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CHAPTER 1 WHAT IS LIFE? DANIEL MAZIA We do not seek to evade the question of the nature of life, but perhaps we should, with stricter discipline, speak rather of living things, organisms, biological objects or the like. For these things exist and they are distinc- tive among things in general. Biology, in common sense and in formal science, may be unrigorous but it is not imbecilic. It is not trivial that we distinguish corals from rocks, babies from dolls, know when to call the doctor and when to call the undertaker. To say that we cannot answer the question "What is Life?" may imply only that we have no simple predicate for the sentence beginning "Life is . . .". That may be a statement of fact, namely that we now believe that the intuitive hypotheses that life is a special "something," a special force or a magic substance, are scientifically incorrect. This experience is not unique in science; the discrediting of the phlogiston theory wiped out the pat definitions of fire and heat, but we did not for that reason deny fire and heat. Perhaps we should not speak of a definition at all. Our task is rather to identify those properties—forms, substances, processes—that are comprehended in the idea of life. The longer the statement, the better, if length is a measure not of prolixity but of the number of features common to living things and distinguishing them from other kinds of things. Such an experimental approach—it is an ex- periment to see whether the facts of life define life—cannot pretend to satisfy the desire for a more analytical and purely logical approach, but at 25

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26 LIFE: ITS NATURE AND ORIGIN least it can make precise what we mean when we talk about living things and what we look for when we examine them. The question "What is Life?" begins to acquire newer implications in a time when the exploration of space is beginning. Whatever the prob- ability of life in other worlds, the possibility of an answer turns old philo- sophical exercises on the definition of life into a concrete observational problem: What criteria are significant, what observations would be prac- ticable, what does this or that kind of measurement or observation tell us about living things, what observations of non-living things could confuse us, what exotic departures from the kind of life we know could go un- recognized? The same questions concern both the advocates of the ex- plorations and their critics. We begin, then, with the modest affirmation that the world of things in general—the domain of physics—includes a class of things that are highly distinctive—the domain of biology. As we shall see, the question of a blurring of the boundaries between the two is far from damaging in an evolving universe; on the contrary, we think we could recognize the transitional stages and learn much about living things from them. Though we seek the distinctiveness of living things by studying them as things, we do not deny alternative approaches to the distinctiveness of life: through instinct, poetry or religion. The present discussion, however, conforms to the limitations of science and exploration. A measure of the distinctiveness of the living world is that it confronts us with phenomena, principles and values that are essential to itself but not to the physical world as a whole. If in the end we must investigate living things as things having a special—a very special—material organi- zation, we begin by noting that the values that govern the investigation are also distinctive. Put more plainly, living things are characterized by the kinds of questions we put to them—or more simply, by what is interesting about them. It is important that the scientific investigation of rock and of cells gives different answers, but just as important that it asks quite differ- ent questions. What is interesting about living things—and the focus of all questions about them—is contained in the idea of survival. Survival contains the idea of an organism. The living world thwarts time by survival, all the rest combats time by endurance. An organism lives; its fossil relic endures. Survival contains the idea of an organism, an individual that can change and replace its atoms and molecules without loss of identity. Survival con- tains a special version of the idea of a species; in the living world the similarity of members of a species derives from common descent and from no other cause. Survival implies a particular version of purpose and value, although these philosophical swear-words are usually concealed prudishly in the term function. There is no biologist, whether he be a biochemist

