The Origins of Life
I suppose that nowadays most people accept, as so obvious that it hardly requires a second thought, the final dethroning of man from the position of focus and summit of the universe, which he for so long, and so confidently, had given himself. We would hardly raise an eyebrow were we to find a new series of books on “The World Natural History,” including among its early volumes a long and quite detailed study on the natural history of the Garden of Eden. Of course, this climate of thought is astonishingly recent. Until a very few decades ago the scientific view of life was completely dominated by the denial that living systems could spontaneously arise from non-living ones. Indeed it would have been claimed that Pasteur had experimentally disproved the possibility of spontaneous generation. He kept a nutritive broth in conditions in which no external living agents could get access to it, and he showed that nothing living appeared within it; hence he concluded that life could not arise spontaneously. Pasteur was of course looking for something obviously recognizable as alive, such as a bacterium or yeast. As the search for the origin of life has gone deeper, we have had to ask more penetrating questions about what is and what is not alive. I shall argue that these are some of the most important issues raised by the book under review.
In the older, “classical,” view life remained a basically mysterious ingredient in nature. Apart from any questions of how living things in their more complex manifestations could be understood, how could one suppose that life appeared in the natural world in the first place? We might suggest that living things reached this earth by traveling through space from other parts of the universe, perhaps as spores or some other resistant particles; but this only shifts the problem of origin geographically to another place, without doing anything to solve it. We are left with nothing more to say than that life is some sort of freak—possibly one produced by God, which we might dignify with the name “special creation,” or possibly just the result of some excessively improbable concatenation of natural events, which occurred for no better reason than that, if enough time goes by, the most unlikely things will happen. In any case, life remains something very, very special.
In the late Twenties and early Thirties the basic thinking was done which led to the view that saw life as a natural and perhaps even inevitable development from the non-living physical world. Future students of the history of ideas are likely to take note that this new view, which amounts to nothing less than a great revolution in man’s philosophical outlook on his own position in the natural world, were first developed by Communists. Oparin of Moscow, in 1924, and J.B.S. Haldane, of Cambridge, England, in 1929, independently argued that recent advances in geochemistry suggested that the conditions on the surface of the primitive earth were very different from those of today, and were of a kind which made it possible to imagine the origin of systems that might be called “living”—but, we shall have to ask later, living in what way? These early biochemical ideas were highly speculative, and little got into print about them until after World War I, when serious discussions began and a spate of books appeared. The two founders, Oparin and Haldane, were joined by others, such as H. J. Muller and J. D. Bernal, and were subjected to the criticisms of another, equally left-wing, biologist, N. W. Pirie. It was not till the Fifties that the subject shed its light odor of politics, and became an arena for biologists of all persuasions.
BERNAL HAS BEEN one of the most important intellectual influences in science ever since the days when this topic was first being considered. He is professionally classified as crystallographer, but to him this science has meant something much more broadly based than a study of neat little geometrically shaped lumps of simple substances. He took crystallography to involve an attempt to discover and understand the spatial arrangement of any assemblage of repeated units, however complex these units might be. He was one of the great pioneers, along with W. T. Astbury and Linus Pauling, in the application of X-ray crystallography to the complex substances found in living things. This work has been central in the development of molecular biology in the last couple of decades, and, although Bernal himself worked at rather too early a stage, at least two groups of his scientific progeny—Dorothy Hodgkin and the Perutz-Kendrew team—have received Nobel prizes for working out completely the structure of very complex molecules, such as Vitamin B-12, or proteins by extending and developing the techniques in which Bernal had been a pioneer.
Beyond this he was in on almost every lively and growing aspect of science, except pure nuclear physics. I remember a discussion group in the mid-Thirties on the nature of the gene, including such people as Muller, Darlington, Bauer, Delbrück, Timofeef-Ressovsky, Astbury, at which we arrived for the first time at two of what have proved to be the gene’s essential features: that the gene consists of a linear arrangement of parts which are as small as small molecules, and that spirals of some kind are essential to its construction. (Unfortunately these two are not the only essential features. We were not clever enough, fifteen years before Watson and Crick, to think of the two major contributions which they made—that it is the DNA and not the protein which is the essential constituent of the genetic material, and that this is not just one spiral, but two wound around each other in a subtle way and fitted neatly together.) At such meetings Bernal was nearly always present, and when he was, he would be playing a key role. Not only did he know more facts about more subjects than anyone else, but he had an outstanding ability both to generalize an observation so that its broad relevance became apparent, and to focus precisely what was the real issue in a complex tangle of problems. He acquired the nickname “Sage,” and anyone who doubts its appropriateness should look at his other major books—The Social Function of Science (1939), the acknowledged source of what is nowadays known as The Science of Science, and of much technological forecasting besides; The Freedom of Necessity (1949), probably the first major study in what is now called Science Policy; and Science in History (first edition 1954, last edition 1965), a major, though controversial, source from which much of the modern study of the history of science has sprung.
