Origin of Life

Background of the origin of life

Theories of the origin of life

The “RNA world” and the origin of life

Panspermia

Resources

There is no direct fossil evidence of how life began on Earth. Such evidence could not exist: the molecular processes that scientists believe preceded the appearance of cells could not have left imprints on rock. However, fossils of single-celled microorganisms are present in rocks over three billion years old, and chemical traces in Greenland rocks show that single-celled life existed as long ago as 3.8 billion years. As Earth itself is only 4.6 billion years old, these data suggest that life appeared within 700 million years after Earth formed. Furthermore, experiments performed since the 1950s suggest that many important ingredients of life, including amino acids and nucleic-acid bases (the molecular building-blocks of deoxyribonucleic acid [DNA] and ribonucleic acid [RNA]), could have formed abundantly under conditions that may have existed on the early Earth, and scientists conjecture that the presence of these molecules facilitated the formation of the first actual life.

Background of the origin of life

All organisms rely on the same kinds of organic (carbon-containing) compounds; the same 20 amino acids combine to make up all the enormous diversity of proteins occurring in living things. DNA and RNA, furthermore, are essential to all life. Into these molecules is encoded the information needed to synthesize specific proteins from amino acids. One class of proteins, known as enzymes, acts to regulate the activity of nucleic acids and other biochemical functions essential to life. Enzymes do this by greatly increasing the speed of (i.e., catalyzing) specific chemical reactions. Other proteins provide structure for cells, regulate the passage of ions through the cellmembrane, and perform numerous other functions. Nucleic acids and proteins are so universally essential to modern life-forms that many scientists assume that either they, or closely related precursor compounds, were present in the first life-forms.

Theories of the origin of life

All cultures have developed stories to explain the origin of life. During the medieval period, for example, European scholars argued that small creatures such as insects, amphibians, and mice, appeared by spontaneous generation—natural self-assembly of nonliving ingredients—in old clothes or piles of garbage. Italian physician Francesco Redi (1626–1698) challenged this belief in 1668, when he showed that maggots come from eggs laid by flies rather than forming spontaneously from the decaying matter in which they are found.

A series of experiments conducted in the 1860s by the French microbiologist Louis Pasteur (1822–1895) also helped to disprove the idea that life originated by spontaneous generation. Pasteur sterilized two containers, both of which contained a broth rich in nutrients. He exposed both containers to the air, but one had a trap in the form of a loop in a connecting tube, which prevented dust and other particles from reaching the broth. Bacteria and mold quickly grew in the open container and made its broth cloudy and rank, but the container with the trap remained sterile. Pasteur interpreted this experiment as indicating that microorganisms did not arise spontaneously in the open container, but were introduced by dust and other airborne contaminants.

Although Redi, Pasteur, and other scientists thoroughly disproved the theory of spontaneous generation as an explanation for the origin of present-day life on whatever scale, they raised a new question: If organisms can arise only from other organisms, how then did the first organism arise?

Charles Darwin (1809–1882), the famous English naturalist, suggested that life might have first occurred in “some warm little pond” rich in minerals and chemicals, and exposed to electricity and light. Darwin argued that once the first living beings appeared, all other creatures that have ever lived could have evolved from them. In other words, spontaneous generation did occur—but only a long time ago, when the first, minimally complex forms of life would have faced no competition from more-competent cells. Many of the laboratory experiments that would eventually be conducted to shed light on the origin of life have been variations on Darwin’s “warm little pond.” first, however, another influential suggestion regarding the origin of life was provided by Russian scientist Aleksandr Oparin (1894–1980) and English scientist J. B. S. Haldane (1892–1964). Oparin and Haldane suggested in the 1920s that the atmosphere of billions of years ago would have been very different from today’s. The modern atmosphere is about 79% nitrogen (N₂) and 20.9% free oxygen (O₂), with only trace quantities of other gases. Because of the presence of oxygen, which combines readily with many other substances, such an atmosphere is termed oxidizing. Oparin noted that oxygen interferes with the formation of organic compounds necessary for life by combining with their hydrogen atoms and reasoned that the atmosphere present when life began must have been a reducing atmosphere, which contained little or no oxygen but had high concentrations of gases that can react to provide hydrogen atoms to synthesize the compounds needed to create life. Oparin and Haldane suggested that this primordial, reducing atmosphere consisted of hydrogen (H₂), ammonia (NH₃), methane (CH₄), and additional simple hydrocarbons (molecules consisting only of carbon and hydrogen atoms). Oxygen could not have been present in large quantities because it is chemically unstable, and is only maintained as a major ingredient of the atmosphere by the action of green plants and algae—that is, by life itself. Before life, Earth’s atmosphere could not have been strongly oxidizing.