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What Is Life? 27 who discovers some small molecule in a microorganism, or a student of behavior examining the social organization of apes, who does not seek, in his observations, for purpose and value in relation to survival. Thus we approach living things as things designed for survival. Our observations and measurements concern the things; our judgments of what we seek, observe and measure concern survival. All this would be rather abstract and, perhaps, marginal to the practice of science, if the world we know contained a great variety of things designed for survival, sharing only the characteristic that they are designed for survival. But that is not the case. The most important fact of the biology that we know is that living things are profoundly similar, and the most important deduction from this fact is that the similarities represent true relationship by com- mon descent. The wonderful thing about evolution on the Earth is that it can produce such a beauty of variety and variety of beauty among things that are fundamentally the same. It is by virtue of that fundamental same- ness that we can surely identify living things on Earth, as well as things that were once alive and (would that we could find them!) things in the process of evolution toward life, if only we are given the means to make a sufficient number of observations. It is the general principles of opera- tion of livmg things on Earth, as well as the general principles of their material organization, that permit us to make judgments about the bio- logical exploration of other worlds. Survival of living things on Earth is observed on two scales of time, and the time itself is biological time (the unit of which is a generation), not clock time. The smaller scale contains those attributes that govern the survival of the individual. Otherwise expressed it contains all the attributes of the individual, judged by the values of survival. It includes the form, organization and anatomy, that identify the individual visibly as the thing that survives. It includes his developmental history, for we are interested in eggs as well as chickens. It includes all the flow of matter through the individual, all the self-sustaining transformations of the invariably simple substances of the non-living world into the often-complex substances making up the fabric of living bodies. It includes the flow of energy by which all the improbable operations of living things become possible operations. It includes all the movement, responses and behavior of the organism, for all of these may be interpreted as purposeful, once we recog- nize purpose as survival. It includes behavior of organisms in relation to other organisms; not only competition, but also social behavior and co- operation for no inference from natural selection is more obsolete than the idea that survival means only tooth-and-claw conflict. The individual on the smaller scale of time is a self that is self-making and self-sustaining at the expense of the world outside. The larger time-scale of survival is imposed by the physical fact of

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28 LIFE: ITS NATURE AND ORIGIN death. Organisms are not very durable; neither are they immune to fatal accident. Any static population of organisms would be destroyed in time or, if aging is an intrinsic property, by time. The means of survival over an indefinite time-span is reproduction: replacement and increase of in- dividuals of a kind by more of the same kind. Most descriptions of life include reproduction as the most important attribute, and correctly so. Other activities that contribute to survival woqld be futile if organisms could not reproduce; indeed the survival of the individual is often sacri- ficed to the propagation of the kind. Reproduction itself allows for indefi- nite survival of a species so long as the external conditions permit survival at all. Species are, however, not absolutely stable, but change in time, for organisms are subject to alterations (mutations) that are propagated by reproduction. The survival and accumulation of inheritable changes are governed by their value for the survival of the reproducing individuals; thus viewed the fruit of Darwin's genius becomes a truism. It has been said that living things are things that can reproduce, mutate and reproduce mutations. We are fairly sure that this statement covers all the living things in nature on this planet and that it is free from ambiguity so far as natural objects are concerned. (We would want such a distinction to apply only to natural objects. The invocation of man-made models that imitate the formal attributes of living things is irrelevant; who would desire a concep- tion of an organism so strict that men could not aspire to imitate it? If we are horrified by imitations of life, it is because the imagination of science fiction and cybernetics seems to run to intelligent machines with obscurely malevolent intentions; most of us would be less horrified by nice, soft, synthetic arteries to replace our sclerosing plumbing, or synthetic yeasts that would produce superior beer.) If an object is most satisfactorily identified as a living thing by showing it to be capable of self-replication, mutation and replication of mutation, that is not necessarily the only or the most practical expedient. It cer- tainly would not be the best means of attributing life to an elephant; it takes much enterprise to observe the reproduction of elephants and exces- sive patience to demonstrate their mutations. On the other hand, it is easy to demonstrate reproduction of a bacterium (and almost as easy to demon- strate mutation). In practice, we would seldom be mistaken in assuming that an elephant is alive because he is wagging his tail, but it takes a good microscopist to observe a bacterium waving its tail. In fact, there has been considerable debate as to whether a bacterium moves its flagellum or whether the flagellum moves the bacterium. The admission that we cannot always appeal to the ideal criteria of an organism—reproduction and mutation—

