in number but much publicized. According to these dissenters, there are intrinsic reasons for believing that life, as we know it, cannot possibly be the fruit of natural phenomena. Worded in apparently irreproachable scientific terms, such affirmations are enthusiastically greeted and fervently propagated, not only by traditional creationist circles, but also by diverse groups who, while claiming to accept the findings of modern biology, emphasize that “science does not explain everything” and defend the thesis, of so-called intelligent design, that detects in the properties, origin, and evolution of life the intervention of an influence other than the simple play of natural laws.
An argument brought forward in favor of this thesis by the American biochemist Michael Behe represents what he calls “irreducible complexity,”9 a notion he defines as the state of “a single system composed of several well-matched interacting parts that contribute to the basic function, wherein the removal of any one of the parts causes the system to effectively cease functioning.” This definition, which he illustrates with the “humble mousetrap,” applies according to Behe to numerous biochemical systems, for example, the flagellum that propels bacteria or the enzymatic cascade that governs blood clotting.
No one will deny that these systems and many others conform to the proposed definition. One cannot remove one of their parts without impairing their functioning. But this in no way proves that, as is claimed by Behe, these systems can have arisen only with the help of an outside intelligence that adjusted the various parts according to a pre-established plan in which their role in the whole was foreseen. Such an affirmation ignores the possibility of an evolutionary process that might, with the help of natural selection, have led to increasing complexity by way of intermediary stages each of which fulfilled a useful function. Many examples of such processes are known. Thus, it is known that the principal proteins of the transparent eye lens were recruited in the course of evolution from enzymatic proteins that played an entirely different role.
Another objection frequently addressed to the theory of a natural origin of life calls on the fact, already mentioned in the preceding chapters, that life uses only an infinitesimal fraction of the possible protein or nucleic acid sequences, or, in more technical terms, occupies only an infinitesimal part of the sequence space. Remember, the number of different protein chains of 100 amino acids that can exist is 10130 (one followed by 130 zeros), that of possible nucleic acid chains of 300 bases 10180 (one followed by 180 zeros).10 Numbers of this size exceed by far anything that can exist in reality, or even be conjured up by our imagination. What, then, of the number of sequences nascent life was able to test, in about 100 million years, with only the materials available on the surface of our planet (or, for that matter, on whatever celestial object provided the cradle of life)? Yet, the bacterial ancestor of all life must have contained hundreds of distinct such molecules, many of them longer than those being considered here. The problem thus arises as to how emerging life could, without guidance, conceivably have selected its constituents from such an immeasurably huge number of possibilities.
To explain the generation of the ancestral proteins—the fact that this process took place by way of nucleic acids makes no difference to the argument—by the natural unfolding of chemical processes, one would have to assume either that almost any random combination of amino acids will produce a collection of proteins adequate to make a viable cell, or that the molecular specificity of the processes involved was such as to almost obligatorily produce the right mixture. The first explanation is ruled out by what we know of biology, which tells us that the functions of proteins often are exquisitely dependent on specific sequences, to the point of being frequently impaired by the replacement of a single amino acid by another. The second explanation is ruled out by what we know of chemistry. Processes of the required precision simply do not take place. Hence, it is claimed, there must have been “something else.” Such is the conclusion arrived at in a solidly argued book by the American mathematician William Dembski significantly titled The Design Inference.11
Here, again, the argument neglects the historical dimension of these phenomena. As will be mentioned later (Chapter 5), there are good reasons for believing that the first sequences were much shorter than today’s and that nascent life has reached its present position in the sequence space by a gradual pathway, each stage of which, honed by natural selection, allowed extensive exploration of the available sequence space. Intervention by a directing intelligence is not mandatory.
Contrary to what is sometimes claimed, a naturalistic view of the origin of life does not necessarily exclude belief in a Creator. The notion, propagated at the same time, though for opposite reasons, by militant atheistic scientists and by many antiscientific circles, that the findings of science are incompatible with the existence of a Creator is false. But these findings at least call for a revision of the image one makes of this Creator. It cannot be a God who, according to the familiar animist saying, “blew life into matter.” This notion is no longer valid now that we know that there is no such thing as a vital principle. Likewise, there is every reason to believe that the elementary constituents of life form spontaneously in many parts of the universe, by the sole operation of physical-chemical phenomena. Thus, if we wish to call on some creative act to explain the origin of life, we are led to imagine a God who got into the act at some precise moment, forcing the molecules of basic constituents to interact against their natural tendency until a machinery capable of functioning under its own steam had been built, following which He would withdraw from the game and allow the sole physical and chemical forces to play freely. This naïve picture of a divine engineer interfering just enough with the laws of his creation to achieve an objective looks too much like a contrived, ad hoc hypothesis to be intellectually acceptable. Why not imagine a God who, from the start, created a world capable of giving rise to life by the sole unfolding of natural laws of His own devising? This view, as we shall see in the last chapter, is now defended by many deists, including a number of scientists.
