Life

Physiological

For many years a physiological definition of life was popular. Life was defined as any system capable of performing a number of such functions as eating, metabolizing, excreting, breathing, moving, growing, reproducing, and being responsive to external stimuli. But many such properties are either present in machines that nobody is willing to call alive, or absent from organisms that everybody is willing to call alive. An automobile, for example, can be said to eat, metabolize, excrete, breathe, move, and be responsive to external stimuli. And a visitor from another planet, judging from the enormous numbers of automobiles on the Earth and the way in which cities and landscapes have been designed for the special benefit of motorcars, might well believe that automobiles are not only alive but are the dominant life form on the planet. Man, however, professes to know better. On the other hand, some bacteria do not breathe at all but instead live out their days by altering the oxidation state of sulfur.

Metabolic

The metabolic definition is still popular with many biologists. It describes a living system as an object with a definite boundary, continually exchanging some of its materials with its surroundings, but without altering its general properties, at least over some period of time. But again there are exceptions. There are seeds and spores that remain, so far as is known, perfectly dormant and totally without metabolic activity at low temperatures for hundreds, perhaps thousands, of years but that can revive perfectly well upon being subjected to more clement conditions. A flame, such as that of a candle in a closed room, will have a perfectly defined shape with fixed boundary and will be maintained by the combination of its organic waxes with molecular oxygen, producing carbon dioxide and water. A similar chemical reaction, incidentally, is fundamental to most animal life on Earth. Flames also have a well-known capacity for growth.

Biochemical

A biochemical or molecular biological definition sees living organisms as systems that contain reproducible hereditary information coded in nucleic acid molecules and that metabolize by controlling the rate of chemical reactions using proteinaceous catalysts known as enzymes (see enzyme). In many respects, this is more satisfying than the physiological or metabolic definitions of life. There are, however, even here, the hints of counterexamples. There seems to be some evidence that a virus-like agent called scrapie contains no nucleic acids at all, although it has been hypothesized that the nucleic acids of the host animal may nevertheless be involved in the reproduction of scrapie. Furthermore, a definition strictly in chemical terms seems peculiarly vulnerable. It implies that, were a person able to construct a system that had all the functional properties of life, it would still not be alive if it lacked the molecules that earthly biologists are fond of—and made of.

Genetic

All organisms on Earth, from the simplest cell to man himself, are machines of extraordinary powers, effortlessly performing complex transformations of organic molecules, exhibiting elaborate behaviour patterns, and indefinitely constructing from raw materials in the environment more or less identical copies of themselves. How could machines of such staggering complexity and such stunning beauty ever arise? The answer, for which today there is excellent scientific evidence, was first discerned by the evolutionist Charles Darwin in the years before the publication in 1859 of his epoch-making work, the Origin of Species. A modern rephrasing of his theory of natural selection goes something like this: Hereditary information is carried by large molecules known as genes, composed of nucleic acids. Different genes are responsible for the expression of different characteristics of the organism. During the reproduction of the organism the genes also reproduce, or replicate, passing the instructions for various characteristics on to the next generation. Occasionally, there are imperfections, called mutations, in gene replication. A mutation alters the instructions for a particular characteristic or characteristics. It also breeds true, in the sense that its capability for determining a given characteristic of the organism remains unimpaired for generations until the mutated gene is itself mutated. Some mutations, when expressed, will produce characteristics favourable for the organism; organisms with such favourable genes will reproduce preferentially over those without such genes. Most mutations, however, turn out to be deleterious and often lead to some impairment or to death of the organism. To illustrate, it is unlikely that one can improve the functioning of a finely crafted watch by dropping it from a tall building. The watch may run better, but this is highly improbable. Organisms are so much more finely crafted than the finest watch that any random change is even more likely to be deleterious. The accidental beneficial and inheritable change, however, does on occasion occur; it results in an organism better adapted to its environment. In this way organisms slowly evolve toward better adaptation, and, in most cases, toward greater complexity. This evolution occurs, however, only at enormous cost: man exists today, complex and reasonably well adapted, only because of billions of deaths of organisms slightly less adapted and somewhat less complex. In short, Darwin's theory of natural selection states that complex organisms developed, or evolved, through time because of replication, mutation, and replication of mutations. A genetic definition of life therefore would be: a system capable of evolution by natural selection.

This definition places great emphasis on the importance of replication. Indeed, in any organism enormous biological effort is directed toward replication, although it confers no obvious benefit on the replicating organism. Some organisms, many hybrids for example, do not replicate at all. But their individual cells do. It is also true that life defined in this way does not rule out synthetic duplication. It should be possible to construct a machine that is capable of producing identical copies of itself from preformed building blocks littering the landscape but that arranges its descendants in a slightly different manner if there is a random change in its instructions. Such a machine would, of course, replicate its instructions as well. But the fact that such a machine would satisfy the genetic definition of life is not an argument against such a definition; in fact, if the building blocks were simple enough, such a machine would have the capability of evolving into very complex systems that would probably have all the other properties attributed to living systems. The genetic definition has the additional advantage of being expressed purely in functional terms: it does not depend on any particular choice of constituent molecules. The improbability of contemporary organisms—dealt with more fully below—is so great that these organisms could not possibly have arisen by purely random processes and without historical continuity. Fundamental to the genetic definition of life then is the belief that a certain level of complexity cannot be achieved without natural selection.

Thermodynamic

Thermodynamics distinguishes between open and closed systems. A closed system is isolated from the rest of the environment and exchanges neither light, heat, nor matter with its surroundings. An open system is one in which such exchanges do occur. The second law of thermodynamics states that, in a closed system, no processes can occur that increase the net order (or decrease the net entropy) of the system (see thermodynamics). Thus the universe taken as a whole is steadily moving toward a state of complete randomness, lacking any order, pattern, or beauty. This fate has been known since the 19th century as the heat death of the universe. Yet living organisms are manifestly ordered and at first sight seem to represent a contradiction to the second law of thermodynamics. Living systems might then be defined as localized regions where there is a continuous increase in order. Living systems, however, are not really in contradiction to the second law. They increase their order at the expense of a larger decrease in order of the universe outside. Living systems are not closed but rather open. Most life on Earth, for example, is dependent on the flow of sunlight, which is utilized by plants to construct complex molecules from simpler ones. But the order that results here on Earth is more than compensated by the decrease in order on the sun, through the thermonuclear processes responsible for the sun's radiation.

Some scientists argue on grounds of quite general open-system thermodynamics that the order of a system increases as energy flows through it, and moreover that this occurs through the development of cycles. A simple biological cycle on the Earth is the carbon cycle. Carbon from atmospheric carbon dioxide is incorporated by plants and converted into carbohydrates through the process of photosynthesis. These carbohydrates are ultimately oxidized by both plants and animals to extract useful energy locked in their chemical bonds. In the oxidation of carbohydrates, carbon dioxide is returned to the atmosphere, completing the cycle. It has been shown that similar cycles develop spontaneously and in the absence of life by the flow of energy through a chemical system. In this view, biological cycles are merely an exploitation by living systems of those thermodynamic cycles that pre-exist in the absence of life. It is not known whether open-system thermodynamic processes in the absence of replication are capable of leading to the sorts of complexity that characterize biological systems. It is clear, however, that the complexity of life on Earth has arisen through replication, although thermodynamically favoured pathways have certainly been used.

The existence of diverse definitions of life surely means that life is something complicated. A fundamental understanding of biological systems has existed since the second half of the 19th century. But the number and diversity of definitions suggest something else as well. As detailed below, all the organisms on the Earth are extremely closely related, despite superficial differences. The fundamental ground pattern, both in form and in matter, of all life on Earth is essentially identical. As will emerge below, this identity probably implies that all organisms on Earth are evolved from a single instance of the origin of life. It is difficult to generalize from a single example, and in this respect the biologist is fundamentally handicapped as compared, say, to the chemist or physicist or geologist or meteorologist, who now can study aspects of his discipline beyond the Earth. If there is truly only one sort of life on Earth, then perspective is lacking in the most fundamental way.

Mechanism and vitalism

Human beings are ambulatory collections of some 10¹⁴ cells. Human cells are in many fundamental respects similar to those that make up all the other animals and plants on the Earth. Each cell typically consists of a central, usually spherical, nucleus and an outer more heterogeneous region, termed the cytoplasm. The substance of nucleus and cytoplasm together has for many decades been called protoplasm. Use of this term implied that there was some special substance underlying living organisms. In the use of the word protoplasm there is occasionally an implication that life cannot be explained solely by physics and chemistry, that some mysterious “vital force” must be invoked. A living cell is a marvel of detailed and complex architecture. Seen through a microscope there is an appearance of almost frenetic activity. On a deeper level it is known that molecules are being synthesized at an enormous rate. Almost any enzyme catalyzes the synthesis of more than 100 other molecules per second. In 10 minutes, a sizable fraction of the total mass of a metabolizing bacterial cell has been synthesized. The information content of a simple cell has been estimated as around 10¹² bits, comparable to about a hundred million pages of the Encyclopædia Britannica. Faced with all this or its equivalent, it is not surprising that early biologists felt despair at ever being able to understand the detailed workings of life.

A Stone Age man, confronted for the first time with a watch, might also deduce that there was some special watch substance in nature, or perhaps even a god of the watch. In ancient times, the most common of biological activities, such as the hatching of an egg or the blooming of a flower, were attributed to the intercession of a deity. After the epochal work of Sir Isaac Newton, when the motion of the planets and comets of the solar system was predictable to some very great precision and understood on the basis of an underlying principle, the idea developed that organisms were also nothing more than a particularly intricate kind of clockwork. But when early investigations failed to unveil the clockwork, a kind of ghostly mainspring was invented—the “vital force.” This force was a rebellion from mechanistic biology, an explanation of all that mechanism could not explain or for which mechanism could not be found. It also appealed to those who felt debased by the implication that they were “nothing more” than a collection of atoms, that their urges and apparent free wills arose merely from the interaction of an enormously large number of molecules in a way that, although too complex to use predictably, was in principle determined.

Not only is there no evidence for a vital force but the idea itself is hardly thought out; it is a sort of catchall concept, covering anything otherwise inexplicable. The alternative approach, that all organisms are made of atoms and nothing else, has proven especially useful and has led to a fundamental new understanding of biological systems. This situation does not imply, of course, that atoms cannot be put together in so complex a way that their collective behaviour is too difficult to understand in terms of the individual atoms; in this sense there may be particular laws of biology not readily derivable from the elementary interaction of atoms. But this is a very different thing from a vital force. Indeed, there is nothing debasing in the thought that a person is made of atoms alone; it means that one is intimately connected with the matter that comprises the inanimate universe. What a wonder that atoms can be put together in so complex a pattern as to produce human beings. Man is a tribute to the subtlety of matter. As the American anthropologist Loren Eiseley has written, “. . . if ‘dead' matter has reared up this curious landscape of fiddling crickets, song sparrows, and wondering men, it must be plain even to the most devoted materialist that the matter of which he speaks contains amazing, if not dreadful powers. . . .” (The Immense Journey, Random House, New York, 1957.)

Nucleic acids

It is now known that many if not all of the fundamental properties of cells are a function of their nucleic acids, their proteins, and the interactions among these molecules. Within the nuclear regions of cells is a mélange of twisted and interwoven fine threads, the chromosomes. During cell division, in all but the simplest organisms, the chromosomes display an elegantly choreographed movement, separating so that each daughter cell of the original cell receives an equal complement of chromosomal material. This pattern of segregation corresponds in all details to the theoretically predicted pattern of segregation of the genetic material implied by the fundamental genetic laws (see heredity). The chromosomes are composed of nucleic acids and proteins in a combination called nucleoprotein. The nucleic acid stripped of its protein is known to carry genetic information and to regulate cellular metabolism; the protein in nucleoprotein undoubtedly plays some secondary, probably regulatory, role.

The specific carrier of the genetic information in higher organisms is a nucleic acid known as DNA, short for deoxyribonucleic acid. DNA is a double helix, two molecular coils wrapped around each other and chemically bound one to another by bonds connecting adjacent bases. Each helix has a backbone that consists of a long sequence of alternating sugars and phosphates. Attached to each sugar is a base. Each sugar-phosphate-base combination is called a nucleotide; a nucleic acid strand can be thought of as a sequence of nucleotides. There is a very significant one-to-one base pairing in the connection of adjacent helices, in the sense that once the sequence of bases along one helix is specified, the sequence along the other is also specified. The specificity of base pairing plays a key role in the replication of the DNA molecule, where each helix makes an identical copy of the other from molecular building blocks in the cell. These nucleic acid replication events are mediated by enzymes, and with the aid of enzymes have been produced in the laboratory.

Ribonucleic acid (RNA) differs from DNA in having a slightly different five-carbon sugar, and in replacing one of the four bases that make up DNA by a slightly different base. RNA does not appear to exist in a double-stranded form. Now DNA, RNA, and the enzymes have a curiously interconnected relation, which appears ubiquitous in all organisms on Earth today.