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What Is Life? 29 is inconvenient, but not very damaging. We are assisted by recognizing that the processes of organisms are so interlocked that the observation of any one of them leads to inferences about the survival of individuals and species. If we are assured that the elephant wagging his tail is a living thing, we will not doubt that it can be shown that the wagging of the tail serves some purpose, however minor, in improving the elephant's chances of surviving to produce more elephants. There is small hazard in assuming that a microscopic object that beats a flagellum will produce a colony of descendants if placed in a suitable nutrient medium. Organisms are surviving things: self-making, self-sustaining, kind-con- serving things. And it is as things in the ordinary sense, as objects denned by their form, that we commonly identify them among things in general and classify them among themselves. The forms of organisms are hardly the sole basis for discriminating decisions about living things, as we shall see, but they can be the most immediate and most subtle ones. We may resist them in scientific principle—while never failing to resort to them in practice—because the judgments are inherently subjective and qualitative, difficult to express and impossible to measure. The values and the pitfalls of morphology have the same cause: our total perceptions are far more subtle and discriminating than are readings of meters, but harder to describe and agree upon. (Thus a romantic poet can charm by his efforts to assert that one girl is exceptional among girls in general, describing her form by surface anatomy and by analogies to astronomical objects, flowers, jewels and the more attractive fauna. We share his feelings, but would not recog- nize that girl from his description, and if we did we might not think she was so very different from girls in general.) In appealing to morphology as a criterion of living things, we are in that ever-embarrassing position of feeling confident that we could recognize something though we are unable to state with great precision what we are looking for. We want a language, a mathematics of form. What we can say is that the organization of living things is expressed in complex form, in form-within-form. The forms are plastic, topological in spirit, recog- nizable and functional despite distortion. (When Donald Duck is flattened by a steamroller, he is still Donald Duck; the plasticity of living form can be a source of naive humor.) Most important, because deriving from the survival principles themselves, is the generic character of biological form; identification is not based solely on the finding of one individual with a given complex form but on the fact that there will be many individuals with similar form. Part of our sense of biological form is its contrast with inorganic form. Crystals are the structures in the non-living world that have regular and

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30 LIFE: ITS NATURE AND ORIGIN repetitive forms. If we had no preconceptions about the structure of crystals, we might perhaps confuse some of them with some biological forms. But we do know a good deal about crystals, have many ways of identifying them and are not likely to be confused. The appeal of morphology in the identification of living things rests on the multiplicity of clues rather than the precision of single criteria. A com- mon paradigm of scientific insight—and one of considerable literary ap- peal—is our ability to deduce from fossil shadows the forms and ways of life of organisms that have long ago surrendered survival to durability. It is a true paradigm. Complex forms are always taken seriously as signs of living things. We can be moved by fossil forms and find a singular beauty in form that is congealed in time. If seeing is at least a strong invitation to believing, it goes without saying that seeing is not limited to naked vision. The clues of complex form hold good for all levels of "seeing," not only for the smallest organisms, but for the smallest parts of organisms. If biological science admitted its dogmas, "function goes with form" would stand high on the list. We have a rather good historical test in the work of the early microscopists: old Leeuwenhoek, an uneducated man who made crude microscopes in the 17th century, had no hesitation in identifying "animalcules" in the world he found beyond the limits of the unaided eye and he did not make many mistakes. In our own world, there are few organisms that cannot be seen with the ordinary light microscope, and there is a good reason for this: even simple surviving things require a considerable number of large molecules for their minimum functions and the smallest living thing is bound to be rather large. There are some forms whose dimensions are just beyond the power of the optical microscope—for example the so-called PPLO (pleuromonia-like) organisms. Their existence tells us that the search is not ended at the limits of the light microscope, but no change of principle is called for: such organisms have regular and complex form, not easily confused with that of any known natural non-biological system when viewed with the electron microscope. We do not think that much smaller, still- undiscovered organisms exist in our world. Viruses are a little aside from the point. They do have complex form but generally are recognized either by chemical criteria (which we shall discuss later) or by the fact that they reproduce in larger organisms. The very idea of a virus implies the existence of larger and more complex or- ganisms; if we were to find something with a virus-like chemistry we would strongly suspect that cellular organisms were also present. The great generalization of morphology is the cell theory, which states that organisms are either cells or societies of cells. Despite the myriad