According to this theory, energy for rearranging atoms and molecules into organic forms that promoted the genesis of life came from sunlight, lightning, and/or geothermal heat. This model of the early environment became especially popular among scientists after a U.S. graduate student of physics named Stanley Miller (1930–), then studying at the University of Chicago, designed an experiment to test it. In 1953, Miller filled a closed glass container with a mixture of the gases that Oparin and haldane suggested were in the ancient atmosphere. In the bottom of the container was a reservoir of boiling water, and above it an apparatus that caused electrical sparks to pass through the gas mixture. After one week of reaction, Miller found that amino acids and other organic chemicals had formed from the gases and water. In the years since Miller reported his results, other researchers have performed more sophisticated “warm little pond” experiments, and have been able to synthesize additional amino acids and even nucleic acids, the molecules that organize into RNA and DNA, which in turn encode the genetic information of organisms.

Subsequent research influenced by these experiments led many scientists to believe that the concentration of organic molecules in the primordial, nutrient-laden, warm “ponds” (which may have been tidal pools, puddles, shallow lakes, or deep-sea hot springs) increased progressively over time. Eventually more complex molecules formed, such as carbohydrates, lipids, proteins, and nucleic acids. The complexity gap between simple nucleic acids and self-replicating RNA or DNA is, however, large; therefore, some scientists have theorized that assembly of more complex compounds from simpler ones may have occurred on the surface of oily drops floating on the water surface, or on the surfaces of minerals—objects whose atomic structure might have provided a template for stringing together nucleic acids and giving them a place to “live” until free-floating cells protected by lipid membranes could evolve.

However, some scientists believe that the young Earth’s surface was too inhospitable a place for life to have developed on its surface at all; lacking O₂, the atmosphere would also have lacked its present-day stratospheric layer of ozone (O₃), which screens large quantities of harmful ultraviolet radiation from the surface. They believe that a more likely environment for abiogenesis (life from prelife) was in the vicinity of deep-sea vents, holes in the crust under the ocean from which hot, mineral-laden water flows.

Furthermore, many scientists today believe that the prelife atmosphere may not have been as strongly reducing as the one proposed by Oparin and Haldane and used in Miller’s experiment. They assert that volcanoes added carbon monoxide (CO), carbon dioxide (CO₂), and nitrogen to the early atmosphere, which may even have contained traces of oxygen. Nevertheless, more recent experiments of the Miller type, run using a less reducing atmosphere, have also resulted in the synthesis of organic compounds. In fact, all 20 of the amino acids found in organisms have been created in the laboratory under experimental conditions designed to mimic what scientists believe the prelife Earth was like billions of years ago—whether using Miller’s model or its less-reducing competitors.

But in the absence of life, how did these amino acids link together into more complex compounds? Living cellular chemistry links amino acids together using specific enzymes to form particular proteins. An amino acid is any compound which contains at least one amino group (–NH₂) and one carboxyl group (–COOH). When amino acids are linked, a hydrogen molecule and a hydroxyl group (OH) are removed from each amino acids, which then link up into a protein chain, while the hydrogen and hydroxyl link up as a water molecule (H + OH = h₂O). Without enzymes, amino acids do not link up in this way—or, as a biochemist might describe it, polymerization does not proceed.

How, then, could amino acids have joined to form proteins without the proteins termed enzymes to help them? One possibility is that amino acids may have joined together on hot sand, clay, or other minerals. Laboratory experiments have shown that amino acids and other organic building blocks of larger molecules, called polymers, will join together if dilute solutions of them are dripped onto warm sand, clay, or other minerals. The larger molecules formed in this way have been named proteinoids. It is easy to imagine some version of Darwin’s “warm little pond”—a soup of spontaneously-formed amino acids—splashing onto hot volcanic rocks. Clay and iron pyrite have particularly favorable properties making them good “platforms” for the formation of larger molecules from smaller building blocks. One recently proposed theory of the origin of life suggests that tiny (=̃.01-mm diameter) hollows in iron sulfide minerals, such as are deposited in the vicinity of deep-sea hot springs, might have incubated the earliest life chemistry. Iron sulfide catalyzes the formation of organic molecules, and is used by some modern bacteria for this purpose. Sheltered in tiny iron-sulfide caverns, prebiotic chemistry might have developed at leisure, leaving this protected environment only after evolving a protective lipid membrane. This theory, however, like all theories of the origin of life, has its scientific opponents, and awaits the production of confirming or disconfirming laboratory evidence.

Proteinoids produced in laboratories can cluster together into droplets that separate, and that may protect their components from degrading influences of the surrounding environment. These droplets are like extremely simple cells, although they cannot reproduce. Such droplets are called microspheres. When fats (i.e., lipids) are present, the microspheres that form are even more cell-like. If a mixture of linked amino acids called polypeptides, sugars called polysaccharides, and nucleic acids is shaken, droplets called coacervates will form. All of these kinds of droplets are called protobionts, and they may represent a stage in the genesis of cellular life.