While scientists generally agree to attribute the origin of life to natural phenomena, the degree of likelihood of these phenomena is very diversely appreciated. According to many scientists, among them some of the most illustrious, life is the product of highly improbable events that are very unlikely ever to occur anywhere else and that could very well not have happened on Earth were it not for an extraordinary combination of circumstances. Any failure to reproduce the phenomenon in the laboratory is thus explained beforehand. It is pointed out that highly improbable events take place all the time without our according them any attention unless there is something special about them. Thus, in the game of bridge, each distribution of the 52 cards among the four players has one chance in 5x1028, that is, in 50 billion billion billion, of being dealt. This guarantees with near certainty that each distribution is a unique event that never occurred previously and will never occur again in a foreseeable future. Nevertheless, bridge players do not spend their time marvelling at their cards with the feeling, at each deal, of being witness to an extraordinary event. They would do so only if there should be something uncommon about the distribution. If, for example, the 13 spades, hearts, diamonds, and clubs should each be gathered in a single hand, the event would cause a sensation, and the whole world would be apprised of it by bridge columnists. And yet, this distribution is no more improbable than any other.
Such, it is claimed, could be the case also with life. As with a bridge deal in which each hand contains a complete suit, the first living system could have been, among innumerable other arrangements of matter of equally low probability but of no particular interest, the outcome of an extremely improbable combination of circumstances, so improbable that it is virtually certain to be unique. In this view, life appears as a cosmic accident devoid of significance. In the words of the late French biologist Jacques Monod, “the Universe was not pregnant with life.”12
Such a conception is acceptable provided the stroke of luck concerns a single event. It could be an extremely improbable event, but there can be only one. Indeed, from the moment several highly improbable events are required to reach a certain goal, the probability of ever getting there soon approaches zero, since the probability of a complex series of events is the product of the probabilities of its individual steps. Thus, the probability of the same bridge distribution being dealt were it only twice in succession is (5x1028)2, that is one chance in 25 followed by 56 zeros; that and zero, practically speaking, amount to the same thing.
It is obvious that life cannot possibly have arisen in one shot. For this to have happened, nothing short of a miracle would have been needed. The process, if it took place naturally, must by necessity have been composed of many steps, most of which, as we have just seen, must have had a high probability of taking place. Thus, the “lucky chance” hypothesis implies that a singular event of extremely low probability occurred in a series in which the great majority of the steps that came before and after followed a highly deterministic course, imposed by the prevailing conditions. Once again, we are faced with a possibility that cannot be ruled out but is hardly conceivable in realistic terms. From what we know of life, it is difficult to see how it could have developed by the succession of a very large number of spontaneous events, broken by a single barrier that could have been surmounted only with an extraordinary assistance of chance. Starting from the basic constituents provided by space chemistry, life must have arisen through a complex fabric of interconnected reactions involving a large number of different substances. This development no doubt relied on numerous discrete events, but not on a single event of extremely low probability.
Another reason for ruling out a critical intervention by chance in the development of life is that chemical processes were involved. Chemistry deals with strictly deterministic, reproducible phenomena that depend on the statistical behavior of trillions of molecules of various kinds. Were it not so, there would be no chemical laboratories, no chemical industries. When substances A and B are mixed under specified conditions, the outcome is always C. If a student fails to get C in the laboratory, the professor does not commiserate: “You have been unlucky. Chance has not favored you.” No, the student is admonished: “You have been sloppy. Go back and try again.” Life, we have seen, is explained in chemical terms; so must its origin be.
For the reasons I have just summarized, I favor the view that life was bound to arise under the physical-chemical conditions that surrounded its birth. This does not necessarily imply that there is life on many other celestial bodies. All depends on the probability of there existing elsewhere in the universe conditions similar to those that allowed life’s emergence on Earth. This question will be examined in Chapter 17.