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What Is Life? 31 variations of cells, ranging from relatively large and elaborately equipped protozoa to degenerate forms such as red blood cells, the microscopist can identify cell structure with reliability and certainly will not confuse it with anything belonging to the inorganic world. He has an abundance of cri- teria and does not need a full-fledged "typical" cell in order to know that he is observing a cell. The prevalence of morphology does not end there. Cells are composed of sub-units: walls, nuclei, various particles, all of which are recognizable as having consistent forms, which is only to say that the principle of sur- vival is a very strict one and that all the finer details of the organization of the living things we know are preserved and propagated. Thus, as a naturalist exploring a strange world, I think I would recognize a cow as a living thing, but if I were not permitted to see the cow but could examine a sample of hamburger with an ordinary microscope I would have little difficulty in knowing that I was observing the vestiges of an animal and might be able to say something about the animal. If the hamburger were so finely ground that I could no longer see cells, I would see nuclei, mitochondria, membranes, etc., all of them bodies of characteristic form. Even small pieces of these pieces would be identifiable with the electron microscope. It is difficult to see how one could be deceived. True, there are imaginable simplicities of biological form that might call for some reservations. The simplest imaginable cell would be a mem- brane-enclosed sphere in which the macromolecules responsible for life- processes would not be organized into formed "organs," but would be dispersed in a liquid internal medium. If one found a population of such limp bags, all very much alike, he would strongly suspect that they had significance for biological evolution, though he could not be so sure he was dealing with an organism as if he had found a cow. Such a discovery would excite us greatly; it would be an ambiguity, but one that could be resolved by appeal to the molecular properties of living things. We are not restricted to morphology. All this appeal to form in perception of the character of living things may be a little embarrassing to the Laputan in us. Facing toward imme- diate experience, it weds the scientific self to the self that responds un- thinkingly to nature and to natural beauty, that loves flowers because they are lovely and delights in the curiousness of the shells of sea animals. Yet we can turn our consideration of biological form in a more analytical direction. We do not think that structure in organisms is ever a play of nature; it is always valuable and meaningful for survival. We think this is so for the gayest plumage of birds, the most sumptuous coloring of flowers, the most exquisite or barbaric sculpturing of molluscan shells, the lace-patterns of diatom shells, the strange forms of fish or insects, some-

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32 LIFE: ITS NATURE AND ORIGIN times charming, sometimes nightmarish. Even when we find our explana- tions of these forms inadequate, we are not prepared to deny their signifi- cance for survival. The maxim that relates form to function holds at the deepest analytical level. The cell, the fundamental unit, is both the unit of biological form and the atom of survival. Its own form, and the form of its parts, can be understood as a device for maintaining a rather precisely defined popula- tion of molecules and maintaining these molecules in proper—for survival —spatial relationships to each other. Conversely, the maintenance of these spatial relations of molecules is the ultimate source of all biological form. Thus we arrive at the idea of a Molecular Biology: namely that the form of living things and the operations that produce and are produced by that form are reflections of a definite and definable organization of matter. The molecular approach to biology is not the only one, but it needs no justification. We study Life as a molecular operation because it is a molecular operation. It would be unfortunate if this outlook seemed to arouse archaic mis- givings about "materialism." The whole point is that molecular biology discovers in organisms potentialities, subtleties, values and purposes in the organization and operation of matter that are not disclosed in the ele- mentary properties that are the domain of physics; we could not know, but had to discover, that matter is not necessarily undignified dirt. We are allowed to make rather firm general statements about the or- ganization of the living things we know by the fact that they are so much alike in so many ways. It is a fact that we can explain on evolutionary grounds. We may even think that they are too much alike for the purposes of a still more general understanding. Natural Selection could tend to eliminate alternatives that would be viable under other conditions; that is one reason why concern about life in other worlds is as relevant to our own biology as it is to a fundamental curiosity about the universe. Let us consider some of the ways in which all the cells we know are fundamentally alike. THE CELL AS AN ENCLOSED AQUEOUS SYSTEM Cells contain water, usually more than 50 per cent by volume, and cease to operate when the water content falls too low. This inability to operate without an ample amount of liquid water is not necessarily fatal; there is the fascinating possibility of dormant life-processes that resume when the liquid water is restored. An ordinary seed is an example of this.