Commonalities among organisms on Earth

The genetic code was broken in the 1960s. It was found that three consecutive nucleotides code for one amino acid of a protein molecule; e.g., an enzyme. By controlling the synthesis of enzymes, the nucleic acids control the functioning of the cell. Of the four different bases taken three at a time, there are 4³ =64 possible combinations. The meaning of each of these combinations, or codons, is known. Most of them represent a particular amino acid. A few of them represent punctuation marks; for example, instructions to start or stop a synthesis. Some of the code is degenerate in the sense that more than one nucleotide triplet may specify a given amino acid. These interactions among nucleic acids and proteins seem absolutely central to living processes on Earth today. Not only are these processes apparently the same in all organisms on Earth but even the particular dictionary that is used for the transcription of nucleic acid information into protein information seems to be essentially the same in all organisms. Moreover, this code has various chemical advantages over other conceivable codes. The complexity, ubiquity, and advantages of these processes clearly argue that the present interactions among proteins and nucleic acids are themselves the product of a long evolutionary history. At the time of the origin of life this very complex replication and transcription apparatus could of course not have been in operation. A fundamental problem in the origin of life is the question of the origin and early evolution of the genetic code.

There are many other commonalities among organisms on Earth. For example, there is only one class of molecules that store energy for biological processes until the cell has use for it, and these molecules are all nucleotide phosphates. The most common example is ATP (adenosine triphosphate). For this very different function, a molecule identical to the building blocks of the nucleic acids is employed. There are metabolically important molecules—e.g., molecules known as FAD (flavin adenine dinucleotide) and coenzyme A—which include subunits similar to the nucleotide phosphates. Porphyrins represent another category of ubiquitous molecules. Porphyrins are the chemical basis of hemoglobin, which carries oxygen molecules through the bloodstreams of animals; of chlorophyll, which is the fundamental molecule mediating photosynthesis in plants; and of the colours that many animals display. The left- or right-handedness of many biological molecules—discussed more fully below—runs identically through all organisms on Earth. In fact, of the billions of possible organic compounds, less than 1,500 are employed by contemporary life on Earth, and these 1,500 are constructed from less than 50 simple molecular building blocks. Similarly, organisms as diverse as paramecia and human sperm cells have little whiplike appendages called cilia or flagella used to propel themselves through their liquid environments. The cross-sectional structure of the cilia and flagella is almost always nine pairs of peripheral and one pair of internal fibres. There is no immediately obvious selective advantage of the 9 : 1 ratio. These commonalities indicate that a few basic chemical and functional patterns are being used over and over again, reflecting the extremely close relations all organisms on Earth have to one another. Many biologists believe that these commonalities—particularly where no obvious selective advantage exists—imply that all organisms on the Earth are descendants of a single common ancestor. But it is possible that there are more subtle selective advantages. The issue may have to await the first detailed study of an extraterrestrial organism.

The number of possible ways of putting nucleotides together in a chromosome is enormous. The renowned geneticist H.J. Muller estimated that in a human chromosome there are about 4 × 10⁹ base pairs. Each base pair position could be filled by any one of four possible bases; accordingly, the number of possible varieties of human chromosomes is 4^{4 × 109} (10^{2.4 × 109}), an inconceivably large number. By contrast, the number of elementary particles (electrons and protons) in the entire physical universe is only about 10⁸⁰. Thus a human being is an extraordinarily improbable object. Most of the 10^{2.4 × 109} possible sequences of nucleotides would lead to complete biological malfunction. Our nucleotides work because natural selection, over a 4,000,000,000-year history of life, has destroyed enormous numbers of combinations that did not work. But there still may be combinations that work far better than any now present, and the future holds the promise that man will be able to assemble nucleotides in any desired sequence to produce whatever characteristics of human beings are thought desirable, an awesome and disquieting prospect.

Metabolism

The chemical bonds that make up living organisms have a certain probability of spontaneous breakage. Accordingly, mechanisms must exist to repair this damage, or to replace the broken molecules. In addition, the meticulous control that cells exercise over their internal activities requires the continued synthesis of new molecules. These processes of synthesis and breakdown of the organic molecules of the cell are collectively termed metabolism, and for synthesis to keep ahead of the thermodynamic tendencies toward breakdown, energy must be supplied to the living system. Organisms acquire this energy by two general methods. Some organisms are heterotrophs, acquiring their energy by the controlled breakdown of pre-existing organic molecules (food)—generally those supplied by other organisms. Human beings and most other animals are heterotrophs. Alternatively, organisms may be autotrophs, acquiring their useful free energy from some other source, either from the energy of sunlight, in which case the organism is called a photoautotroph, or from the controlled chemical reaction of inorganic materials, in which case the organism is known as a chemoautotroph. Organisms that use both modes are called photochemoautotrophs.

A green plant is a typical example of a photoautotroph. It uses sunlight to break water into oxygen and hydrogen. Hydrogen is then combined with carbon dioxide to produce such energy-rich organic molecules as ATP and carbohydrates, and the oxygen is released back into the atmosphere. Many animals, on the other hand, utilize the atmospheric oxygen to combine chemically with organic materials they have eaten and release carbon dioxide and water as waste products in extracting energy from the organic materials. This is an example of an ecological cycle in which a material (here carbon) is pumped through two different organisms.

More generally, such metabolic cycles—used by the organism to extract useful energy from the environment—can be described in terms of oxidation-reduction reactions. In the case of respiration, molecular oxygen accepts electrons from glucose or other sugars. The oxygen is said to be an electron acceptor (it has a great affinity for electrons), the glucose an electron donor. This is the prototype of oxidation-reduction reactions, but not all such reactions necessarily involve oxygen. Biological electron acceptors other than oxygen include nitrates, sulfates, carbonates, nitrogen, and methanol. Biological electron donors other than sugars include nitrogen, sulfides, methane, ammonia, and methanol. For acceptor-donor transformations to occur over any period of time, biological cycles are necessary. It is possible that, for geologically short periods of time, organisms have lived off a finite supply of material, but for any long-term continuance of life, a dynamic cycling of matter, involving at least two different varieties of organisms, is necessary. If there is life on other planets, a similar cycling must exist. A search for such molecular transformations is one method of detecting extraterrestrial life.

On the Earth, all such useful biological electron transfer reactions lead to the net production of one or more molecules of ATP. Two of the three phosphates of this molecule are held by “energy-rich” bonds sufficiently stable to survive for long periods of time in the cell, but not so strong that the cell cannot tap these bonds for energy when needed. ATP and very similar molecules, all of them having a base, a five-carbon sugar, and three phosphates, are, so far as is known, the general and unique energy currency of living systems on Earth.

Metabolic processes do not occur in one step. The ordinary sugar, glucose, is not oxidized to carbon dioxide and water by living cells in the same way that occurs if a flame is applied to glucose in air. The resulting release of energy would be much too sudden, and concentrated in too small a volume, for such a process to be utilized safely by the cell. Instead, the glucose is broken down by a series of successive and coordinated steps, each mediated by a particular and specific enzyme. In almost all organisms that metabolize glucose, the sugar is first broken down in a set of anaerobic steps (that is, in the absence of oxygen). The total number of such steps is about 11. Some organisms are anaerobes; that is, they do not utilize molecular oxygen. In them the anaerobic steps are as far as the glucose metabolism is carried. Other organisms, including man, carry the oxidation of glucose further, gingerly combining glucose breakdown products with molecular oxygen. Such aerobic oxidation of glucose requires about 60 more enzymatically catalyzed steps. Another indication of the relative simplicity of the anaerobic breakdown of sugar is that all the enzymes used are free in solution in the cell; the aerobic steps use enzymes that are localized in specific regions of the cell. The complete aerobic breakdown of sugar to carbon dioxide and water is about 10 times more efficient than the breakdown accomplished by anaerobes; 10 times as many ATP molecules are produced. Similar themes and variations exist for the metabolism of other molecules (see metabolism).

The energy made available in this way to ATP is used in a variety of ways by the cell; for example, for motility. When an amoeba extends pseudopods, or a person walks, ATP molecules are being tapped for their energy-rich phosphate bonds. In addition, ATP molecules are used for the synthesis of molecules that the organism needs and does not have available. Among such molecules may be amino acids, the particular five-carbon sugars involved in nucleic acids, the nucleic acid bases, and so on. Each of these synthetic processes is again controlled and enzymatically mediated and may start from a variety of building blocks available to the organism, some simple, some more complex. For example, the amino acid L-leucine is produced from pyruvic acid, itself the product of the anaerobic breakdown of glucose. Synthesis of L-leucine from pyruvic acid involves eight enzyme-mediated steps and the addition of acetic acid and water.

These exquisitely interlocked and controlled metabolic steps are not usually performed in a diffuse manner all over the cell. Instead there is, at least in all higher organisms, a marvellously architectured cellular interior with particular specialized regions where particular chemical reactions are performed. Those oxidation-reduction reactions that involve molecular oxygen occur in an inclusion within the cytoplasm called the mitochondrion. The mitochondrion itself has an intricate substructure, and particular enzymes are thought to reside in particular sites within it; the molecule being metabolized may be passed on from one enzyme to another as through a conveyor belt in a factory. Similarly, photosynthesis occurs in a cytoplasmic inclusion called a chloroplast, which contains the chlorophyll and other pigments that absorb visible light, as well as the detailed enzymatic apparatus for the photosynthetic process. Chloroplasts and mitochondria, as well as other cytoplasmic inclusions at the base of flagella and cilia, all contain DNA. Moreover, this DNA has a somewhat different distribution of bases from that of the nucleus. It has been suggested that the cytoplasmic inclusions are the remnants of once free-living forms that, because of the favourable conditions found there, have taken up residence in the insides of other organisms.

Nucleic acids are known to pass from cell to cell and to perform their replication and coding functions efficiently in the new cell. In fact, viruses are essentially strands of nucleic acid, with a protein coat, that operate in just this way. It is also known that pieces of the genetic material from one cell may migrate into another cell of the same species and produce genetic and permanently heritable changes there. Alternatively, part of the virus nucleic acid may be permanently bound to the nuclear DNA of a host cell. It is likely that a virus is a degenerate form, now highly specialized to live off a specific host, of an organism once free-living and much more generally capable of performing a wide range of metabolic tasks. A virus must use the genetic transcription apparatus of its host cell. Many viruses do this extremely efficiently, turning a bacterium from a factory for making other bacteria into a factory for making viruses. In some cases it takes no more than 10 minutes for a bacterium infected by a single virus to produce a hundred new virus particles, which then burst forth from the host bacterium, destroying it. The line between benign or useful cytoplasmic inclusions and infective agents is not a very sharp one (see virus).

Eucaryotes and procaryotes

In the very simplest one-celled organisms one may distinguish between eucaryotic and procaryotic cells. Many familiar one-celled organisms, such as paramecia and amoebas, as well as the cells of all higher organisms including man, are eucaryotic. Such cells undergo mitosis, a fundamental sequence of events that occurs after DNA replication and that ensures that the DNA is precisely and equally distributed to the daughter cells. Eucaryotic cells have nucleoprotein in their nuclei. There is a membrane that separates the nucleus from the cytoplasm. Mitochondria are generally present in the cytoplasm, as is a very intricately convoluted structure, called the endoplasmic reticulum, that probably serves as the anchoring point for many cytoplasmic enzymes not contained in such inclusions as mitochondria or chloroplasts.

On the other hand there are procaryotic cells, which are most generally typified by the bacteria and the blue-green algae. In these cells nuclear division is nonmitotic, there is no nucleoprotein, and a nuclear membrane is absent. While eucaryotic cells may have more than one chromosome, procaryotic cells have one chromosome only, and that one is dispersed in the cytoplasm. Mitochondria, chloroplasts, and the endoplasmic reticulum are always absent. It is clear from this description that the procaryotes are in many respects more primitive than the eucaryotes. A basic unsolved evolutionary question concerns the evolution of procaryotes into eucaryotes.

An interesting subject of biological speculation concerns what the smallest and simplest contemporary free-living organism might be. The smallest free-living cells now known are the pleuropneumonia-like organisms (PPLO). While an amoeba has a mass of 5 × 10^-7 grams (1 gram = 0.035 ounce), a PPLO weighs 5 × 10^-16 grams and is only about 110 of a micrometre across. It can be seen only in the electron microscope. Such organisms grow very slowly. There may be smaller organisms that grow even more slowly, but they would be extremely difficult to detect. Even an organism of the size of PPLO has room for only about a hundred enzymes. A much smaller organism would have room for many fewer enzymes, and its ability to accomplish the functions that contemporary living systems must accomplish would be severely compromised. Were there, however, an environment in which all the necessary organic building blocks and such energy sources as ATP were provided “free,” a functioning organism could be substantially smaller than PPLO. In fact the inside of a cell provides just such an environment; this is why infectious agents, such as viruses, can be substantially smaller than PPLO. But it must be emphasized that such agents are not free-living organisms.