The formation of amino acids and other organic compounds is presumed to have been a necessary step in the genesis of life; it is certain, at least, that somewhere along the line all life became dependent on DNA and RNA for reproduction. Scientists thus presume that the first self-replicating molecules were similar to the nucleic acids of modern organisms. (These early molecular systems need not have been as complex as the self-replicating systems that comprise modern cells. Researchers have recently shown, by deleting genes, that even the genetically simplest bacteria alive today can reproduce with much less than their full natural complement of DNA.) Once molecules that could self-replicate were formed, the process of evolution would account for the subsequent development of life. The particular molecules best adapted to the local environmental conditions would have duplicated themselves more efficiently than competing molecules. Eventually, primitive cells appeared; perhaps coacervates or other protobionts played a role at this stage in the genesis of life. Once cells became established, evolution by natural selection could have resulted in the development of all of the life-forms that have ever existed on Earth.

The “RNA world” and the origin of life

Most living cells today store genetic information in the long-ribbon-shaped molecules of DNA. The information stored in DNA’molecular components is transferred to another ribbon-shaped molecule, RNA, by a process termed transcription. Proteins, including enzymes, are then formed by cellular structures that translate the information on the DNA. The enzymes thus produced facilitate the biochemical cellular functions necessary to maintain life and reproduce. Many scientists believe it is unlikely that all of the components of this complex sequence of events, DNA to RNA to protein, evolved simultaneously. Some scientists propose that, in fact, RNA appeared before DNA. This view has been strengthened by the discovery that some forms of RNA, called ribozymes, can act like non-protein enzymes to catalyze biological reactions. RNA thus may have been capable of ordering amino acids into forming proteins and of replicating itself in an RNA-based arrangement termed the “RNA world.”

Scientists who favor the RNA world hypothesis suggest that RNA might have been able to self-replicate even before DNA and protein enzymes had evolved. Single-stranded RNA might have been able to assume a shape that allowed it to line up amino acids in specific sequences to create specific protein molecules. rNA molecules capable of causing amino acids to link up to form a protein could have had an advantage in replication and survival, compared with other RNA molecules. At that point, molecular evolution and natural selection could have taken over in furthering the development of life. RNA that produced useful protein enzymes, for example, would have survived better than that which did not.

Critics of these ideas say that the evidence for self-replicating RNA is weak. Instead, they suggest that other organic molecules, rather than nucleic acids, were the first self-replicating chemicals capable of storing genetic information. According to this idea, these simple hereditary systems were later replaced by nucleic acids during the course of evolution. Since laboratory results in this field are hard to come by, this debate is likely to persist for a long time to come.

Panspermia

Radio astronomers have found that organic molecules (including amino acids), which might have played an important role in the formation of life, are present in dust clouds in outer space. Organic molecules are also known to be present in meteors that have fallen to Earth’s surface. These observations provide further evidence that chemicals important for the genesis of life may have been present on the early earth. The presence of complex organic compounds outside of our solar system suggests that the formation of compounds important for life is more likely than once thought.

The presence of organic compounds in outer space also suggests to a few scientists that life on Earth may not have originated on Earth. Instead, they suggest that abiogenesis may have occurred somewhere in outer space, and that organisms later arrived on Earth. Most researchers discount this “panspermia” hypothesis, because they feel that ionizing radiation and the great extremes of temperature in space would have killed any organisms before they could have reached Earth. However, the discovery of living bacteria that can survive intense extremes of radiation and heat has made this objection less compelling in recent years. The suggestion of an extraterrestrial origin of life suffers from the greater drawback that it merely shifts the mystery of abiogenesis from Earth to another place in the universe.

Other researchers suggest that organic precursors to life arrived on Earth aboard meteors or comets. Once here, these organic compounds arranged themselves into molecules that eventually led to the development of life. This theory simplifies the problem of explaining the origin of life by suggesting that the

KEY TERMS

Coacervates —A cluster of polysaccharides, nucleic acids, and polypeptides formed when a solution of these molecules is shaken. Coacervates are a type of protobiont.

Organic compound —A molecule containing carbon atoms.

Protobiont —Cell-like aggregates of organic molecules capable of maintaining a separate environment slightly different from its surroundings. Protobionts are not capable of reproduction but may have been a step toward the formation of life on Earth.

formation of simple organic compounds did not have to take place on Earth.

The genesis of organisms is not yet satisfactorily explained by any extant theory of the origin of life. However, given that life is the most complex chemical process in the universe and that its chemical basis of reproduction (DNA and RNA) has been known to scientists for only about 50 years (with many details still unrevealed), it would be foolish to conclude that science cannot explain the origin of life. The Miller-type experiments and astronomical observations show that chemicals essential to life occur spontaneously under many conditions, and have established an essential point in the case for abiogenesis. The fact that the earliest-known life-forms (eukaryotic cells) were far simpler than later life-forms is also highly suggestive of an even earlier, simpler stage—a bridge between non-reproducing chemistry and life chemistry. Scientists still seek to understand the likely nature of that bridge.