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What Is Life? 33 The cell is bounded by a membrane that separates the inside aqueous solution from the outside aqueous medium. The membrane is a lipid-pro- tein complex about 50-100 A thick and is rather similar in form in every kind of cell. The similarities extend to fine details, which will not be described here. A living system does not conform even in its simple internal chemistry to the chemistry of its surroundings, but selects and rejects among even the simplest components of its world. The chief constituent of the internal solution is the potassium ion. This ion is relatively scarce in the environ- ment and the internal concentration of potassium is almost invariably higher than that in the medium. Other ions are either concentrated or excluded and it can be said that the composition of the internal solution is invariably different from that of the bathing environment. ENZYME PRINCIPLE The entire chemistry of the cell is governed by the principle that all reactions are catalyzed by enzymes and that each reaction is catalyzed by a different enzyme. This principle, exceptions to which are rare, sets bio- chemistry apart from the natural chemistry of the non-living world and even from chemical technology, which may employ catalysts but does not possess such a variety and specificity of catalysts. (The enzyme principle is seldom named as such in the biological literature, apparently because it developed gradually and is not associated with a single sensational discov- ery.) In our own world, all the enzymes are proteins. In any other world in which the principle applies, they would have to be a class of molecules with the same possibility of variety. The fundamental meaning of the enzyme principle is not merely that life-processes are very rapid, even though they take place at low temperatures, but that their rates are governed and balanced by the nature of the enzymes and are not at the mercy of their spontaneous rates. Sometimes we exaggerate when we say that the life of a cell could be completely described if we knew enough about its enzymes, but it is not an outrageous exaggeration. FLOW OF MATTER AND ENERGY The conservation principle of living things is different from, in fact opposite to, the main conservation principles of physics. Survival is a synonym for the "conservation of individual and kind" and it takes place by

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34 LIFE: ITS NATURE AND ORIGIN an expenditure of both matter and energy by the individual and the kind. (Only if we had some quantitative expression for what is conserved in sur- vival would biology cease to be an essentially qualitative science.) The flow of matter in living systems in our world is ultimately gov- erned by the capture of simple constituents of the Earth and its atmosphere, such as carbon dioxide, nitrogen or its simpler compounds, water and a small number of inorganic elements. These are built into molecules much more complex than any that exist in the non-living world itself and if we find com- plex molecules outside organisms we always assume that they were put there by organisms. The life-death cycle returns matter to the non-biological world, so that the total flow of matter is describable in terms of cycles. It is, of course, incorrect to say that living things consume matter; what we mean is that their mere survival demands an enormous flow of matter through them. The source of energy for the transformations of matter in our kind of living things is chemically reduced molecules and the energy is made avail- able by catalyzed oxidation of reduced molecules. The major ultimate source of energy is sunlight, which is used in photosynthesis to reduce car- bon compounds. But there are other sources available to specialized organ- isms, such as reduced inorganic elements. It is also postulated in contemporary discussion that the early history of our planet, and perhaps of others, included the production of reduced organic compounds by proc- esses not requiring organisms, so that a ready source of energy and of prefabricated organic molecules could have been available at primitive stages of evolution (cf., Chapter 2). The fundamental principles of our biological energetics may first be stated in negative form by contrasting organisms with familiar engines: organisms cannot use directly the energy released by oxidation, nor can they exploit temperature differences to perform work. They employ what may be called a principle of chemical energy coupling, a two-step process. Energy is "stored" in so-called high-energy compounds and "used" by breaking down these compounds, much as an old-fashioned submarine burned fuel in its engines to charge its batteries on the surface, then used the stored energy to propel it under water. In cells, the energy made available by oxidation of food (reduced compounds) is used to form high-energy compounds. The latter supply energy to the systems of the cell, where it is expended in many ways: synthesis, "pumping" of matter, movement, pro- duction of electricity, production of light, communication, etc. All the organisms we know use the same high-energy compound for carrying out most of their work—adenosine triphosphate, familiarly called ATP. Thus living things are quite different kinds of machines from those made for us by our engineers. An automobile is a device for oxidizing reduced com-

