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:

               1. The origin of life is a result of a supernatural event; that is, one permanently beyond the
               descriptive powers of physics and chemistry.

               2. Life--particularly simple forms--spontaneously and readily arises from nonliving matter
               in short periods of time, today as in the past.

               3. 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. (see also Index: spontaneous generation)

               4. 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

                    [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? (see also Index:
               germ theory)

               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.
               (see also Index: Darwinism)

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. (see also Index: ultraviolet

               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. (see also Index: genetic code)

               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. (see also Index: polarization)

               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. (see also Index: carotene)

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. (see
               also Index: fossil record)

               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

               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
               THEORY OF: The concept of natural selection).


               It is notknown what aspects of living systems are necessary in the sense that living systems
               everywhere must have them; it is not known what aspects of living systems are contingent
               in the sense that they are the result of evolutionary accident, so that somewhere else a
               different sequence of events might have led to different characteristics. In this respect the
               possession of even a single example of extraterrestrial life, no matter how seemingly
               elementary in form or substance, would represent a fundamental revolution in biology. It is
               not known whether there is a vast array of biological themes and counterpoints in the
               universe, whether there are places that have fugues, compared with which our one tune is a
               bit thin and reedy. Or it may be that our tune is the only tune around. Accordingly the
               prospects for life on other planets must be considered in any general discussion of life.
               (see also Index: exobiology)

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. (see
               also Index: chemical bonding)

               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

               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. (see also Index: molecule)

               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. (see also Index: carbon cycle, carbon)

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. (see also Index: space

               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. (see also Index: isoprenoid)

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. (see also Index:
               celestial photography, satellite communication)

               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. (see also Index: radio source)

               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

               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
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. (see also Index:
               Mars, canals of)

               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 SOLAR SYSTEM: The quest for life on Mars).
               (see also Index: Mariner)

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. (see also Index: ultraviolet radiation)

               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. The environment of Pluto is almost
               completely unknown. In addition to these planets, the solar system contains 32 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

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. (see
               also Index: Milky Way Galaxy)

               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
               N of extant technical civilizations in the galaxy can be expressed by the following equation
               (the so-called Green Bank formula): (see also Index: star, Green Bank equation)

               N = R*fpne fl fi fcLwhere R* is the average rate of star formation over the lifetime of the
               galaxy; fp is the fraction of stars with planetary systems; ne is the mean number of planets
               per star that are ecologically suitable for the origin and evolution of life; fl is the fraction of
               such planets on which life in fact arises; fi is the fraction of such planets on which intelligent
               life evolves; fc is the fraction of such planets on which a technical civilization develops; and
               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 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 fpne 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 fl is
               also about one. For the quantities of fi and fc 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 fi fc taken as
               10-2 is a fairly conservative estimate.

               Still more uncertain is the value of the final parameter, 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 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

               How is it possible to enter into communication with another technical civilization?
               Independent of the value of 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. (see also Index: communication system, extraterrestrial life , radio

               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.