See also Chemical evolution.

Resources

BOOKS

Crick, Francis. Life Itself, Its Origin and nature. New York: Simon and Schuster, 1981.

Luise, Pier Luigi. The Emergence of Life: From Chemical Origins to Synthetic Biology. Cambridge, UK: Cambridge University Press, 2006.

Popa, Radu. Between Necessity and Probability: Searching for the Definition and origin of Life. New York: Springer, 2004.

Rosen, Robert. Life Itself : A Comprehensive Inquiry into the Nature, Origin, and fabrication of Life. New York: Columbia University Press, 2005.

PERIODICALS

Fry, Iris. “Search for Life’s Beginnings.” Science. 312 (2006): 1140-1141.

OTHER

Washington University in St. Louis. “Calculations favor reducing atmosphere for early earth: Was miller-Urey experiment correct?.” September 7, 2005. <http://newsinfo.wustl.edu/news/page/normal/5513.html> (accessed November 9, 2006).

Dean Allen Haycock

LIFE, ORIGIN OF

Theories of how life originated on Earth are of two sorts. Biogenetic theories hold that living things always arise through the agency of preexisting organisms. Abiogenetic theories hold that living things arise from inanimate sources. Spontaneous generation, an abiogenetic theory, postulates the origin of lower plants and animals from the slime of Earth and microorganisms from nutrient broth. Biopoesis holds that only the first living form or forms arose in the remote past from inorganic matter by spontaneous generation.

This article will review some of the theories of how life may have arisen on Earth and consider the philosophical problems associated with them. These theories can be categorized under the headings of spontaneous generation, cosmozoic processes, creation, and biopoesis.

Spontaneous Generation. Until the mid-17th century it was generally held that living things could arise spontaneously, as well as by sexual or nonsexual reproduction. In 1668 Francesco Redi provided experimental evidence that maggots thought to be spontaneously generated had actually been produced from the eggs of adult flies. Redi put a snake, eels, a slice of veal, and some fish into two sets of four large, wide-mouthed flasks, covered one set of jars with fine gauze, and left the other set open. Flies entered and left the open jars, and maggots soon appeared in them. No maggots were discovered in the covered jars, although a few fly deposits and maggots appeared on the gauze cover. Redi noticed that the adult flies that finally emerged were like those crawling on the meat before the appearance of the maggots. Redi's experiments discouraged belief in the spontaneous generation not only of maggots, but of macroscopic animals and plants in general.

In the 18th century the controversy over spontaneous generation was reopened when Anton van Leeuwenhoek made a number of important discoveries about microorganisms using a microscope. An Italian abbot, L. Spallanzani, performed hundreds of experiments to show that no animalcules appeared when nutrient broth was heated in phials and sealed off from the air. The English Jesuit John Turberville countered that Spallanzani had heated his liquids too vigorously and had in this way destroyed the "vital force" of the infusions and of the air in the sealed container.

In the 1860s Louis Pasteur ended the debate over whether or not microorganisms developed de novo without parents in nutrient solutions. After heating flasks of nutrient materials to the boiling point, he sealed off the necks of some and drew the necks of others out into an S-shaped curve and left them open to the air. In the cases where he sealed the portals of entry no microorganisms appeared, whereas in the open flasks the microorganisms were trapped in the moisture in the S-shaped necks. As a result of Pasteur's experiments many biologists ruled out spontaneous generation as a theory of the origin of life. Omne vivum e vivo became the accepted dictum: yet this did not preclude the possibility of life's having arisen spontaneously in the remote past.

Cosmozoic Processes. H. von Helmholtz and Lord Kelvin were among the scientists who speculated that viable spores floating through interstellar space may have accidentally seeded life on Earth when conditions were favorable. Adherents of this theory held that life, like matter, is eternal. Therefore, it was not the origin of life that needed explanation, but the passage of the seeds of life from one planet to another. Helmholtz postulated that live germs were brought to Earth in meteorites. In 1908S. A. Arrhenius published a similar theory, known as panspermia, which included careful calculations of the pressure of the Sun's rays acting on live germs to bring them to Earth.