Metazoa, embryology, and sex

The distinction between single-celled and many-celled organisms (in animals, between protozoa and metazoa) is far from a sharp one. An interesting illustration is the slime molds, which undergo an extraordinary sequence of events during their life cycle. The cycle begins with single cells, somewhat like amoebas, which swarm, or combine, into a slimy mass with many nuclei called a plasmodium. The plasmodium in turn forms a sluglike mass that is certainly a multicelled organism. The slug develops into a stalked, fruitlike sporangium, still multicellular. The sporangium produces spores with cellulose cell walls similar to those of plants. The spores in turn germinate into small cells bearing flagella. The flagella are lost and the life cycle is completed with the production of an amoeboid form.

Biology is replete with life cycles of comparable complexity. It is possible that the swarming of individual cells to form a plasmodium may in fact be an example of the events that led to the production of metazoa in the early history of the Earth. Such life cycles, while apparently very exotic, are shared by many organisms, including man, where a one-celled, free-swimming sperm stage is part of the life cycle.

The life cycle of slime molds, or humans, or any other multicellular organism, brings up a fundamental and still largely unsolved problem. These organisms develop from a single cell that has a single complement of the genetic material. These cells then divide, forming many identical cells. The very early embryology of man goes through stages with 2, 4, 8, 16, etc., cells. Since the genetic information is identical in each cell, how does it ever happen that cells become specialized, forming hair cells, teeth cells, liver cells, blood cells, or bone cells? How can any given cell “know” what sort of specialized cell it must become, since all cells contain identical nucleic acids? Possibly the answer to this question has to do with geometry. After the 16- or 32-cell stage, there is a distinct difference between a cell on the inside of the embryo, which is entirely surrounded by cells, and a cell on the outside of the embryo, which is not entirely surrounded by other cells. One of the earliest major steps in embryonic development is a distinction in function between interior cells (the endoderm) and exterior cells (the ectoderm). There are physical and chemical interactions among adjacent cells. Perhaps any cell then has the capability of becoming any specialized cell, but cells are, as a result of their external cellular environment, called upon to develop in different ways. Occasional embryonic anomalies or cysts occur in which, for example, hair or teeth develop in totally inappropriate portions of the body. Similarly, eyes have been caused to develop on the limbs of frogs. Such incidents demonstrate the capability of the “wrong” cells to produce particular cellular specializations (see malformation).

Much of the beauty and diversity of contemporary life on Earth is due to sex. A totally asexual organism will be genetically identical to its (single) parent except for occasional mutations. The development of any major new adaptation would then require the acquisition of large numbers of appropriate mutations. Consider, for example, how the ability of an organism to metabolize a given molecule depends on the interaction of many enzymes, each produced by the transcription of the genetic information in hundreds of nucleotides, each nucleotide being the product of a single mutation. Thus, the chance development of any advantageous adaptation in an asexual organism requires the mutations to wait in line for a fortuitous juxtaposition.

Sex solves this problem in an elegant way. The genetic material of the parents is reassorted so that totally new combinations of genes are produced. In this way mutations acquired by any member of the population are rather quickly distributed to other members, and mutations arising in separate organisms can be combined. The likelihood of producing a useful sequence of mutations is thereby greatly enhanced. The advantages of sexual reproduction are so great that even many simple forms, such as bacteria or protozoa, which largely reproduce asexually, have occasional sexual encounters. While two sexes are clearly adequate for such a random assortment of genetic material, some organisms have developed more sexes: paramecia, for example, have somewhere between five and 10 sexes, defined in terms of the elaborate taboos about which organisms can combine their genetic material. In the process of genetic reassortment, some organisms make a very large number of attempts; for example, frogs lay millions of eggs at a time, and the number of sperm cells in a single human ejaculation is about 3 × 10⁸ (see sex).

The varieties of organisms and environments

The environment of the Earth is heterogeneous. There are mountains, oceans, and deserts, extremes of temperature and humidity. In addition, there are diverse microenvironments: oxygen-depleted oceanic oozes, ammonia-rich soils, mineral deposits with a high radioactivity content, and so on. The environment of an organism also includes the other organisms in its surroundings. For each of these environmental situations there are corresponding ecological niches, and the variety of ecological niches populated on the Earth is quite remarkable. Furthermore, ecological niches can be filled independently several times. For example, quite analogous to the ordinary mammalian wolf is the marsupial wolf that lives in Australia; the two have striking similarities in physical appearance and in predation behaviour. As another example, the same streamlined shape for high-speed marine motion has evolved independently at least three times: in Stenopterygius and other Mesozoic reptiles; in the tuna, which are fish; and in the dolphins, which are mammals. This case of convergent evolution must arise from the fact that hydrodynamics admits a narrow range of solutions to the problem of high-speed marine motion by large animals. Similarly, the eye has independently evolved several times among animals on the Earth; apparently such a structure is the best solution to the problem of visual recording. In those cases where physics or chemistry establishes one most efficient solution to a given ecological problem, natural selection will often tend to reach the solution, but not always. Some adaptations of undoubted utility, such as tractor treads in swampy environments, have never been evolved by natural selection on the Earth.

Life in extreme environments

There is an extraordinarily wide range of ecological niches to which organisms have adapted through the operation of natural selection. The same basic fabric of life has been used to produce very diverse organisms. The alga Cyanidium caldarium can grow in concentrated solutions of hot sulfuric acid. Other bacteria, algae, and fungi can live in extremely acidic (pH of 0) or extremely alkaline (pH near 13) environments. Procaryotic bacteria live in pools at Yellowstone National Park at temperatures above 90° C (194° F), almost at the boiling point of water. Sulfate-reducing bacteria are reported to grow and reproduce at 104° C (219° F) under very high pressures. Many organisms employ organic or inorganic antifreezes to lower the freezing point of their internal liquids, so that they can live at several tens of degrees below 0° C (32° F). Some insects use dimethyl sulfoxide as an antifreeze. Other organisms live in briny pools in which dissolved salts lower the freezing point. For example, Don Juan Pond in Antarctica has about one molecule of calcium chloride for every two water molecules and does not freeze until -45° C (-49° F). It contains a possibly unique microflora that continues to metabolize at least down to -23° C (-9° F). Biological activity does not cease at the freezing point of water; in fact some enzymes are actually more active in ice than in water. Many single-celled organisms can be frozen indefinitely to extremely low temperatures—the temperature of liquid air for example—and then be thawed with no decrease in activity. The primary damage that freezing causes is apparently due to the unavailability of liquid water and to the expansion and contraction attendant to freezing and thawing. Some arthropods can be severely dehydrated and then revived simply by adding water. In the dehydrated state they can be brought to any temperature from close to absolute zero to above the boiling point of water without apparent damage. When encysted in response to dehydration, some such organisms seem indistinguishable from a weathered grain of sand.

The great majority of familiar organisms on the Earth, however, are much more sensitive to the temperature of their surroundings. Warm-blooded animals internally regulate their temperatures for this reason. A human being whose body temperature drops below 30° C (86° F) or rises above 40° C (104° F) soon dies. Organisms that inhabit cold climates have special insulating layers of fat and fur. Other organisms adapt to seasonal temperature changes by producing dormant forms, such as spores or eggs, to survive the low temperatures. In all cases dormancy appears to be accompanied by dehydration.

Since organisms are composed largely of water, the availability of water is clearly a limiting factor. Here also, however, remarkable adaptations exist. Certain microorganisms can live on the water adsorbed on a single crystal of salt. Other organisms, such as the kangaroo rat and the flour beetle, obtain no water at all in the liquid state, relying entirely on metabolic water; that is, on water released from chemical bonds through the metabolism of food. A variety of plants, including Spanish moss, live in environments where they have no contact with groundwater—for example, on telephone wires—apparently extracting water directly from the air, although such plants require a relatively high humidity. Plants that live in deserts and other very dry environments have evolved wide-spreading root systems that adsorb subsurface water from a great volume of adjacent soil.

Organisms have been found from the stratosphere to the ocean depths. Bacteria and fungal spores have been discovered near the base of the stratosphere by balloons, and searches for organisms at much greater altitudes (up to 100,000 feet) have been attempted with ambiguous results. Birds have been observed flying at altitudes as great as 27,000 feet, and jumping spiders have been found at 22,000 feet on Mt. Everest. At the opposite extreme, microorganisms, fish, and a variety of other metazoa have been recovered from the ocean depths down to thousands of feet, where the corresponding pressures are hundreds of times that at sea level. At these depths no light can penetrate and the organisms, some of which are quite large and include unique phosphorescent adaptations to the dark, ultimately live off particles of organic matter raining down from the upper reaches of the oceans.

There is a range of adaptations to the radiation environment of the Earth. Some microorganisms are readily killed by the small amount of solar ultraviolet light that filters through the Earth's atmosphere at wavelengths near 3,000 angstrom units (Å; 1 Å = one ten-billionth of a metre). On the other hand, the bacterium Pseudomonas radiodurans thrives in the large neutron flux at the cores of swimming-pool reactors, to the continuing annoyance of nuclear physicists. Organisms can avoid radiation by shielding. For example, some algae and some desert plants live under a superficial coating of soil or rocks that are more transparent to visible light than to ultraviolet light. In addition, organisms have active methods of undoing the damage produced by radiation. Some of these repair mechanisms work in the dark; others require visible light. The usual reason for the ultraviolet sensitivity of organisms is that their nucleic acids absorb ultraviolet light very effectively at a wavelength near 2,600 Å. Generally speaking, there is an upper limit to the amount of ionizing radiation (such as gamma rays, X-rays, electrons or protons) that an organism can receive without being killed: in the vicinity of 1,000,000 roentgens. Such a lethal dose applies only to extremely radiation-resistant microorganisms; mammals, for example, are killed by much lower doses because there is more that can go wrong with such complex organisms. A lethal dose of ionizing radiation for human beings is a few hundred roentgens applied to the whole body. A thermonuclear weapon dropped on a populated area may deliver, through direct radiation and fallout, doses of a few hundred roentgens or more to people within a radius of some tens of miles of the target. Much smaller doses can produce a variety of diseases and predominantly deleterious mutations in the hereditary material. Moreover, the effect of small doses is cumulative. But until very recently human beings have not lived in environments characterized by large doses of ionizing radiation (see radiation: Biologic effects of ionizing radiation).

The sizes of organisms on the Earth vary greatly. As discussed above, the smallest free-living organisms on the Earth, PPLO, are about 1,000 Å in diameter; a limitation on the size of the smallest free-living organism is its volume: it must contain all the molecules necessary for metabolism. A variety of influences place an upper limit to the size of organisms. One is the strength of biological materials. Galileo calculated in 1638 that a tree taller than roughly 300 feet (91.4 metres) would, when displaced slightly from the vertical (for example, by a breeze), buckle under its own weight. (Sequoias, some of which exceed 300 feet, are apparently near the upper limit of height for an organism.) Because of the buoyancy of water, large whales are not presented with such stability problems, but other difficulties arise. For a fixed shape, the volume of tissues to be nourished increases as the cube of the characteristic length of the organism, but the surface of the gut, which adsorbs the ingested food, increases only as the square of the length. As the length is increased, a point of diminishing returns is ultimately reached.

The range of organic molecules that organisms on Earth can metabolize is very wide and occasionally includes such foods as formaldehyde or petroleum, which seem unlikely from a human point of view. Pseudomonas bacteria are capable of using almost any organic molecule as a source of carbon and of energy, provided only that the molecule is at least slightly soluble in water. Microorganisms cannot metabolize plastics, not because of any fundamental chemical prohibitions but probably because plastics have not been part of the environment of microorganisms for very long. Man tends to think of oxygen as extremely important for life, but there are facultative anaerobes that can take their oxygen or leave it, and obligate anaerobes that are actually poisoned by oxygen. Such organisms use a variety of alternative electron acceptors, as previously discussed.

The water content of organisms usually represents between 50 and 90 percent of the live weight. Unless there is a massive mineral skeleton, the dry matter of organisms constitutes about one-half carbon by weight, reflecting the fact that organic molecules are based upon carbon. A wide variety of other chemical elements are used for diverse functions. Amino acids are made of nitrogen and sulfur in addition to carbon, hydrogen, and oxygen. Nucleic acids, as has been seen, employ phosphorus in addition to hydrogen, nitrogen, oxygen, and carbon. Sodium and potassium are used in maintaining the electrolyte balance, and calcium and silicon as structural materials. Iron plays a fundamental role in the transport of molecular oxygen as part of the hemoglobin molecule. In some ascidians (sea squirts), however, vanadium replaces iron. Ascidian blood also contains unusually large amounts of niobium, tantalum, titanium, chromium, manganese, molybdenum, and tungsten. The vanadium and niobium compounds in ascidian blood may be adaptations to low oxygen levels. Occasional organisms use selenium or tellurium as electron acceptors; others may produce the fully saturated gas hydrides of arsenic, phosphorus, or silicon, as metabolic wastes. Still others form compounds of carbon with such halogens as chlorine or iodine. Many of the foregoing elements, plus copper, zinc, cobalt, and possibly gallium, boron, and scandium, perform particular functions in the enzymatic apparatus of cells. Many of these elements, both the uncommon ones and those as common as phosphorus, are very highly concentrated in organisms over their general availability in the environment. This concentration must indicate that such chemicals play unique functional roles where other more abundant elements will not serve.