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What Is Life? 35 pounds by combustion, using the heat to drive pistons. A horse is a cool device in which the muscles oxidize sugar without combustion, storing the chemical energy (originally solar energy) in ATP. The ATP then reacts with a system of molecular fibers in the muscle, causing motion as the ATP is broken down. We would expect the principle of chemical energy coupling to apply to any form of life; to imagine surviving systems that solve their energetic problems in any other way is to appeal to an extreme of science fiction that is beyond the asymptotes of our knowledge. Still, the details we know about such systems are not so demanding. We cannot say why ATP in par- ticular was chosen during evolution of life on our world, since chemical considerations suggest that many kinds of compounds may be suitable for energy storage. THE DESIGN FOR SURVIVAL Survival, both of the individual and of its kind, implies a conservation of character in the face of a continuous flux of matter. Stability of the matter itself is secondary to the character by which we acknowledge the individual or species; whether this dog is still the same dog named Igor or whether he is a descendant or an ancestor of dogs, does not depend at all on whether he still has any of the atoms he was born with. (The conservation of self and of kind is most vividly expressed in consciousness; conversely, the temptation to impute consciousness to dogs or to bacteria comes from the evidence of behavior that can be explained only in terms of survival of self and kind.) Modern biology has accepted the task of accounting for what is con- served, and for the means of conserving it, in terms of the molecules of which living things are made. It is superfluous to say that living systems are molecular systems; the objective of molecular biology is to translate the meanings of survival into molecular terms. The skeleton of the design is now perceived; the cardinal intellectual sin is to think the story complete. The essential points are these: (1) The unit organism, the biological atom capable of surviving in a non-living environment, is the cell. If we abstract to the essentials, all cells are the same. (2) The two main domains of action in the cell are what we may call, loosely, a genome and a cytoplasm. The genome embodies the hereditary material. It is the embodiment of conservation and survival, that part of the organism which remains constant in character and is transmitted from gen- eration to generation. The replication of genomes is the ultimate basis of

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36 LIFE: ITS NATURE AND ORIGIN survival, for so long as exact copies of the genome can be made and exact copies of the copies can be made, survival becomes independent of the flux of matter. (3) The replication of genomes is an extraordinarily exact process, but mistakes can be made. The mistakes are then replicated indefinitely. These are the hereditary changes or mutations, that are thought to be the sources of variation at the disposal of the evolutionary process. (4) The cytoplasm—all the structure and working-equipment of the cell —forms itself and grows under the government of the genome. Its con- stancy is a reflection of the constancy of the genome; its ability to vary in a consistent way is a reflection of the versatility of the genome. The genome seems to be programmed to give a consistent pattern of instructions over the life of the organism, yet also is responsible in its commands to "needs" of the organism. (5) The molecular basis of the genome is deoxyribonucleic acid (DNA); the inherited instructions to the organism are coded by the sequence of subunits (nucleotides) in DNA as letters of a language are coded by their sequence in words and sentences. DNA is the self-replicating molecule, and many of the chemical principles of its replication are known. (6) The main molecular basis of the form and functions of the cytoplasm —both factory and machinery—is expressed in protein molecules, whose individual character depends on a sequence of small component molecules, the amino acids. This is clearer in the chemical operations of the cell which, as we have seen, are governed by the amounts and kinds of enzymes. All known enzymes are proteins. The genome determines the character of the cell as a machine for capturing and transforming matter by determining the kinds of enzymes, the amounts of enzymes, and the time when each enzyme is made. (7) The instructions in the genome, which is DNA, are translated into the formation of proteins by a system of molecular transcription, many of whose features are known. Thus, the minimum design for survival can be expressed in the proposi- tion that the cell is a device for maintaining and propagating a genome. The genome determines the structures and the operations of a cytoplasm which in turn provides the genome with all the goods and services it needs for its maintenance and propagation. The genome makes only two things: copies of itself and copies of commands to the cytoplasm. The cytoplasm makes everything else, structures, enzymes, ATP, etc., carries on the business with the outside world and plays peasantry and proletariat to the autocratic (and parasitic) dynasties of the genome. These features of the living things we know are common to all of them, and each contains details that are equally common. We may repeat once