At the beginning of the 21st century, the cosmozoic theory draws mixed reactions from scientists. Those in favor of it, such as Francis Crick, point to the rapidity with which life arose on Earth. Earth is approximately 4.6 billion years old. The earliest fossils currently known are3.5 billion years old, and life is thought to have originated around 3.8 billion years ago. Until about 3.9 billion years ago Earth was bombarded by meteorites, which in some cases may have been large enough to sterilize Earth's surface to the depth of several kilometers. The numerous chemical reactions needed for the first living thing to form on Earth seem incompatible with so short a period. Other scientists, however, think that there may have be more time available than is generally assumed. Their claim is that life may not have had to wait until Earth ceased to be bombarbed, but may have have arisen deep in the subsurface of Earth. Also, it is far from certain that life could not have arisen more quickly than is generally supposed. Thus panspermia does not seem to be the only option. Another objection to panspermia is that the extreme cold, the absence of moisture and oxygen, the intense ultraviolet radiation, and the vast interstellar and interplanetary distances to be traveled make the passage of highly organized living things through space virtually impossible. On the other hand, it must be noted that life forms such as bacteria are amazingly resistant to extreme conditions. For example, Deinococcus radiodurans is able to resist 3,000 times the dose of radiation lethal to humans. Bacteria inside a camara left on the Moon were found to still be alive two-and-a-half years later despite the absence of atmosphere on the Moon. Still, proponents of panspermia fail to explain what it is about sites in the universe other than Earth such that one would have reason to believe that life began elsewhere. Panspermia does not solve the problem of the origin of life; it merely locates the origin on another planet or in space, and leaves the question unsolved. At present it is debated whether definite traces of living things have been found in meteorites. Some of the apparent trace fossils in the meteorites examined were ultimately determined to be earthly contaminants (e.g., in the 1969 Murchison meteorite). Examination of meteorite ALH 84001 and other Martian meteorites, however, has led some researchers to conclude that the features observed are best explained as biogenic in origin.

Creation. The author of the book of Genesis states that God created the heavens and the earth and that God then said, "Let the earth bring forth vegetation: seedbearing plants and all kinds of fruit trees that bear fruit containing their seed" (Gn 1.11–12). St. augustine (De Genesi ad litteram 5.4–5, Patrologia Latina, ed. J. P. Migne, 271 v., indexes 4 v. [Paris 1878–90] 34:323–372) interprets Genesis to mean that God created animals and plants only virtually, in the sense that the earth was given the power to bring forth living things in time. Although this idea concurs with modern evolutionary theories, Scripture's purpose is not to pass judgment on scientific theories of the universe. The sacred writer teaches only that a transcendent God called the cosmos into being and set man, made in His own image and likeness, over visible creation. (see creation.)

Biopoesis. Theories of biopoesis attempt to explain how the first living things evolved naturally from inorganic matter. There is perhaps no other area of investigation in biology in which the words "conjectural," "speculative," and "as yet lacking experimental support" come up so frequently. Despite great advances in biology we remain ignorant of many aspects of the processes carried on by extant living things, and thus it is not surprising that we experience difficulties in explaining the origin of life processes in organisms that may be different from those we can observe. Compounding this difficulty is our lack of certitude regarding the conditions of early Earth at the time that life originated.

One of the earliest theories of the origin of life was that of A. I. Oparin. In 1924 Oparin theorized that the complex properties of living things arose in the natural process of the evolution of matter. In his view, large amounts of complex organic compounds in the oceans of primitive Earth reacted to form yet more complex molecules until one or more evolved that could be designated as alive. Oparin later showed that by mixing solutions of different proteins and other substances of high molecular weight he could produce coacervate droplets that readily adsorb organic substances from the surrounding medium. He proposed that the first primitive cells may have been much like these coacervate droplets. J. B. S. Haldane speculated (1926) that ultraviolet light acting on a mixture of water, carbon dioxide, and ammonia could produce a wide variety of organic compounds in the primitive oceans. If the first living things were formed in such a medium, the nutrient material there would sustain them.

In the 1950s, experimental work to establish the possibility of a prebiological formation of "building blocks" for living things originated in the laboratories of Melvin Calvin of the University of California and of Harold Urey at the University of Chicago. Calvin and his associates treated carbon dioxide and water in a cyclotron and produced formaldehyde and formic acid. S. L. Miller, a student of Urey, then exposed a mixture of water vapor, methane, ammonia, and hydrogen—gases believed to have been present on primitive Earth—to a silent electric discharge for a few days. Analysis of the results by the method of paper chromatography revealed a mixture of amino acids, several of which are essential components of proteins.

By heating concentrated solutions of hydrogen cyanide in aqueous ammonia for several days, J. Oró was able to produce adenine, an essential building block of nucleic acids. C. Ponnamperuma exposed to ultraviolet light a dilute solution of hydrogen cyanide and produced guanine as well as adenine, the only purines found in RNA (ribonucleic acid) and DNA (deoxyribonucleic acid).

One problem with these experiments that produce amino acids abiotically is that they require reducing conditions in order to work. Many scientists no longer think that the atmosphere of early Earth at the time that life is thought to have appeared was a reducing atmosphere. Banded iron formations have been found as far back as the Archean, and it is thought that they could not have formed under reducing conditions. There are other lines of evidence as well pointing to a neutral rather than a reducing atmosphere. Some scientists remain unconvinced by this evidence and hold to a Miller-type scenario. Others turn to outer space as a source of life's building blocks. Ninety different amino acids have been discovered in meteorites, eight of which belong to the set of 20 that are found in organisms on Earth. Meteorites including microscopic ones called micrometeorites may have deposited a substantial amount of organic material on early Earth's surface. Comets and cosmic dust clouds that early Earth passed through may also have been other significant sources of organic materials.