Behaviour and sensory capabilities

Analogous to the wide range of physiological adaptations and the great variety of elements used by organisms on Earth, there is an enormous range of behaviour patterns and sensory capabilities. Coded into its nucleic acids is the information that allows a bird raised from the egg in the absence of other birds to migrate when migration time arrives, to build a nest characteristic of its species, or to engage in elaborate courtship rituals. Those birds that do not perform acceptably do not leave descendants. Such behavioral information must itself have evolved. Rats that pass through mazes easily can be interbred, as can rats that pass through with difficulty; eventually two populations with inherited characteristics called “maze-smart” and “maze-dumb” will be produced. Fruit fly populations attracted to the light can be separated from those that avoid light. Classical genetic crossing experiments reveal that the two populations differ largely in a small number of genes for phototropism. Similar genetic determinants of behaviour exist in man. Possession of a supernumerary Y-chromosome in males is strikingly correlated with aggressive tendencies—which may, however, have been a selective advantage in more primitive societies. Myopia may have had strong survival value in earlier times: near-sighted males, useless in the hunt, stayed home and painted the walls of the cave. As technology develops, natural selection enters new behavioral arenas; for example, in an age of artificial contraception, the clumsy and forgetful preferentially reproduce.

Human beings use only a small part of the total electromagnetic spectrum, the part called visible light, which extends from about 4,000 to about 7,000 Å in wavelength. While many plants and animals are sensitive to this same range of wavelengths, many of them are sensitive to other wavelengths as well. Most insects are sensitive to ultraviolet light at wavelengths below 4,000 Å, and many flowering plants take advantage of this fact and present patterns visible only in the ultraviolet range. Honeybees use polarized light, which the unaided human eye is quite unable to detect, for direction finding on partly cloudy days. The “pit” of such pit vipers as the rattlesnake is an infrared receptor and direction finder. These reptiles can sense the thermal radiation emitted by warm-blooded prey, radiation to which human beings are completely insensitive.

It is common knowledge that some animals (for example, dogs) are sensitive to sounds that the human ear cannot detect. Bats emit and detect sound waves at ultrahigh frequencies, in the vicinity of 100,000 cycles per second, about five times the highest frequency to which the human ear is sensitive. Bats use these sounds not so much to communicate, however, as to echolocate their prey and were doing this for millions of years before radar and sonar were invented. The audio receptors of many moths that are prey to bats are responsive only to the frequencies emitted by the bats. When the bat sounds are heard, the moths take evasive action. Dolphins have a very wide frequency range and several communication channels, as well as a “click” echolocator. Dolphins and whales use their blowholes rather than their mouths to utter these sounds. Sharks and other marine predators are said to locate their prey by the low-frequency sounds the prey makes when in distress. Some animals develop highly specialized and exotic organs for the detection or transmission of sound—for example, a European grasshopper has a relatively large parabolic antenna on its back that looks very much like a small radio telescope. This antenna is used for producing noises evidently thought attractive by the female of the species.

Many organisms are capable of smell and taste; that is, the detection of specific chemical molecules. According to one theory of smell, there are particular olfactory sensors, each receptive only to a specific chemical group on airborne molecules. The ultimate in olfactory specialization is probably the male silkworm moth: with its feathery antennae it is able to smell essentially nothing except the chemical sex attractant discharged by the female of the species. But it can detect this molecule very well, needing an impact of only 40 molecules per second on its antennae to produce a marked response. One female silkworm moth need release only 10^-8 grams of sex attractant per second in order to attract every male silkworm moth in a volume hundreds of metres to kilometres on a side.

Besides the senses of sight, hearing, smell, taste, and touch, various animals have a wide variety of other senses (see sensory reception). Man has an inertial orientation system and accelerometer in the cochlear canal of the ear. The water scorpion (Nepa) has a fathometer sensitive to hydrostatic pressure gradients. Most higher plants have chemically amplified gravity sensors. Fireflies and squids communicate with their own kind by producing time sequences or patterns of light on their bodies. The African freshwater fish Gymnarchus niloticus operates a dipole electrostatic field generator and a sensor to detect the amplitude and frequency of disturbances in the impressed field, an adaptation well suited for its nocturnal activities in turbulent waters. Other organisms have salinity sensors, or humidity sensors. There may be sensors involved in homing instincts of animals that have not yet been discovered. All of these senses confer upon their possessors an awareness of the environment that may be very different from that of such other organisms as man. Man, however, has the remarkable ability to extend his sensory and intellectual capabilities artificially, through the use of instrumentation.

Hypotheses of origins

Perhaps the most fundamental and at the same time the least understood biological problem is the origin of life. It is central to many scientific and philosophical problems and to any consideration of extraterrestrial life. Most of the hypotheses of the origin of life will fall into one of four categories:The origin of life is a result of a supernatural event; that is, one permanently beyond the descriptive powers of physics and chemistry.Life—particularly simple forms—spontaneously and readily arises from nonliving matter in short periods of time, today as in the past.Life is coeternal with matter and has no beginning; life arrived on the Earth at the time of the origin of the earth or shortly thereafter.Life arose on the early Earth by a series of progressive chemical reactions. Such reactions may have been likely or may have required one or more highly improbable chemical events.

Hypothesis 1, the traditional contention of theology and some philosophy, is in its most general form not inconsistent with contemporary scientific knowledge, although this knowledge is inconsistent with a literal interpretation of the biblical accounts given in chapters 1 and 2 of Genesis and in other religious writings. Hypothesis 2 (not of course inconsistent with 1) was the prevailing opinion for centuries. A typical 17th-century view follows:[May one] doubt whether, in cheese and timber, worms are generated, or, if beetles and wasps, in cowdung, or if butterflies, locusts, shellfish, snails, eels, and such life be procreated of putrefied matter, which is to receive the form of that creature to which it is by formative power disposed[?] To question this is to question reason, sense, and experience. If he doubts this, let him go to Egypt, and there he will find the fields swarming with mice begot of the mud of the Nylus [Nile], to the great calamity of the inhabitants.

It was only in the Renaissance, with its burgeoning interest in anatomy, that such transformations were realized to be impossible. A British physiologist, William Harvey, during the mid-17th century, in the course of his studies on the reproduction and development of the king's deer, made the basic discovery that every animal comes from an egg. An Italian biologist, Francesco Redi, in the latter part of the 17th century, established that the maggots in meat came from flies' eggs, deposited on the meat. And an Italian priest, Lazzaro Spallanzani, in the 18th century, showed that spermatozoa were necessary for the reproduction of mammals. But the idea of spontaneous generation died hard. Even though it was proved that the larger animals always came from eggs, there was still hope for the smaller ones, the microorganisms. It seemed obvious that, because of their ubiquity, these microscopic creatures must be generated continually from inorganic matter.

Meat could be kept from going maggoty by covering it with a flyproof net, but grape juice could not be kept from fermenting by putting over it any netting whatever. This was the subject of a great controversy between the famous French bacteriologists Louis Pasteur and F.A. Pouchet in the 1850s, in which Pasteur triumphantly showed that even the minutest creatures came from germs floating in the air, but that they could be guarded against by suitable filtration. Actually, Pouchet was arguing that life must somehow arise from nonliving matter; if not, how had life come about in the first place?

Toward the end of the 19th century Hypothesis 3 gained currency, particularly with the suggestion by a Swedish chemist, S.A. Arrhenius, that life on Earth arose from panspermia, microorganisms or spores wafted through space by radiation pressure from planet to planet or solar system to solar system. Such an idea of course avoids rather than solves the problem of the origin of life. In addition, it is extremely unlikely that any microorganism could be transported by radiation pressure to the Earth over interstellar distances without being killed by the combined effects of cold, vacuum, and radiation.

Pasteur's work discouraged many scientists from discussing the origin of life at all. Moreover they were anxious not to offend religious feeling by probing too deeply into the subject. Although Darwin would not commit himself on the origin of life, others subscribed to Hypothesis 4 more resolutely, notably the famous British biologist T.H. Huxley in his Protoplasm, the Physical Basis of Life (1869), and the British physicist John Tyndall in his “Belfast Address” of 1874. Although Huxley and Tyndall asserted that life could be generated from inorganic chemicals, they had extremely vague ideas about how this might be accomplished. The very phrase “organic molecule” implies that there exists a special class of chemicals uniquely of biological origin, despite the fact that organic molecules have been routinely produced from inorganic chemicals since 1828. In the following discussion the word organic carries no imputation of biological origin. In fact the problem largely reduces to finding an abiological source of appropriate organic molecules.

The primitive atmosphere

Darwin's attitude was: “It is mere rubbish thinking at present of the origin of life; one might as well think of the origin of matter.” The two problems are, in fact, curiously connected, and modern scientists are thinking about the origin of matter. There is convincing evidence that thermonuclear reactions and subsequent explosions in the interiors of stars generate all the chemical elements more massive than hydrogen and helium and then distribute them into the interstellar medium from which subsequent generations of stars and planets form. Because of the commonality of these thermonuclear processes, and because some thermonuclear reactions are more probable than others, there exists a cosmic distribution of the major elements, so far as is known, throughout the universe. Table 1 compares, for some atoms of interest, the relative numerical abundances in the universe as a whole, on the Earth, and in living organisms. There is of course some variation in composition from star to star, from place to place on the Earth, and from organism to organism, but such comparisons are nevertheless very instructive. The composition of life is intermediate between the average composition of the universe and the average composition of the Earth. Ninety-nine percent both of the universe and of life is made of the six atoms, hydrogen (H), helium (He), carbon (C), nitrogen (N), oxygen (O), and neon (Ne). Can it be that life on Earth arose when the chemical composition of the Earth was much closer to the average cosmic composition, and that some subsequent events have changed the gross chemical composition of the Earth?

The Jovian planets (Jupiter, Saturn, Uranus, and Neptune) are much closer to cosmic composition than is the Earth. They are largely gaseous, with atmospheres composed principally of hydrogen and helium. Methane (CH₄) and ammonia (NH₃) have been detected in smaller quantities, and neon and water are suspected. This circumstance very strongly suggests that the Jovian planets were formed out of material of typical cosmic composition. They have very large masses, and because they are so far from the sun their upper atmospheres are very cold. Therefore it is impossible for atoms in the upper atmospheres of the Jovian planets to escape from their gravitational fields; escape was probably very difficult even during planetary formation. The Earth and the other planets of the inner solar system, however, are much less massive and most have hotter upper atmospheres. It is possible for hydrogen and helium to escape from the Earth today, and it may well have been possible for much heavier gases to have escaped during the formation of the Earth. It is reasonable to expect that in the very early history of the Earth a much larger abundance of hydrogen prevailed, which has subsequently been lost to space. Thus the atoms carbon, nitrogen, and oxygen were present on the primitive Earth, not as CO₂ (carbon dioxide), N₂, and O₂ as they are today but rather in the form of their fully saturated hydrides, CH₄ (methane), NH₃ (ammonia), and H₂O. In the geological record, the presence of such reduced minerals as uraninite (UO₂) and pyrite (FeS₂) in sediments formed several billions of years ago implies that conditions then were considerably less oxidizing than they are today.

In the 1920s J.B.S. Haldane in Britain and A.I. Oparin in the Soviet Union recognized that the abiological production of organic molecules in the present oxidizing atmosphere of the Earth is highly unlikely; but that, if the Earth once had more reducing (in this context, hydrogen-rich) conditions, the possible abiogenic production of organic molecules would have been much more likely. If large numbers of organic molecules were somehow synthesized on the primitive Earth, there would not necessarily be much trace of them today. In the present oxygen atmosphere, largely produced by green-plant photosynthesis, such molecules would tend, over geological time, to be oxidized to carbon dioxide, nitrogen, and water. In addition, as Darwin recognized, the first microorganisms would consume prebiological organic matter produced prior to the origin of life.

Production of simple organic molecules

The first deliberate experimental simulation of these primitive conditions was carried out in 1953 by a U.S. graduate student, S.L. Miller, under the guidance of the eminent chemist H.C. Urey. A mixture of methane, ammonia, water vapour, and hydrogen was circulated through a liquid water solution and continuously sparked by a corona discharge elsewhere in the apparatus. The discharge may be thought to represent lightning flashes on the early Earth. After several days of exposure to sparking, the solution changed colour. Subsequent analysis indicated that several amino and hydroxy acids, intimately involved in contemporary life, had been produced by this simple procedure. The experiment is in fact so elementary, and the amino acids can so readily be detected by paper chromatography, that the experiment has been repeated many times by high school students. Subsequent experiments have substituted ultraviolet light or heat as the energy source or have altered the initial abundances of gases. In all such experiments amino acids have been formed in large yield. On the early Earth there was much more energy available in ultraviolet light than in lightning discharges. At long ultraviolet wavelengths, in which methane, ammonia, water, and hydrogen are all transparent, but in which the bulk of the solar ultraviolet energy lies, the gas hydrogen sulfide (H₂S) is a likely ultraviolet absorber.