I HAVE WRITTEN at some length about Bernal’s position in the intellectual world, because this is very relevant to an assessment of The Origin of Life. The importance of this book lies not so much in its account of the particular biochemical processes which, he suggests, may have been involved in the Origin of Life. The book is valuable because, coming from this depth of knowledge and scientific experience, it puts beyond argument the major point that life could indeed have originated spontaneously from the non-living. There are in fact a number of other possible pathways that can be suggested, and it may be some considerable time before we can decide which of these was the path actually followed on this earth; and some time longer before we discover whether any of the others have been followed elsewhere in the Universe. But that life is a natural phenomenon, developing out of the non-living world through processes which can, in principle, be understood, is no longer to be doubted, and we have to adjust our thinking to this condition.
It may seem odd to claim that there are a number of other ways in which life could have originated. To understand what is involved we really have to tackle the question—which may have been worrying the reader already—what is “living”? Can we define “life”? I think the answer is No; certainly there is no single snappy phrase which can be applied to decide unequivocally whether a system is living or not. But we can describe a few major properties which, if possessed by any system, would unhesitatingly determine that that system be called living—though doubts would arise about systems which had only some of them. The properties usually quoted in this connection are metabolism, growth, reproduction, and evolution. These differ in importance. A capacity for growth would surely not be enough in itself to qualify a system for being called living—a cloud, for instance, can grow under suitable atmospheric conditions. Again, simple reproduction, in the sense of increasing in numbers, is not enough—a raindrop, condensing out of a cloud, will split into two and “reproduce” when it reaches a certain upper limit of size.
The other two qualities, metabolism and evolution, are much more crucial. It is a major characteristic of all things which we commonly consider alive to “metabolize,” that is, take in simple substances from their surroundings and build these up into more elaborate substances out of which the body is made. Many biologists accept this biochemical ability as the overriding criterion for life. Others argue that the capacity to undergo evolution is even more important, since any system that can evolve will thereby gradually become more complex and will eventually acquire these capacities for metabolism. If one can think of a way by which natural processes produce a system that starts evolving, then it may be permissible to think of such a system as being on an escalator that may take it to any level of biological complexity.
The Origin of Life is a rather complex work. The first part—rather more than half the total—is a rediscussion, much elaborated and brought up-to-date, of topics which Bernal treated much earlier in his short book, The Physical Basis of Life (1951). After introductory sketches on the history and the modern development of the subject, Bernal distinguishes three major stages during the origin of life: first, the formation of small molecules of the kind we find in living things; second, the assemblage of these small building blocks into larger macro-molecules; and, third, the transition from isolated molecules to organized assemblages of them in living cells.
Bernal and others report important new ideas and experiments related to each of these three phases. We now know that extreme natural conditions, produced for instance by lightning flashes, or volcanoes, or high intensity ultra-violet, can bring about the formation, from such simple raw materials as water and ammonia, of unexpectedly complex compounds of the kind used in modern living systems. (There was a report only a few weeks ago of the production in this way of porphyrins, the class of compounds which includes chlorophyll, the basic molecule used by plants to capture the energy of sunlight.) The buildup of these “lifelike” molecules into larger macro-molecules, and the architectural job of putting these together into organized assemblages, have become considerably less mysterious than they were a few years ago. We now know a good many examples of relatively small molecules spontaneously coming together to form much larger aggregates of a quite definite and specific structure: for instance, protein sub-units join together in precise ways to form multiple units; and similar processes are the daily bread of polymer chemists synthesizing nylon, terylene, and similar materials.
Of the formation of the larger units, there have been two main views. Oparin thought that little blobs of jelly-like substances would be produced, with no definite internal organization. Bernal, in his earlier book, produced the stimulating idea that some substances, such as clays, have properties which would cause them to act as rallying points where the small proto-biological molecules would settle and adhere; but which would also give these substances an ability to facilitate some of the basic metabolic processes of life—although of course their efficiency in this would be much less than that of the highly effective enzyme molecules which living things produced at a later stage in their evolution.