Among those who think that the building blocks of life had a terrestrial source, some suggest that they were formed at other sites on Earth other than its surface, namely, in thermal vents in the ocean floor that provide a reducing environment. Yet other scientists suggest a hot environment even deeper in Earth's crust. Still others propose a cold environment, because high temperature tends to break down protein, nucleic acids, and many of their building blocks. In short, theories abound and there is no consensus.

Even if there were a plausible explanation for how the building blocks of life were generated, there still would remain the question of how they became assembled into the more complex units, the proteins and nucleic acids that are found in even the simplest life forms. A "chicken and egg problem" arises here: in cells amino acids are assembled into proteins on ribosomes following instructions on mRNA that was copied from DNA. Thus to get proteins it seems that there has to be DNA. Nevertheless, in order for the DNA to be transcribed into RNA one needs proteins (proteins are also need for DNA replication). A number of approaches have been taken to get around this problem.

One is to say that originally proteins were assembled by some other means. Sidney Fox had some success in forming proteinlike polypeptides under anhydrous conditions in vitro. With carefully controlled conditions of temperature and hydration he produced proteinlike polymers. The majority of origin of life scientists, however, remain unimpressed, and some find Fox's proteinoids to be more like gunk than proteins. Freeman Dyson is also of the view that proteins arose first, followed by cells that carried on metabolism and reproduced without any genetic material, but simply by breaking in two. Nucleic acids arose independently, and eventually came together with these cells, eventually directing their replication. This gets around the chicken and egg problem, but the questions remain as to how proteins and nucleic acids arose without each other, and then how the two got working together in so highly an orchestrated manner. There are other current theories of abiotic protein formation, such as the thioester hypothesis advanced by Christian De Duve, but even their own authors regards them as speculative.

Another group of scientists holds that proteins did not come first, but rather RNA did. This theory gained popularity when it was discovered that certain forms of RNA can self-replicate as well as catalyze other reactions. This seemed to offer a way out of the protein-nucleic acid dilemma. Recently, however, much of the initial enthusiasm for this theory has been lost, largely because of the failure to discover a way that RNA could be synthesized under plausible abiotic conditions, and because of RNA's instability.

A further question remains as to how these larger units (proteins, nucleic acids) are assembled into cells. Oparin formed coascervate droplets by shaking a mixture of large protein and a polysaccharide. These droplets divide into an interior and exterior phase, and the conditions of interior phase were unlike those in the surrounding medium. Although some scientists find this an attractive model of a precursor of the cell, it is far from being a consensus view. Fox proposed as a better alternative "microspheres" that spontaneously form upon adding water to abiotically formed protenoids. Acccording to Fox microspheres manifested lifelike characteristics. Many scientists, however, remained unconvinced that there are sufficient likenesses between microspheres and cells such that microspheres could have been the precursor of cells.

There are many other theories of how life got started that have not been mentioned, e.g., theories such as those of J. D. Bernal and A.G. Cairns-Smith suggesting that clay initially catalysed the formation of complex biological molecules. This brief review should make it clear that scientists are nowhere close to an abiotic explanation of life's origin. This is only to be expected, however, given the relative newness and the special difficulties of this area of investigation.

Philosophical Problems

Philosophical problems associated with the origin of life are closely related to the definition one adopts for life itself. The lower forms of life have manifest affinities with the nonliving as well as with the higher forms of the living, and depending on which affinities one emphasizes, less or greater difficulty is encountered in explaining the production of life from inorganic matter. What follows is a discussion of the possibility of biopoesis in the context of Thomistic teaching on life and its need for an adequate cause, and the possible role of chance in the biopoesic process.

Possibility. St. thomas aquinas admitted the possibility of biopoesis. He did not reject St. Augustine's teaching that in the six days of creation Earth received from God the power to produce plants and trees (Summa theologiae 1a, 69.2). Thomas Aquinas acknowledged that the potentiality for such living forms could be present in primary matter; what is problematic is identifying an efficient cause of sufficient power to educe such from matter (see matter and form). Earth does not seem to be an adequate efficient cause of plants, since plants, like all living things, are superior to the nonliving elements in that they are able to move themselves.

Thomas Aquinas appears to offer two different solutions to this problem. One answer is given when he addresses the question of whether the heavenly bodies are animate. In an objection it is argued that since a cause must be superior to its effect, the generation of animals from decaying materials cannot be adequately accounted for by inanimate causes, but requires an animate cause. Given that decay is caused by the motions of the celestial bodies, especially the Sun, it follows that the celestial bodies must be animate. Thomas Aquinas responds in part by proposing an alternate explanation, namely, that spontaneous generation is caused by spiritual beings who use the celestial bodies as their instruments (De spiritualibus creaturis 6 ad 12; Contra gentiles 3.23).