Following such reasoning, a U.S. astrophysicist, Carl Sagan, and his colleagues made amino acids by long wavelength ultraviolet irradiation of a mixture of methane, ammonia, water, and H₂S. The amino acid syntheses, at least in many cases, involve hydrogen cyanide and aldehydes (e.g., formaldehyde) as gaseous intermediaries formed from the initial gases. It is quite remarkable that amino acids, particularly biologically abundant amino acids, can be made so readily under simulated primitive conditions. When laboratory conditions become oxidizing, however, no amino acids are formed, suggesting that reducing conditions were necessary for prebiological organic synthesis.

Under alkaline conditions, and in the presence of inorganic catalysts, formaldehyde spontaneously reacts to form a variety of sugars, including the five-carbon sugars fundamental to the formation of nucleic acids and such six-carbon sugars as glucose and fructose, which are extremely common metabolites and structural building blocks in contemporary organisms. Furthermore, the nucleotide bases as well as porphyrins have been produced in the laboratory under simulated primitive Earth conditions by several investigators. While there is still debate on the generality of the experimental synthetic pathways and on the stability of the molecules produced, most if not all of the essential building blocks of proteins, carbohydrates, and nucleic acids can be readily produced under quite general primitive reducing conditions, plus probably ATP as well.

Production of polymers

The construction of polymers, long-chain molecules made of repeating units of these essential building blocks, however, is a much more difficult experimental problem. Polymerization reactions are generally dehydrations, in which a molecule of water is lost in the formation of a two-unit polymer. Dehydrating agents must be used to initiate polymerization. The polymerization of amino acids to form long protein-like molecules was accomplished through dry heating by a U.S. investigator, S.W. Fox. The polyamino acids that are formed are not random polymers and have some distinct catalytic activities. The geophysical generality of dry heating and return to solution, however, has been questioned. Long polymers of amino acids can also be produced from hydrogen cyanide and anhydrous liquid ammonia. Some evidence exists that nucleotide bases and sugars can be combined in the presence of phosphates or cyanides under ultraviolet irradiation. Some condensing agents such as cyanamide are efficiently made under simulated primitive conditions. Despite the breakdown by water of molecular intermediates, condensing agents are often quite effective in inducing polymerization, and polymers of amino acids, sugars, and nucleotides have all been made this way.

A famous British scientist, J.D. Bernal, suggested that adsorption of molecular intermediates on clays or other minerals may have concentrated these intermediates. Such concentration could offset the tendency for water to break down polymers of biological significance. Of special interest is the possibility that such concentration matrices included phosphates, for this would help explain how phosphorus could have been incorporated preferentially into prebiological organic molecules at a time when biological concentration mechanisms did not yet exist. Mineral catalysis implies that organic synthesis could also occur in deep water where ultraviolet light had been filtered out.

Quite apart from concentration mechanisms, the primitive waters themselves may have been a not very dilute solution of organic molecules. If all the surface carbon on the Earth were present as organic molecules in the contemporary oceans, or if many known ultraviolet synthetic reactions producing organic molecules were permitted to continue for a billion years with products dissolved in the oceans, a 1 percent solution of organic molecules would result. For similar reasons, Haldane suggested that the origin of life occurred in a “hot dilute soup.” In addition, concentration mechanisms do exist, such as evaporation or freezing of pools or adsorption on interfaces or the generation of colloidal enclosures called coacervates.

The origin of the code

It has been shown that all the essential building blocks for life and their polymers may have been produced in some fair concentration on the primitive Earth. This possibility is certainly relevant to the origin of life, but it is not the same thing as the origin of life. By the genetic definition of life discussed above in Definitions of life, a self-replicating, mutable molecular system, capable of interacting with the environment, is required. In contemporary cells the nucleic acids are the sites of self-replication and mutation. Laboratory experiments have already shown that polynucleotides can be produced from nucleotide phosphates in the presence of a specific enzyme of biological origin and a pre-existing “primer” nucleic acid molecule. If the primer molecule is absent, polynucleotides are still formed, but they of course contain no genetic information. Once such a polynucleotide spontaneously forms, it then acts as primer for subsequent syntheses.

Imagine a primitive ocean filled with nucleotides and their phosphates and appropriate mineral surfaces serving as catalysts. Even in the absence of the appropriate enzyme it seems likely, although not yet proved, that spontaneous assembly of nucleotide phosphates into polynucleotides occurred. Once the first such polynucleotide was produced, it may have served as a template for its own reproduction, still of course in the absence of enzymes. As time went on there were bound to be errors in replication. These would be inherited. A self-replicating and mutable molecular system of polynucleotides, eventually leading to a diverse population of such molecules, may have arisen in this way. Alternatively, the primitive hereditary material may have involved some other molecule altogether, but no concrete suggestion for such a molecule has ever been proposed.

In any case, a population of replicating polynucleotides cannot quite be considered alive because it does not significantly influence its environment. Eventually, all the nucleotides in the ocean would have been tied in polynucleotides and the entire synthetic process would then have ground to a halt. So far as is known, polynucleotides have no an catalytic properties, and proteins have no reproductive properties. It is only the partnership of the two molecules that makes contemporary life on Earth possible. Accordingly, a critical and unsolved problem in the origin of life is the first functional relation between these two molecules, or, equivalently, the origin of the genetic code. The molecular apparatus ancillary to the operation of the code—the activating enzymes, adapter RNAs, messenger RNAs, ribosomes, and so on—are themselves each the product of a long evolutionary history and are produced according to instructions contained within the code. At the time of the origin of the code such an elaborate molecular apparatus was of course absent.

It has been proposed that a weak but selective chemical bonding does exist, even in the absence of any of this apparatus, between amino acids and nucleotides. There need not be a very great selectivity; a given nucleotide sequence might in primitive times have coded for many different amino acids or, conversely, the same amino acid may have been coded for by several different nucleotide sequences. All that is required is that a particular linear sequence of nucleotides must code for some nonrandom sequence of amino acids. The active sites largely responsible for the catalytic activity of contemporary enzymes are generally only five or six amino acids long; the remainder of the enzyme is devoted to more sophisticated functions, such as arranging for the enzyme to be turned on and off by the machinery of the cell. With, say, 20 different varieties of amino acids available in the primitive environment, the chance of any given active site being produced by a random sequence of nucleotides is one in 20⁵, or one in about 3,000,000. But 3,000,000 combinations to form units five amino acids long is not a very large number for the chemistry and time periods in question. To conclude this speculation, then, if polynucleotides were initially capable of crude, nonenzymatic replication, and if a crude primitive genetic code existed, then any one of a very large number of catalytic properties was available to some self-replicating polynucleotides on the primitive Earth. This situation is all that would be necessary for the origin of life; those polynucleotides that could code for a primitive protein having catalytic properties furthering the replication of the polynucleotide would preferentially replicate. Other polynucleotides coding for less effective proteins would have replicated more slowly. The foregoing is one of several possibilities for the origin of the first living systems. Many separate and rather diverse instances of the origin of life may have occurred on the primitive Earth, but competition eventually eliminated all but one line. Every organism on Earth today would be a descendant of that line.

The earliest living systems

One curious feature of biological organic molecules is their optical activity: they rotate the plane of a beam of plane-polarized light. Organic molecules produced abiologically do not show optical activity. Molecules made of the same units can be put together in complementary ways like a left- and right-handed glove. The same building blocks can be used to produce molecules that are three-dimensional mirror images of each other. This asymmetry is responsible for optical activity. At the time of the origin of life, organic molecules, corresponding both to left- and right-handed forms, were produced. The laboratory simulation experiments always produce both types. But the first living systems could have been made only of one type, for the same reason that carpenters do not use random mixtures of screws with left- and right-handed threads. Whether left- or right-handed activity was adopted was probably purely a matter of chance, but once a particular asymmetry was established in the first living systems, it maintained itself. This belief implies that optical activity should be a feature of life on any planet, and also that the chances should be equal of finding a given terrestrial organic molecule or its mirror image molecule in extraterrestrial life forms.

The first living systems probably resided in a molecular garden of Eden, where all the building blocks that contemporary organisms must work hard at synthesizing were available free. Under such conditions the numbers of organisms must have increased very rapidly. But such increases cannot go on indefinitely. In time the supply of some molecular building block must have become short. Those primitive organisms that had the ability to synthesize the scarce building block, say A, from a more abundant one, say B, clearly had a competitive advantage over those organisms that could not perform such a synthesis. In time, however, the secondary source of supply, B, would have also become depleted and those organisms that could produce it from a third building block, C, would have preferentially replicated. A U.S. biochemist, N.H. Horowitz, proposed that in this way the enzymatic reaction chains of contemporary organisms—each step catalyzed by a particular enzyme—originally evolved.

Even the evolution of enzymatic reaction chains may have occurred in free nucleic acids before the origin of the cell. The cell may have arisen in response to the need for maintaining a high concentration of scarce building blocks or enzymes, or as protection against the gradually increasing abundance of oxygen on the primitive Earth. Oxygen is a well-known poison to many biological processes, and in contemporary higher organisms the mitochondria that handle molecular oxygen are kept in the cytoplasm, far from contact with the nuclear material. Even today processes are known whereby polyamino acids form small spherical objects, microns to tens of microns across, with some of the properties of cells. These objects, called proteinoid microspheres by Fox, are certainly not cells, but they may indicate processes by which the ancestors of cells arose. Procaryotic cells almost certainly preceded eucaryotic cells, and the evolution of so extremely complex an apparatus as the mitotic spindle (which ensures equal segregation of replicated chromosomes) must have taken very long periods of time to evolve. The development of mitochondria and chloroplasts (each of which contains its own DNA) in the eucaryotic cell may have been the result of a symbiosis, a cooperative arrangement entered into at first tentatively by originally free-living cells.

As the competition for building blocks increased among early life forms, and also perhaps as the abiological production of organic molecules dwindled because of the increasing oxygen abundance, the strictly heterotrophic way of life became more and more costly. The utilization of porphyrins, which are also made abiologically, by primitive photoautotrophs would have had great selective advantage. Many of the intermediates and enzymes in photosynthesis and in the anaerobic breakdown of carbon compounds are similar, but there is no generally accepted view of the origin of the photosynthetic process. Photosynthesis in procaryotes is more primitive than in such eucaryotes as green plants. In bacteria, water is not the ultimate source of hydrogen atoms for reducing carbon dioxide, and therefore oxygen is not produced. In addition, when a chlorophyll-containing cell is exposed both to light and to oxygen, it is killed unless it also contains an accessory carotenoid pigment. Thus green-plant photosynthesis had to wait until the appearance of carotenoids while bacterial photosynthesis, which does not produce oxygen, could function without carotenoids.

The antiquity of life

Among the oldest known fossils are those found in the Fig Tree chert from the Transvaal, dated at 3,100,000,000 years old. These organisms have been identified as bacteria and blue-green algae. It is very reasonable that the oldest fossils should be procaryotes rather than eucaryotes. Even procaryotes, however, are exceedingly complicated organisms and very highly evolved. Since the Earth is about 4,500,000,000 years old, this suggests that the origin of life must have occurred within a few hundred million years of that time.

By performing chemical analyses on the oldest sediments, it is possible to say something about the sorts of organic molecules produced, either biologically or abiologically, in primitive times. Thus, amino acids and porphyrins have been identified in the oldest sediments, as have pristane and phytane, typical breakdown products of chlorophyll. There are several indications that these organic molecules, dating from 2,000,000,000 to more than 3,000,000,000 years ago, are of biological origin. For one thing their long-chain hydrocarbons show a preference for a straight chain geometry, whereas known abiological processes tend to produce a much larger proportion of branched chain and cyclic hydrocarbon molecular geometries than have been found in these sediments. Abiological processes tend to produce equal amounts of long-chain carbon compounds with odd and with even numbers of carbon atoms. But the oldest sediments show a distinct preference for odd numbers of carbon atoms per molecule, as do products of undoubted biological origin. Finally, a C¹² enrichment, for which no abiological process seems able to account, has been discovered in the oldest sediments, evidence that suggests that plantlike life, which concentrates the carbon isotope C¹² preferentially to C¹³, was present very early. These departures from thermodynamic equilibrium are often considered to be compelling signs of biological activity. Such evidence again points to the great antiquity of life on Earth.

The fossil record, in any complete sense, goes back only about 600,000,000 years. In the layers of sedimentary rock known by geological methods and by radioactive dating to be that old, most of the major groups of invertebrates appear for the first time. All these organisms appear adapted to life in the water, and there is no sign yet of organisms adapted to the land. For this reason, and because of a rough similarity between the salt contents of blood and of seawater, it is believed that early forms of life developed in oceans or pools. With no evidence for widespread oxygen-producing photosynthesis before this time, and for cosmic abundance reasons described above, the oxygen content of the Earth's atmosphere in Precambrian times was very likely less than today. Accordingly, in Precambrian times, solar ultraviolet radiation, especially near the wavelength of 2,600 Å, which is particularly destructive to nucleic acids, may have penetrated to the surface of the Earth, rather than being totally absorbed in the upper atmosphere by ozone as it is today. In the absence of ozone, the ultraviolet solar flux is so high that a lethal dose for most organisms would be delivered in less than an hour. Unless extraordinary defense mechanisms existed in Precambrian times, life near the Earth's surface would have been impossible. Sagan suggested that life at this time was generally restricted to some tens of metres and deeper in the oceans, at which depths all the ultraviolet light would have been absorbed, although visible light would still filter through. As the amount of atmospheric oxygen and ozone increased, due both to plant photosynthesis and to the photodissociation of water vapour and the escape to space of hydrogen from the upper atmosphere, life increasingly close to the Earth's surface would have been possible. It has been suggested that the colonization of the land, about 425,000,000 years ago, was possible only because enough ozone was then produced to shield the surface from ultraviolet light for the first time.