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What Is Life? 37 more that in the life we know it is not only the principles that are shared, but the material embodiments of the principles. Instead of stating princi- ples we could list the corresponding substances: water, potassium and other characteristic elements, the molecules of cell membranes, ATP and lesser high-energy compounds, an immense number of protein catalysts (all of which we would expect to find in every cell), DNA, the chemical constitu- ents involved in the replication of DNA, the substance ribonucleic acid (RNA) into which the messages of the genome are transcribed, the chemical machinery of genome-directed protein synthesis . . . and the list would go on with molecules we have not mentioned. If we think of living things as chemical systems, evolution, which has produced such a vast variety of forms, has been rather conservative in the kinds of molecules it seems to have used from the first. It is not that there has been no molecular evolu- tion, but that molecular evolution leaves its imprint mainly in the finer details of molecules and in those molecules (the pigments of flowers, for example) that are significant to the special problems of survival of special organisms, but are not fundamental to the state-of-being-a-living-thing. Anyone can observe the difference between a man and a chimpanzee, but it takes rather clever chemistry to establish the fact that the molecules of the two are different. On the other hand, it takes only very simple chem- istry to decide that both are organisms. If we contemplate living things over the longest time scale, we will not confine our thoughts to self-maintaining, kind-maintaining organisms. We will acknowledge that something came before them, and find force for that opinion in the very fact that existing organisms have so much in common. For the quintessential point of evolutionary reasoning is that similarity implies common descent and, if that point has sometimes been misleading, we still are inclined to think that the common material features of living things imply common descent beginning with a common supply of molecules out of which the first organisms were made. Therefore, an evolutionary frame of thought leads to the conclusion that a biological exploration of this world at one time would have been a search for the molecular features of a planet on its evolutionary way to the produc- tion of full-fledged organisms. Some of these features are contained in a familiar word that has lost some of its original implications but not all of them: the term organic chemistry. Originally implying that certain kinds of molecules could be produced only by the action of living things, it lost its vitalistic meaning when the chemist in his laboratory began to learn how to make similar molecules and so became an organic chemist. In a way, that event which was so portentous in the history of science and technology seems rather insignificant from our immediate standpoint; it merely says that one kind of organism, which we call an organic chemist, is capable of

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38 LIFE: ITS NATURE AND ORIGIN imitating in the laboratory the molecules made by other organisms. But evolution gives it a deeper meaning: that there must have been a time in this world when molecules now made in nature by organisms (which have a sumptuous battery of enzymes at their disposal) were made directly and without the guidance of enzymes by natural processes of the planet itself. Organic chemistry defines itself as the study of that immense class of compounds that is made possible by the ability of carbon atoms to bond to other carbon atoms in a great variety of linear and cyclic forms. On the planet Earth, the presence of such molecules can be taken as traces of life: signs of living things, the remains of living things that have lost their investment in survival, or products made and cast off by living things. But the chemical traces of life can be much more definite than the mere presence of organic compounds and more compelling as evidence of the presence or transit of organisms. If we limit our consideration of biochemistry only to that chemistry which is characteristic and indispensable to all the organisms we know, we can appeal to distinct classes of carbon compounds, to extra- ordinarily important linkages of carbon to nitrogen or to phosphorus or to sulfur, can search for those vital complexes of organic molecules with metals, iron or magnesium or others, that play such a part in the ener- getics of organisms. We need not be more specific here; any elementary text of biochemistry lists dozens of compounds that belong only to the world of organisms in the world we know. Another general principle of the chemistry of biological systems is that of polymerization. We have seen that the design for survival and reproduc- tion calls for operations of very large molecules; all the naturally occurring molecules at the long end of the chemists' size-spectrum are products of organisms. The crucial classes of such large molecules are the nucleic acids, by which the genetic record is coded, replicated and translated, and the pro- teins, whose subtle and varied chemical shapes are adapted to recognizing particular molecules, combining with those molecules, and catalyzing their transformations. The nucleic acids are the genes, the regulators of the genes, the messengers of the genes; the proteins are the enzymes. This variety of immense molecules is made by stringing together smaller mole- cules. In the case of nucleic acids the huge molecules represent merely permutations of the sequence of four kinds of small molecules called nucleotides. In the case of the proteins, the variety and subtlety is achieved by the patterned folding of chains that themselves comprise permutations of the sequence of about 20 types of smaller molecules, the amino acids, molecules of the same kind but different in important ways. Thus, all the variety of living things as we know them is derived essentially from four kinds of nucleotides (or 8 kinds by a different reckoning that is mainly of technical interest), and from 20 kinds of amino acids. The trick lies