Other passages of Thomas Aquinas indicate that he does not categorically reject the notion that natural material agents alone may be sufficient causes of the generation of plants and lower animals. (Following Aristotle, he did not believe that higher organisms could be generated without semen.) He did see living things as differing from nonliving things in performing activities that go beyond those of the nonliving, either simply as to the manner in which they act, or as to what they accomplish as well (De Anima 13). Plants stay in existence through their own activities, whereas inanimate things do not. Animals are not only able to do things to maintain their existence, they are also capable of knowing things through their senses. In these ways plants and animals exceed nonliving natural things in perfection. On the other hand, the souls of plants and animals (see soul) are not forms separable from matter; the activities that plants and animals perform all involve physical interaction. E.g., a life activity proper to higher organisms such as hearing requires both having organs (ears, brain), and the presence of physical entities (sound waves) that interact with the organ. And so Thomas Aquinas in commenting on the verse from Genesis which speak of Earth producing reptiles says that "it seems that the sensitive souls of reptiles and other animals are from the action of the corporeal elements….Souls of this sort do not exceed the principles of natural things. And this is manifest from considering the operation of them." (De potentia 3.11, s.c. and corp.) This latter line of thought coincides with some versions of contemporary anthropic reasoning that hold that active principles sufficient for the development of life were built into the universe by an intelligent agent from its very inception.

On either view, it is reasonable to think that Thomas Aquinas would maintain that human intelligence accompanied by our ability to manipulate natural forces may allow us someday to produce life from chemicals by artificial means.

Role of Chance. Can chance account for the origin of life? Charles Darwin wrote in a letter to Asa Gray that the problem of the origin of living things is too profound for the human intellect. Nevertheless he concluded: "I am inclined to look at everything as resulting from designed laws, with the details, whether good or bad, left to the working out of what we may call chance."

There is much debate about whether the origin of life is fortuitous or necessary, and several confusions. A position at one extreme, emphasizing the contingency of natural things, regards life as a cosmic accident. At the other extreme are those who regard the finality of living things as sufficient evidence that they are the product of design, and who go on from there to deny that there is any element of chance in life's origin. Both extremes regard chance and design as mutually exclusive. This is not, however, always the case. Thomas Aquinas gives the example of a head of the household who wants the maid to meet the butler, and with this in mind sends them to the same place, each with a different task. Since the two intend something other than meeting each other, from their point of view they meet by chance. But from the householder's point of view, the meeting was not chance, but prearranged.

Another common confusion lies in opposing fortuitous occurrences with lawlike occurrences, as Oparin does when he says that "The origin of life is not a 'fortunate,' extremely improbable event, but quite a regular phenomenon subject to a deep scientific analysis." Those who favor this view of the workings of nature hold that, where conditions were favorable, life had to originate. Now chance occurrences are not mere coincidences, such as the Moon undergoing eclipse as one steps into the shower. Chance involves causality in keeping with the laws of nature. When a person fails to notice a small step, and moves his foot upwards, of necessity he stubs his toe. We call that bad luck not because it is not in keeping with the laws of nature, but because it was not his intention to stub his toe. Thus that something be fortuitous does not mean that it is not due to causes knowable through scientific analysis. Moreover, whether life has originated once or a few times or frequently in the universe, adequate efficient causality is in all cases necessary.

Another common misconception is the notion that if a thing arises but once, it must be an unintended fluke. From this some of those who think that life on Earth was intended go on to conclude that life must be abundant throughout the universe. If the first premise was correct, however, then many of the world's great art masterpieces would be accidents, since they are unique.

Thomas Aquinas held that spontaneously produced plants and animals, unlike those that were produced from seeds or semen, were produced by chance (In Metaphys. no. 1403). Their production required the causality of the Sun, and the fortuitous coming together of matter not determinately ordered to the production of that life form, in the way that the material in the gametes is. That the Sun gives rise to different spontaneously generated species is due to differences in the matter the Sun shines on (Summa theologiae 1a2ae, 60.1). Thus that the Sun any given day causes the production of an individual of this particular species is by chance. In the greater scheme of things, however, the Sun is a universal cause meant to bring into act all the forms that are in the potency of the matter, and from this point of view that it produces individual of various species is not chance, but intended (In Metaphys. no.1403). Thus in a similar way Thomas Aquinas might have held that the exact time life originated in the universe was a matter of chance as to its immediate causes, but that it arise at some given time was not.

Chance is not seen by Thomas Aquinas as opposed to Divine Providence, but indeed as intended by the Creator in order to have a richer universe than one in which all things were necessary. Chance events, however, are, from God's point of view, part of the order of divine providence; in this sense "nothing in the world happens by chance," as Augustine declares (Summa theologiae 1a, 103.7 ad 2). Thus Thomas Aquinas would also assert a divine determinism overarching the very real contingencies of nature according to which chance events occur in the production of life and, especially, of human life.