Life then had insinuated itself between the sun and the Earth. It diverted solar energy to its own uses and contrived more and more ways of exploiting more and more environments. Some experiments were faulty and the lines became extinct; others were more successful and the lines filled the Earth. Evolution through natural selection directed the proliferation of a growing array of life forms throughout the biosphere (see evolution: The concept of natural selection).

The chemistry of extraterrestrial life

What are the methods and prospects for a search for life beyond the Earth? Each of the definitions of life described in Definitions of life (see above) implies a method of searching for life. Particular physiological functions, particular metabolic activities, such specific molecules as proteins and nucleic acids, self-replication and mutation, processes not in closed-system thermodynamic equilibrium—all these might be sought. All the search methods significantly depend upon chemistry.

Life on Earth is structurally based on carbon and utilizes water as an interaction medium. Hydrogen and nitrogen have significant accessory structural roles; phosphorus is important for energy storage and transport, sulfur for three-dimensional configuration of protein molecules, and so on. But must these particular atoms be the atoms of life everywhere, or might there be a wide range of atomic possibilities in extraterrestrial organisms? What are the general physical constraints on extraterrestrial life?

In approaching these questions several criteria can be used. The major atoms should tend to have a high cosmic abundance. A structural molecule for making an organism at the temperature of the planet in question should not be extremely stable, because then no chemical reactions would be possible; but it should not be extremely unstable, because then the organism would fall to pieces. There should be some medium for molecular interaction. Solids are not appropriate because the diffusion times are very long. Such a medium is most likely a liquid (but could possibly be a very dense gas) that is stable in a number of respects. It should have a large temperature range (for a liquid, the temperature difference between freezing point and boiling point should be large). The liquid should be difficult to vaporize and to freeze; in fact, it should be very difficult to change its temperature at all. In addition it should be an excellent solvent. There should also be some gas on the planet in question that could be used in various biologically mediated cycles, as CO₂ is in the carbon cycle on Earth.

The planet, therefore, should have an atmosphere and some near-surface liquid, although not necessarily an ocean. If the intensity of ultraviolet light or charged particles from the sun is intense at the planetary surface, there must be some place, perhaps below the surface, that is shielded from this radiation but that nevertheless permits useful chemical reactions to occur. Since after a certain period of evolution, lives of unabashed heterotrophy lead to malnutrition and death, autotrophs must exist. Chemoautotrophs are, of course, a possibility but the inorganic reactions that they drive usually require a great deal of energy; at some stage in the cycle, this energy must probably be provided by sunlight. Photoautotrophs, therefore, seem required. Organisms that live very far subsurface will be in the dark, making photoautotrophy impossible. Organisms that live slightly subsurface, however, may avoid ultraviolet and charged particle radiation and at the same time acquire sufficient amounts of visible light for photosynthesis.

Thermodynamically, photosynthesis is possible because the plant and the radiation it receives are not in thermodynamic equilibrium; for example, on the Earth a green plant may have a temperature of about 300 K while the sun has a temperature of about 6,000 K. (K = Kelvin temperature scale, in which 0 K is absolute zero; 273 K, the freezing point of water; and 373 K, the boiling point of water at one atmosphere pressure.) Photosynthetic processes are possible in this case because energy is transported from a hotter to a cooler object. Were the source of radiation at the same (or at a colder) temperature as the plant, however, photosynthesis would be impossible. For this reason the idea of a subterranean plant photosynthesizing with the thermal infrared radiation emitted by its surroundings is untenable, as is the idea that a cold star, with a surface temperature similar to that of the Earth, would harbour photosynthetic organisms.

It is possible to approach some of the foregoing chemical requirements and see just which atoms are implied. When atoms enter into chemical combination, the energy necessary to separate them is called the bond energy, a measure of how tightly the two atoms are bound to each other. Table 2 gives the bond energies of a number of chemical bonds, mostly involving abundant atoms. The energies are in electron volts (eV; 1 eV = 1.6 × 10^-12 ergs). The symbols are as follows: H, hydrogen; C, carbon; N, nitrogen; O, oxygen; S, sulfur; F, fluorine; Si, silicon; Bi, bismuth (very underabundant, biologically uninteresting, and present only as an illustration of the relatively weak chemical bonds in some metals). Bond energies generally vary between 10 eV and about 0.03 eV; double and triple bonds where two or three electrons are shared between two atoms tend to be more energetic than single bonds, single bonds more energetic than hydrogen bonds where a hydrogen atom is shared between two other atoms, and hydrogen bonds more energetic than the very weak (van der Waals) forces that arise from the attraction of the electrons of one atom for the nucleus of another. At room temperature, atoms, free or bound, move with an average kinetic energy corresponding to about 0.02 eV. Some of the atoms have greater energies, some lesser. At any temperature a few will have energies greater than any given bond energy; hence bonds occasionally will break. The higher the temperature, the more atoms there are moving with sufficient energy to spontaneously break a given bond.

Suppose it is decided arbitrarily (although the decision will not critically affect the conclusions) that for life to exist at any time the fraction of bonds broken by random thermal motions must be no larger than 0.0001 percent. It then turns out that any hypothetical life where the structural bonds are based upon van der Waals forces can only exist where the temperature is below 40 K, for hydrogen bonds below about 400 K, for bonds of 2 eV below 2,000 K, and for bonds of 5 eV below 5,000 K. Now, 2,000 to 5,000 K are typical surface temperatures of stars; 400 K is somewhat above the highest surface temperature found on Earth; and 40 K is about the cloud-top temperature of distant Neptune. Thus, over the entire range of temperatures, from cold stars to cold planets, there seem to exist chemical bonds of appropriate structural stability for life, and it would appear premature to exclude the possibility of life on any planet on grounds of temperature.

Life on Earth lies within a rather narrow range of temperature. Above the normal boiling point of water, much loss of configurational structure or three-dimensional geometry occurs. At these temperatures proteins become denatured, in part because above the boiling point of water the hydrogen bonding and van der Waals forces between water and the protein disappear. Also, similar bonds within the protein molecule tend to break down. Proteins then change their shapes, their ability to participate in lock-and-key enzymatic reactions is gravely compromised, and the organism dies. Similar structural changes, some of them connected with the stacking forces between adjacent nucleotide bases, occur in the heating of nucleic acids. But it is significant that these changes are not fragmentations of the relevant molecules but rather changes in the ways they fold. There appears to be no reason that configurational bonds should not have been evolved that are stable at higher temperatures than terrestrial organisms experience. On planets hotter than the Earth there seems to be no reason that slightly more stable configurational forces should not be operative in the local biochemistry.

Molecular factors

While the bonds that characterize life on Earth are too weak at high temperatures, they are too strong at low temperatures, tending to slow down the rates of chemical reactions generally. There are less stable bonds (e.g., hydrogen bonds, silicon-silicon bonds, and nitrogen-nitrogen bonds), however, that might play structural roles at significantly lower temperatures. At higher temperatures, multiple bonds (e.g., in aromatic, or ring-shaped, hydrocarbons) might be utilized for life. There clearly is a rich variety of little-studied chemical reactions that proceed at reasonable rates either at much lower or at much higher temperatures than those on Earth.

Except for bismuth and fluorine, all the atoms in Table 2 have relatively high cosmic abundances. At terrestrial temperatures, carbon is the unique atom for biological structure. Not only does it have high abundance but it forms a staggering variety of compounds of great stability, it lends itself to compounds that are configured by weaker bonds, and it enters into multiple bonds. These double- and triple-bonded molecules, among other useful properties, absorb long-wavelength ultraviolet light, a process leading to the synthesis of a variety of more complex molecules. A photon of ultraviolet light at a wavelength of 2,000 Å has an energy of 6.2 eV, capable of breaking many bonds, and permitting more complex reactions among the resulting molecular fragments. Photons of blue light have energies of about 3 eV, and of red light about 2 eV.

Silicon compounds do not form double bonds. Silicon-oxygen bonds are slightly more stable than carbon-carbon bonds, but they tend to produce molecules like the silicates, which are crystals of the same unit repeated over and over again, rather than molecules with aperiodic side chains with potential information content. On low-temperature planets, silicon-silicon bonds are more promising than carbon bonds in terms of reaction times, but they do not form double bonds and the carbon abundance is likely to be greater. Nevertheless, silicon compounds may be of limited biological importance both on high-temperature and low-temperature worlds.

Hydrogen bonding confers on liquids the stability properties necessary for life. There seem to be very few reasonable candidates for liquid interaction media. By all odds water is the most suitable. The other candidates, all to some extent hydrogen bonded, are ammonia, hydrogen fluoride, hydrogen cyanide, and mixtures of liquid hydrocarbons. Hydrogen fluoride can be excluded because it is too scarce cosmically. The hydrocarbons are not good solvents of salts, but life elsewhere may not be based on the same acid-base chemistry as life on Earth. The liquid range of water is larger than commonly thought, ranging from about 210 K in saturated salt solutions to 647 K at enormous atmospheric pressures. Water is the biological liquid medium of choice above 200 K, particularly in view of its extremely high cosmic abundance. At lower temperatures ammonia or hydrogen cyanide could serve as a liquid medium.

There are functional roles for specific atoms in biology, but except for considerations of structure and a liquid interaction medium they do not seem fundamental. For example, the energy-rich phosphate bonds in ATP are in fact of relatively low energy; they are about as energetic as the hydrogen bonds (see Table 2). The cell must store up large numbers of these bonds to drive a molecular degradation or synthesis. On high-temperature worlds the energy currency may be much more energetic per bond, and on low-temperature worlds much less energetic per bond.

It may be concluded that, in our present state of ignorance, it is premature to exclude life on grounds of temperature on any other planet, particularly when account is taken of the temperature heterogeneity of the other planets. But life does require an interaction medium, an atmosphere, and some protection from ultraviolet light and from charged particles of solar origin.

The conclusion that for the Earth, carbon-based aqueous life is the most appropriate may be slightly suspect, since terrestrial life is manifestly carbon-based and aqueous. In 1913 a U.S. biochemist, L.J. Henderson, published The Fitness of the Environment in which the biological advantages of carbon and water were stressed for the first time in terms of comparative chemistry. He was struck by the fact that those very atoms that are needed are just those atoms that are around; it remains a remarkable fact that atoms most useful for life do have very high cosmic abundances.

The search for extraterrestrial life

Exobiology, a term coined by a U.S. biologist, J. Lederberg, for the study of extraterrestrial life, has been called a science without a subject matter. It is certainly true that, as yet, no strong evidence for life beyond the Earth has been adduced. Exobiology, however, has deep significance even if extraterrestrial life is never found. The mere design of exobiological experiments forces man to examine critically the generality of his assumptions about life on Earth. In addition, a lifeless neighbouring planet presents a very interesting quandary: How is it that life has originated and evolved on Earth, but not on the planet in question? There is an entire spectrum of possibilities. A given planet may be lifeless and have no vestiges of primitive organic matter and no fossils of extinct life. It may be lifeless but may have either organic chemical or fossil relics. It may possess life of a simple sort or life of a quite complex biochemistry, physiology, and behaviour. It may possess intelligent life and a technical civilization. Establishment of any one of these five possibilities would be of fundamental biological importance.

The difficulties and opportunities inherent in exobiological exploration, in determining which of these five possibilities applies to a given planet, is most clearly grasped by imagining the situation reversed, with man on some neighbouring planet, say Mars, examining the Earth for life with the full armoury of contemporary scientific instrumentation and knowledge. First a distinction must be made between remote and in situ testing. In remote testing light of any wavelength reflected from or emitted by the target planet can be examined, but with in situ studies samples of the planet must be acquired by visiting them or by sending instruments that land on the planet, perform experiments, and radio back their findings. Since biological exploration involves the detailed characterization of any life found, rather than its mere detection, in situ experiments are necessary.