Bernal later describes some of the other explanations which have been suggested, apart from those which he himself favors. For instance, there is a not implausible hypothesis that the essential concentration of proto-biological molecules into aggregates “thick” enough to be effective, which must initially have been rare and widely scattered, took place not in lumps of jelly, as Oparin thought, or on clays, as Bernal suggests, but that they first accumulated in the surface of the sea—this is just the sort of thing most “interesting” molecules tend to do—and then the wind and the waves produced masses of foam in which these molecules became highly concentrated.
Finally, Bernal considers the influence on contemporary thought of the discussions on the origin of life. This section is provided with magnificent illustrations showing the forms of macro-molecules and how they became assembled to produce the parts and substructures of cells. A postscript follows which brings the discussion still more up to date. There are four appendices, which include the two pioneering articles by Oparin and Haldane (the former appears for the first time in an English translation), and an account by G. Mueller of his studies on meteorites containing carbon (which provide perhaps the most suggestive evidence we have of organized living systems in other parts of the Universe.) Bernal also describes in non-technical terms the development of crystallography from a science which dealt only with regular crystalline aggregates of simple substances to one which can begin to cope with the much more complicated and fluid spatial arrangements of the parts of living systems. Finally the nonbiological reader may be glad to hear that besides the usual indices, bibliography, etc. there is a large well-illustrated glossary of technical terms.
All of this makes a splendid book, but perhaps I may now confess that, in my opinion, Bernal takes his stand on the wrong side of one of the most important divides in this field. He seems to me to side too much with the biochemists and to give too little importance to the problem of the origin of hereditary specificity. This gets to the center of the most crucial phase in the origin of life. As we have seen, there is plenty of evidence now that many of the small to middling molecules required for present-day living systems, such as aminoacids, sugars, nucleotides, would actually have been formed under the conditions of the earth’s surface in early times. The great question is how to take the next step and assemble these into the larger aggregates which could perform effectively one of the tasks of living matter. The size of the problem is vividly illustrated by two sentences from a recent paper by Cairns-Smith (Journal of Theoretical Biology, 1966, Vol. X, p. 54):
Even if the whole earth had been made of nothing but aminoacids which had rearranged themselves randomly and completely 10 times a second, in the whole period of the earth’s history, there would have been little chance of producing even once, for a tenth of a second, one molecule of insulin! …A given 50-aminoacid sequence would be expected to take about a billion years, while a given 40-aminoacid sequence should turn up every few minutes.
For those who, like Bernal, adopt the biochemical point of view, the task is to account for the appearance of a large molecule, like an enzyme or insulin, which can act within the metabolic machinery of the living system.
Those who adopt the more genetical point of view would want to pose the question differently. For them the problem is not so much to get active enzymes, but rather to see how a system could be set up which can reproduce, which can suffer alterations in its material and reproduce those also, and thus provide the basis for a true process of natural selection, which we know can lead to rapid evolutionary change. The substances that living things use at present for their genetical systems are much too complex to have arisen by chance. There must have been a “pre-biotic evolution” to bring them into being. Oddly enough, thought on this problem has also led to the suggestion that clay may have had a key role. The point here is that crystals, which contain a number of different atoms arranged with something less than perfect regularity (and clays are good examples), can transmit these irregularities to the next layer of atoms which settles down as the crystal grows. Thus crystal irregularities could provide a primitive type of genetic system, on which natural selection could operate. Cairns-Smith discussed, in the paper that I have cited above, how clay-like, irregularly crystalline materials—growing in some fissure of the rocks where there was a stream of dirty water contaminated by the middle-sized molecules produced by lightning or ultra-violet—might absorb on their surface a layer of jellyfied polymers. These would assist in capturing the necessary materials for further growth as they flowed past. Different crystal irregularities might be more or less efficient in recruiting such jelly-filters; and so there could be a “proto-natural selection” and a “proto-evolution” at a pre-biotic stage, before any of the biochemist’s criteria for life, such as the ability to build up really complex molecules, could be met. It is of course unfair to fault Bernal for not reviewing these notions, which probably did not appear in print until his book was set up in proof; but these are the ideas I myself find most enticing in this fascinating field.
One way or another, mythologies that state that man was made out of clay may turn out to be quite near the mark. But we are beginning to see how the clay could become living without the need of a special puff of God’s breath to enliven it. Still, if you like, you can take a side bet, that life did not begin with the quickening of Adam’s heavy clay but with Aphrodite, born of the foam and the spindrift.