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What Is Life? 39 in how they are lined up, much as all the meanings and operations of all the languages using our alphabet depend on the ordering of 26 or so letters. The spoor of life is in such chemical traces, and the means for observing them are powerful and objective. It makes sense to think that instruments detecting the constellations of atoms that go with living things could give us news about life, though it might say very little about organisms. The chemical traces would be expected in a world at the early stages of biological evolution, before organisms; according to prevailing ideas, the molecules might be accumulated in even greater abundance than in an evolved bio- logical world, precisely because of the absence of reproducing organisms to "eat them up." The traces might be found in a world in which organisms were now extinct; we do not always think of an oil well as a trace of life, though we know it is. To think of a universal biology is to test the generality of propositions that apply to our own biological world. A basic example is a question that has often been discussed since it can be approached through theoretical chemistry: must organic chemistry be a chemistry of carbon, or can one think of a comparable variety of biochemical compounds and reactions that is based on some other element? (cf., Chapter 14.) For example, could a comparable chemistry be achieved with the silicon atom? The answer gen- erally has been negative, although such questions will continue to be asked. At an even more fundamental level, it has been asked whether water should be regarded as a unique medium for organic reactions, or whether other solvents (for example, liquid ammonia) might not serve the same purpose in another world. Again, the answer has seemed dubious, yet no one would exclude categorically the possibility of other solvents though we probably would insist that biological systems would have to be liquid systems. As we turn to molecules with a more particular significance for survival, the estimates of their universality are even less certain. In principle, a bio- logical system will require "high-energy compounds" but how similar to ATP would they have to be? Heredity would require a coded replicable polymer (unless governed by a principle not yet imagined); what properties of DNA could be embodied in a polymer that was quite different from DNA—so different that it would not give any of the chemical tests for a DNA-like molecule? What kinds of large molecules that did not test as proteins could have the same versatility of catalytic powers? Perhaps theory can expose possibilities and probabilities; the explorer will make the tests. We ask "What is Life?" but find that we can define only the attributes and material traces of living things. The definition is not succinct, for it comprehends all the principles and material facts that distinguish the life we know, and gains precision from the specification of additional detail.

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40 LIFE: ITS NATURE AND ORIGIN Nevertheless, the distinction between the living and the non-living is no less real because the boundary seems blurred. Indeed, the evolutionary ap- proach to biology implies that we should expect the boundary to be diffuse (in the past, if not now), though the domains on either side are quite dif- ferent. The problem is not that our conception of a living thing is vague; on the contrary, our concern is that it is too definite because it is too provincial. It may seem curious that the biologist has so little to say about death in his talk about life, but he has his reasons. The failures of individual sur- vival are determined by a multitude of causes: caprices of the environment, accident and predation, excessive fecundity, and senescence in those few individuals that can enjoy the luxury of old age. There is no common de- nominator. The powers of reproduction are so efficacious that death seldom leads to the extinction of species unless it is assisted by the singular lethality of the most advanced of species. Death is a private affair; the poet and the prophet have more to tell us about it than does the biologist.