The bulk of the remote sensing methods are directed toward finding some thermodynamic disequilibrium on the planet. This may be a chemical disequilibrium, a mechanical disequilibrium, or a spectral disequilibrium. For example, it would be quite easy to determine spectroscopically from Mars that the Earth's atmosphere contains large amounts of molecular oxygen and about one part per million (10⁶) of methane. It would also be possible to calculate that, at thermodynamic equilibrium, the abundance of methane should be less than one part in 10³⁵. This huge discrepancy implies the existence of some process continuously generating methane on the Earth so rapidly that methane increases to a very large steady-state abundance before it can be oxidized by oxygen. Now such a methane-production mechanism need not be biological. It is conceivable that relatively stable aromatic hydrocarbons were produced abiologically in the early history of the Earth and that their slow thermal degradation leads to a continuous loss of methane from the planetary subsurface. But this and similar nonbiological explanations of the observed disequilibrium are unlikely. From Mars this thermodynamic discrepancy would be considered not as proof of life on Earth but as a significant hint of life on Earth. In fact the methane abundance on the Earth is produced by bacteria that, in the course of the reduction of a more oxidized form of carbon, release methane. Some methane bacteria live in swamps (hence, the term marsh gas for methane), and others—a significant fraction—live in the intestinal tracts of cows and other ruminants. The methane abundance over India is probably larger than over most other areas of the world, and if an extraterrestrial observer knew how to interpret the methane disequilibrium accurately (which is unlikely) it would be possible for him to deduce cows on Earth by spectrochemical analysis. The existence of relatively large quantities of methane in the presence of an excess of oxygen would remain a tantalizing but enigmatic hint of life on Earth. Similarly, the large amount of oxygen might itself be a sign of life if one could reliably exclude the possibility that the photodissociation of water and the escape to space of hydrogen were the source of oxygen. Also such relatively complex reduced organic molecules as terpenes, a hydrocarbon given off by plants, might conceivably be detected spectroscopically, perhaps by a spectrometer in orbit about the Earth. Not only would the chemical disequilibrium of terpenes in an excess of oxygen be suggestive of life, but equally suggestive would be the fact that terpenes are much more abundant over forested areas than over deserts.

Photographic observation

Photographic observations of the daytime Earth from Mars would give equivocal results. Even with a resolution of 100 metres (that is, an ability to discriminate fine detail at high contrast only if its components are more than 100 metres apart), it would be extremely difficult to discern cities, canals, bridges, the Great Wall of China, highways, and other large-scale accoutrements of the Earth's technical civilization. In satellite photographs with 100-metres (one metre = 1.0936 yards) resolution only about one in a thousand random photographs of the Earth yields features even suggestive of life. As the ground resolution is progressively improved, it becomes increasingly easy to make out the regular geometrical patterns of cultivated fields, highways, airports, and so on. But these are only the products of a civilization recently developed on Earth, and even photographs of the Earth with a ground resolution of 10 metres, but taken 100,000 years ago, would still have shown no clear sign of life. The lights of the largest cities might be just marginally detectable from Mars at night. Seasonal changes in the colour or darkness of plants would be detectable from Mars, but such cycles might easily have nonbiological explanations.

To detect individual animals a ground resolution of a few metres is required, and even here a low sun and long shadows are generally necessary. This detection could be accomplished with a large telescope in Earth orbit. It would then be possible to determine, for example, that objects with the general shape of cows are frequent on the Earth. But suppose that members of the civilization examining the Earth thus remotely are not even approximately quadrupedal and do not immediately associate the shape of cows with life. They would nevertheless be able to deduce life. They would observe that certain locales on Earth have a quantity of raised lumps connected to the ground by four stilts. It would be possible to calculate that wind and water erosion would cause the lumps to topple to the ground in geologically short periods of time. Such stilted lumps are mechanically unstable; they are not in equilibrium; if pushed hard, they fall. Accordingly, there must be a process for generating stilted lumps on the Earth in short periods of time. It would be very difficult to avoid the implication that this generation process is biological.

A third detection technique arises upon scanning the radio spectrum of the Earth. Because of domestic television transmission, the high-frequency end of the AM broadcast band, and the radar defense networks of the United States and various other countries, the amount or energy put out by the Earth to space at certain radio frequencies is enormous. At some frequencies, if this radiation were to be interpreted as ordinary thermal emission, the temperature of the Earth would have to be hundreds of millions of degrees, according to an estimate made by a Russian astrophysicist, I.S. Shklovskii. Moreover, it would be possible to determine that this radio “brightness temperature” of the Earth had been steadily increasing with time over the last several decades. Finally, it would be possible to analyze the frequency and the time variation of these signals and deduce that they were not purely random noise.

Now imagine in situ studies by vehicles that enter the Earth's atmosphere and land at some predetermined locale. There are many places on the Earth (the ocean surface, the Gobi Desert, Antarctica) where large organisms are infrequent and a life-detection attempt based solely on television searches for large life forms would be a risky investment. On the other hand, if such an experiment were successful (the camera records a dolphin cavorting, a camel chewing its cud, a penguin waddling), it would provide quite convincing evidence of life.

Although the oceans, the Gobi Desert, and Antarctica are relatively devoid of large life forms, they are in many places replete with minute life forms. Therefore, microorganism detectors would be a good investment. A television camera coupled to a microscope (optical or electron) would be a promising life detector if the sample acquisition problem could be solved: the early Dutch microscopist Antonie van Leeuwenhoek had no difficulty at all in identifying as alive the little “animalcules” that he found in a drop of water, although nothing similar had previously been seen in human history.

In addition to morphological criteria for the detection of microorganisms, there are metabolic and chemical criteria. For example, a sample of terrestrial soil, or seawater, say, might be acquired and introduced into a chamber containing food the investigators guess the earthlings might find tasty. Such food might be an abundant product of prebiological organic synthetic experiments. It could then be determined whether any characteristic molecules, such as carbon dioxide or ethanol, are produced metabolically or whether the medium containing food and terrestrial sample changes its acidity or becomes cloudy because of the growth of microorganisms, or it might be investigated whether there is heat given off in the chamber containing sample and food. Alternatively, photosynthesis could be tested by measuring the fixation of some gas, say carbon dioxide, as a function of illumination provided artificially to the sample by the instrument. Along chemical lines a direct test of terrestrial soil or seawater for optical activity might be made. Organic molecules could certainly be searched for with a combined gas chromatograph and mass spectrometer or by a remote analytic chemistry laboratory. The detection of any amount of organic matter would of course be interesting and relevant, whether or not it was biological in origin. Such criteria as have been used in the analysis of Precambrian sediments (described in The antiquity of life, above) might be used to test for biological origin.

Ambiguities of tests for life

It is remarkable, however, that many of these tests are ambiguous. It would be possible, for example, for the Martian investigator to guess wrong about what terrestrial organisms eat and to make incorrect assumptions about their structural chemistry or their interaction medium. If forms of regular geometry that do not move were detected microscopically, there might be serious questions of biological versus mineralogical origin. Chemical criteria (such as the expectation that if odd-numbered carbon chains are more prominent than even-numbered carbon chains, then life is detected) might not be valid unless it was certain which processes actually occurred in the prebiological organic chemistry of the planet in question. In addition, there might be the galling problem of contamination. The Martians' spacecraft might carry living organisms from their own planet and report them as detected on the planet Earth. For this reason great care would have to be taken that spacecraft were rigorously sterilized.

In fact, many of these problems have already arisen in an analysis of a variety of meteorite called carbonaceous chondrites. These meteorites, which fall on the Earth probably from the asteroid belt, contain about 1 percent organic matter by mass, far too much to be largely the result of terrestrial contamination. The most abundant organic molecules, however, are not clearly of biological origin, and some of the biologically more interesting molecules may be contaminants. Reports of optical activity have been contested and might alternatively be due to contamination. Geometrically interesting microscopic inclusions have been detected in these bodies. The most abundant inclusions, however, are probably mineralogical in origin, while the most highly structured and lifelike are very rare and, at least in some cases, are obviously the result of contamination (in one case by ragweed pollen). Finally, claims have been made of the extraction of viable microorganisms from the interiors of carbonaceous chondrites. These meteorites are porous, however, and “breathe” air in and out during their entry into the atmosphere. There also have been significant opportunities for their contamination after arrival on the Earth. Moreover, one of the organisms extracted was a facultative aerobe. Since, as yet, no planet in the solar system besides the Earth is known to contain significant quantities of molecular oxygen, it seems quite curious that the complex electron-transfer apparatus required for oxygen metabolism would be evolved out on the asteroid belt in expectation of ultimate arrival on the Earth. Here, again, contamination has proved a serious hazard. The large amounts of organic matter that are found in carbonaceous chondrites, however, suggest that the production of organic molecules occurred with very great efficiency in the early history of the solar system.

From such a hypothetical exercise as the instrumental detection of life on Earth by an extraterrestrial observer and from the actual experience acquired in the analysis of carbonaceous chondrites, the following conclusions can be drawn: There is no single and unambiguous “life detector.” There are instruments of great generality that make few ambiguous assumptions about the nature of extraterrestrial organisms, particularly their chemistry. These systems, however, require a fair degree of luck (an animal must walk by during the operating lifetime of the instrument), or they require the solution of difficult instrumental problems (such as the acquisition and preparation of samples for remote microscopic examination). Other instruments, such as metabolism detectors, have great sensitivity and are directed at the more abundant microorganisms. They are quite specific, however, and are critically dependent upon certain assumptions (for example, that extraterrestrial organisms eat sugars) that are no better than informed guesses. Therefore, an array of instruments, both very general and very specific, seems required. Stringent sterilization of such spacecraft appears necessary, both to avoid confusion of the life-detection experiments and to prevent interaction of contaminants with the indigenous ecology. Many of the instruments and strategies discussed in the preceding paragraphs continue to be adapted by the United States in attempts to search for life on the Moon and the nearby planets.

An exobiological survey of the solar system

A brief survey of the physical environments and biological prospects of the moons and planets of the solar system, so far as is known, follows. The Moon's surface seems inhospitable to life of any sort. The diurnal temperatures range from about 100 to about 400 K. In the absence of any significant atmosphere or magnetic field, ultraviolet light and charged particles from the Sun penetrate unimpeded to the lunar surface, delivering in less than an hour a dose lethal to the most radiation-resistant microorganism known. For other reasons already mentioned, the absence of an atmosphere and of any liquid medium on the surface also argues against life. The subsurface environment of the Moon is not nearly so inclement. About a metre or so subsurface there is no penetration of ultraviolet light or solar protons, and the temperature is maintained at a relatively constant value about 230 K. Even there, however, the absence of an atmosphere and the probable absence of abundant liquids make the biological prospects rather dim.

It is not out of the question, however, that prebiological organic matter, produced in the early history of the Moon, might be found sequestered beneath the lunar surface. Such organic matter may have been produced either in an original lunar atmosphere that has subsequently been lost to space, or in a secondary lunar atmosphere produced by release of gases after the formation of the Moon, and also subsequently lost to space. The depth at which such organic matter may be found depends upon the unknown history of the early lunar atmosphere, if any, and upon whether the Moon has, on the whole, gained or lost matter due to meteoritic impact. An apparent gaseous emission near the lunar crater Alphonsus was recorded in 1958 and a spectral identification was made of the molecule C₂, a likely organic fragment, but this identification subsequently has been disputed.

Because of contamination by unmanned spacecraft, the lunar surface had accumulated a microbial load estimated by the late 1960s at some 100,000,000 microorganisms. Since such organisms will be immediately killed unless shielded from radiation, and since the likelihood of their growth seems remote, such contamination may not be a serious problem in subsequent microbial analysis of returned lunar samples. A much more serious contamination problem occurs during the acquisition of such samples by astronauts. Samples obtained during the historic Apollo 11 Moon landing in July 1969 were tested for possible organic molecules, but results were inconclusive. Such a finding might shed significant light on the early history of organic molecules in the solar system.

The environment of Mercury is rather like that of the Moon. Its surface temperatures range from about 100 to about 620 K, but about a metre subsurface the temperature is constant, very roughly at comfortable room temperature on Earth. But the absence of any significant atmosphere, the unlikelihood of bodies of liquid, and the intense solar radiation make life unlikely.

Martian “vegetation” and “canals”

Direct evidence for life on Mars has been claimed for many decades. The first such argument was posed by a French astronomer, E.L. Trouvelot, in 1884: “Judging from the changes that I have seen to occur from year to year in these spots, one could believe that these changing grayish areas are due to Martian vegetation undergoing seasonal changes.” The seasonal changes on Mars have been reliably observed, not only visually but also photometrically. There is a conspicuous springtime increase in the contrast between the bright and dark areas of Mars. Accompanying colour changes have been reported, but their reality has been disputed. While such changes have been attributed to the growth of vegetation, seasonally variable dust storms are an equally convincing possibility.

The most famous case, historically, for life on Mars is the discovery of the “canals,” a set of apparent thin straight lines that cross the Martian bright areas and extend for hundreds and sometimes thousands of kilometres. They change seasonally as do the Martian dark areas. These lines, first systematically observed by an Italian astronomer, G.V. Schiaparelli, in 1877, were further cataloged and popularized by a U.S. astronomer, Percival Lowell, around the turn of the century. Lowell argued from the unerring straightness of the lines that they could not be of geological origin but must instead be the artificial constructs of a race of intelligent Martians. He suggested that they might be channels carrying water from the melting polar caps to the parched equatorial cities of Mars. While considerable skepticism has been expressed about these straight lines, there is no doubt that approximately rectilinear features do exist on the Martian surface. More probable explanations, however, include crater chains, terrain contour boundaries, faults, mountain chains, and ridges analogous to the suboceanic ridge systems that are features of the Earth.

In July and August 1976, two U.S. probes bearing equipment designed to detect the presence or remains of organic material made successful landings on Mars. Analyses of atmospheric and soil samples met with procedural difficulties and yielded initially ambiguous and inconclusive results, although the data were later generally interpreted as negative, at least for the vicinity of the probe (see Mars).

Venus and the superior planets

According to both ground-based and space-borne observations, the average surface temperatures of Venus are around 750 K. It does not seem likely, either at the poles or on the tops of the highest Venus mountains, that the surface temperature will be below 400 K, and noontime temperatures are probably significantly hotter than 700 K. Thus, quite apart from the other surface conditions, the temperatures on Venus seem too hot for terrestrial life. It is still not possible to exclude a Venus surface life with a rather different chemistry, although hydrogen bonding would be much less suitable for the geometrical configuration of polymers on Venus than it is on Earth. The clouds of Venus, however, are another matter. There, carbon dioxide, sunlight, and (according to the results of the Venera space vehicles) water are to be found. These are the prerequisites for photosynthesis. Some molecular nitrogen also is expected at the cloud level, and some supply of minerals can be expected from dust convectively raised from the surface. The cloud pressures are about the same as on the surface of the Earth, and the temperatures in the lower clouds also are quite Earthlike. Despite the fact that there is little oxygen, the lower clouds of Venus are the most Earthlike extraterrestrial environment known. While there are no recorded cases of organisms on Earth that lead a completely airborne existence throughout their life cycle, it is not impossible that such organisms could exist in the vicinity of the Venus clouds, perhaps buoyed, as is a fish by its swim bladder, to avoid downdrafts carrying them to the hotter lower atmosphere.

A similar speculation can be entertained with regard to the lower clouds of Jupiter. On Jupiter the atmosphere is composed of hydrogen, helium, methane, ammonia, and probably neon and water vapour. But these are exactly those gases used in primitive-Earth simulation experiments directed toward the origin of life. Laboratory and computer experiments have been performed on the application of energy to simulated Jovian atmospheres. In addition to the immediate gas-phase products, such as hydrogen cyanide and acetylene, more complex organic molecules, including aromatic hydrocarbons, are formed in lower yield. The visible clouds of Jupiter are vividly coloured, and it is possible that their hue is attributable to such coloured organic compounds. There is also an apparent absorption feature near 2,600 Å, in the ultraviolet spectrum of Jupiter, which has been attributed both to aromatic hydrocarbons and to nucleotide bases. In any event it is likely that organic molecules are being produced in significant yield on Jupiter; it is possible that Jupiter is a vast planetary laboratory that has been operating for 5,000,000,000 years on prebiological organic chemistry.

The other Jovian planets, Saturn, Uranus, and Neptune, are similar in many respects to Jupiter, although much less is known about them. Their cloud-top temperatures progressively decrease with distance from the Sun. In the case of Saturn, microwave studies have indicated that the atmospheric temperature increases with depth below the clouds; similar situations are expected on Jupiter, Uranus, and Neptune. Thus, it is by no means clear that the low temperatures of the upper clouds of the Jovian planets apply to the lower clouds, or to the underlying atmosphere. In addition to these planets, the solar system contains many natural satellites, some of which, such as Titan, a satellite of Saturn, and Io, a satellite of Jupiter, appear to have atmospheres. There are also tens of thousands of comets, which, judging from their spectra, contain organic molecules, as well as some thousands of asteroids and asteroidal fragments revolving about the Sun between the orbits of Mars and Jupiter. These are the presumed sources of the carbonaceous chondrites, which contain organic matter.

In short, there is a wide range of environments of biological interest within the solar system. There is no direct evidence for extraterrestrial life on these planets, but, on the other hand, there is no strong evidence against life on many of these worlds. Beyond this is the near certainty that biologically interesting organic molecules will be found throughout the solar system.

Intelligent life beyond the solar system

For thousands of years man has wondered whether he is alone in the universe or whether there might be other worlds populated by creatures more or less like himself. The common view, both in early times and through the Middle Ages, was that the Earth was the only “world” in the universe. Nevertheless, many mythologies populated the sky with divine beings, certainly a kind of extraterrestrial life. Many early philosophers held that life was not unique to the Earth. Metrodorus, an Epicurean philosopher in the 3rd and 4th centuries &BC;, argued that “to consider the Earth the only populated world in infinite space is as absurd as to assert that in an entire field sown with millet, only one grain will grow.” Since the Renaissance there have been several fluctuations in the fashion of belief. In the late 18th century, for example, practically all informed opinion held that each of the planets was populated by more or less intelligent beings; in the early 20th century, by contrast, the prevailing informed opinion (except for the Lowellians) held that the chances for extraterrestrial intelligent life were insignificant. In fact the subject of intelligent extraterrestrial life is for many people a touchstone of their beliefs and desires, some individuals very urgently wanting there to be extraterrestrial intelligence, and others wanting equally fervently for there to be no such life. For this reason it is important to approach the subject in as unbiased a frame of mind as possible. A respectable modern scientific examination of extraterrestrial intelligence is no older than the 1950s. The probability of advanced technical civilizations in our galaxy depends on many controversial issues.

A simple way of approaching the problem, which illuminates the parameters and uncertainties involved, has been devised by a U.S. astrophysicist, F.D. Drake. The number &math.N; of extant technical civilizations in the galaxy can be expressed by the following equation (the so-called Green Bank formula):&math.N; = &math.R;_*&math.f;_&math.p;&math.n;_&math.e; &math.f;_&math.l; &math.f;_&math.i; &math.f;_&math.c;&math.L; where &math.R;_* is the average rate of star formation over the lifetime of the galaxy; &math.f;_&math.p; is the fraction of stars with planetary systems; &math.n;_&math.e; is the mean number of planets per star that are ecologically suitable for the origin and evolution of life; &math.f;_&math.l; is the fraction of such planets on which life in fact arises; &math.f;_&math.i; is the fraction of such planets on which intelligent life evolves; &math.f;_&math.c; is the fraction of such planets on which a technical civilization develops; and &math.L; is the mean lifetime of a technical civilization. What follows is a brief consideration of the factors involved in choosing numerical values for each of these parameters, and an indication of some currently popular choices. In several cases these estimates are no better than informed guesses and no very great reliability should be pretended for them.

There are about 2 × 10¹¹ stars in the galaxy. The age of the galaxy is about 10¹⁰ years. A value of &math.R;_* = 10 stars per year is probably fairly reliable. While most contemporary theories of star formation imply that the origin of planets is a usual accompaniment of the origin of stars, such theories are not well enough developed to merit much confidence. Through the painstaking measurement of slight gravitational perturbations in the proper motions of stars, it has been found that about half of the very nearest stars have dark companions with masses ranging from about the mass of Jupiter to about 30 times the mass of Jupiter. The nearest of these dark companions orbit Barnard's star, which is only six light-years from the sun and is the second nearest star system. The most direct indication that planetary formation is a general process throughout the universe is the existence of satellite systems of the major planets of our own solar system. Jupiter, with 16 satellites, Saturn with 20 or more, and Uranus with five each closely resemble miniature solar systems. It is not known what the distribution of distances of planets from their central star are in other solar systems and whether they tend to vary systematically with the luminosity of the parent star. But considering the wide range of temperatures that seem to be compatible with life, it can be tentatively concluded that &math.f;_&math.p;&math.n;_&math.e; is about one.

Likelihood of life

Because of the apparent rapidity of the origin of life on Earth, as implied by the fossil record, and because of the ease with which relevant organic molecules are produced in primitive-Earth simulation experiments, the likelihood of the origin of life over a period of billions of years seems high, and some scientists believe that the appropriate value of &math.f;_&math.l; is also about one. For the quantities of &math.f;_&math.i; and &math.f;_&math.c; the parameters are even more uncertain. The vagaries of the evolutionary path leading to the mammals, and the unlikelihood of such a path ever being repeated has already been mentioned. On the other hand, intelligence need not necessarily be restricted to the same evolutionary path that occurred on the Earth; intelligence clearly has great selective advantage, both for predators and for prey.

Similar arguments can be made for the adaptive value of technical civilizations. Intelligence and technical civilization, however, are clearly not the same thing. For example, dolphins appear to be very intelligent, but the lack of manipulative organs on their bodies has apparently limited their technological advance. Both intelligence and technical civilization have evolved about halfway through the relevant lifetime of the Earth and Sun. Some, but by no means all, evolutionary biologists would conclude that the product &math.f;_&math.i; &math.f;_&math.c; taken as 10^-2 is a fairly conservative estimate.

Still more uncertain is the value of the final parameter, &math.L;, the lifetime of a technical civilization. Here, fortunately for man, but unfortunate for the discussion, there is not even one example. Contemporary world events do not provide a very convincing counterargument to the contention that technical civilizations tend, through the use of weapons of mass destruction, to destroy themselves shortly after they come into being. If we define a technical civilization as one capable of interstellar radio communication, our technical civilization is only a few decades old. If then &math.L; is about 10 years, multiplication of all of the factors assumed above leads to the conclusion that there is in the second half of the 20th century only about one technical civilization in the galaxy—our own. But if technical civilizations tend to control the use of such weapons and avoid self-annihilation, then the lifetimes of technical civilizations may be very long, comparable to geological or stellar evolutionary time scales; the number of technical civilizations in the galaxy would then be immense. If it is believed that about 1 percent of developing civilizations make peace with themselves in this way, then there are about 1,000,000 technical civilizations extant in the galaxy. If they are randomly distributed in space, the distance from the Earth to the nearest such civilization will be several hundred light-years. These conclusions are, of course, very uncertain.

How is it possible to enter into communication with another technical civilization? Independent of the value of &math.L;, the above formulation implies that there is about one technical civilization arising every decade in the galaxy. Accordingly, it will be extraordinarily unlikely for man soon to find a technical civilization as backward as his. From the rate of technical advance that has occurred on the Earth in the past few hundred years, it seems clear that man is in no position to project what future scientific and technical advances will be made even on Earth in the next few hundred years. Very advanced civilizations will have techniques and sciences totally unknown to 20th-century man. Nevertheless man already has a technique capable of communication over large interstellar distances. This technique, already encountered in the discussion of life on Earth, is radio transmission. Imagine that we employ the largest radio telescope available on Earth, the 1,000-foot-diameter dish of Cornell University, the Arecibo Observatory in Puerto Rico, and existing receivers, and that the identical equipment is employed on some transmitting planet. How distant could the transmitting and receiving planets be for intelligible signals to be transmitted and received? The answer is a rather astonishing 1,000 light-years. Within a volume centred on the Earth, with a radius of 1,000 light-years, there are more than 10,000,000 stars.

There would of course be problems in establishing such radio communication. The choices of frequency, of target star, of time constant, and of the character of the message would all have to be selected by the transmitting planet so that the receiving planet would, without too much effort, be able to deduce the choices. But none of these problems seem insuperable. It has been suggested that there are certain natural radio frequencies (such as the 1,420-megacycle line of neutral hydrogen) that might be tuned to; the first choice might be to listen to stars of approximately solar spectral type; in the absence of a common language there nevertheless are messages whose intelligent origin and intellectual content could be made very clear without making many anthropocentric assumptions.

Because of the expectation that the Earth is relatively very backward, it does not make very much sense to transmit messages to hypothetical planets of other stars. But it may very well make sense to listen for radio transmissions from planets of other stars. Project Ozma, a very brief program of this sort, oriented to two nearby stars, Epsilon Eridani and Tau Ceti, was organized in 1960 by Drake. On the basis of the Green Bank formula, it would be very unlikely that success would greet an effort aimed at two stars only 12 light-years away, and Project Ozma was unsuccessful. It remains, however, the first pioneering attempt at interstellar communication. Related programs were organized on a larger scale and with great enthusiasm in the 1960s in the U.S.S.R., where a state scientific commission devoted to such an effort was organized. Other communication techniques including laser transmission and interstellar spaceflight have been discussed seriously and may not be infeasible, but if the measure of effectiveness is the amount of information communicated per unit cost, then radio is the method of choice.

The search for extraterrestrial intelligence is an extraordinary pursuit, in part because of the enormous significance of possible success, but in part because of the unity it brings to a wide range of disciplines: studies of the origins of stars, planets, and life; of the evolution of intelligence and of technical civilizations; and of the political problem of avoiding man's self-annihilation. But at least one point is clear. In the words of Loren Eiseley (also from The Immense Journey),Lights come and go in the night sky. Men, troubled at last by the things they build, may toss in their sleep and dream bad dreams, or lie awake while the meteors whisper greenly overhead. But nowhere in all space or on a thousand worlds will there be men to share our loneliness. There may be wisdom; there may be power; somewhere across space great instruments, handled by strange, manipulative organs, may stare vainly at our floating cloud wrack, their owners yearning as we yearn. Nevertheless, in the nature of life and in principles of evolution we have had our answer. Of men [as are known on earth] elsewhere, and beyond, there will be none forever.