Bibliography: c. de duve, Vital Dust (New York 1995). f. dyson, Origins of Life (Cambridge 1999). j. heidmann, Extraterrestrial Intelligence (Cambridge 1997). p. d. ward and d. brownlee, Rare Earth: Why Complex Life Is Uncommon in the Universe (New York 2000).

[a. m. hofstetter/

m. i. george]

Origin of life

There is not direct fossil-like evidence of how life originated on Earth , the molecular processes that preceded the appearance of cells do not leave such tangible evidence. However, fossils of single-celled microorganisms are present in rocks 3.0–3.5 billion years old (with some scientific controversy over which rocks contain the oldest true bacterial fossils), and chemical traces in Greenland rocks establish that single-celled life existed as long ago as 3.8 billion years. As the Earth itself is approximately 4.6 billion years old, these data suggest that life evolved within 700 million years after the Earth formed. Experiments performed over the past 50 years suggest that many important ingredients of life, including amino acids and nucleic-acid bases (the molecular building-blocks of deoxyribonucleic acid [DNA] and ribonucleic acid [RNA]), could have formed abundantly under conditions that may have existed on the early Earth, and scientists conjecture that the presence of these molecules facilitated the formation of the first actual life.

Background of the origin of life

All organisms rely on the same kinds of organic (carbon-containing) compounds; the same 20 amino acids combine to make up all the enormous diversity of proteins occurring in living things. DNA and RNA, furthermore, are essential to all life. Into these molecules is encoded the information needed to synthesize specific proteins from amino acids. One class of proteins, known as enzymes, acts to regulate the activity of nucleic acids and other biochemical functions essential to life. Enzymes do this by greatly increasing the speed of (i.e., catalyzing) specific chemical reactions . Other proteins provide structure for cells, regulate the passage of ions through the cell membrane , and perform numerous other functions. Nucleic acids and proteins are so universally essential to modern life-forms that many scientists assume that either they, or closely related precursor compounds, were present in the first life-forms.

Theories of the origin of life

All cultures have developed stories to explain the origin of life. During medieval ages, for example, European scholars argued that small creatures such as insects , amphibians , and mice appeared by "spontaneous generation"—natural self-assembly of nonliving ingredients—in old clothes or piles of garbage. Italian physician Francesco Redi (1626?–1698) challenged this belief in 1668, when he showed that maggots come from eggs laid by flies , rather than forming spontaneously from the decaying matter in which they are found.

A series of experiments conducted in the 1860s by the French microbiologist Louis Pasteur (1822–1895) also helped to disprove the idea that life originated by spontaneous generation . Pasteur sterilized two containers, both of which contained a broth rich in nutrients . He exposed both containers to the air, but one had a trap in the form of a loop in a connecting tube, which prevented dust and other particles from reaching the broth. Bacteria and mold quickly grew in the open container and made its broth cloudy and rank, but the container with the trap remained sterile. Pasteur interpreted this experiment as indicating that microorganisms did not arise spontaneously in the open container, but were introduced by dust and other airborne contaminants.

Although Redi, Pasteur, and other scientists thoroughly disproved the theory of spontaneous generation as an explanation for the origin of present-day life on whatever scale, they raised a new question: If organisms can arise only from other organisms, how then did the first organism arise?

Charles Darwin (1809–1882), the famous English naturalist, suggested that life might have first occurred in "some warm little pond" rich in minerals and chemicals, and exposed to electricity and light . Darwin argued that once the first living beings appeared, all other creatures that have ever lived could have evolved from them. In other words, spontaneous generation did occur—but only a long time ago, when the first, minimally complex forms of life would have faced no competition from more-competent cells. Many of the laboratory experiments that would eventually be conducted to shed light on the origin of life have been variations on Darwin's "warm little pond." First, however, another influential suggestion regarding the origin of life was provided by Russian scientist Aleksandr Oparin (1894–1980) and English scientist J. B. S. Haldane (1892–1964). Oparin and Haldane suggested in the 1920s that the atmosphere of billions of years ago would have been very different from today's. The modern atmosphere is about 79% nitrogen (N2) and 20.9% free oxygen (O2), with only trace quantities of other gases. Because of the presence of oxygen, which combines readily with many other substances, such an atmosphere is termed oxidizing. Oparin noted that oxygen interferes with the formation of organic compounds necessary for life by combining with their hydrogen atoms and reasoned that the atmosphere present when life began must have been a reducing atmosphere, which contained little or no oxygen but had high concentrations of gases that can react to provide hydrogen atoms to synthesize the compounds needed to create life. Oparin and Haldane suggested that this primordial, reducing atmosphere consisted of hydrogen (H2), ammonia (NH3), methane (CH4), and additional simple hydrocarbons (molecules consisting only of carbon and hydrogen atoms). Oxygen could not have been present in large quantities because it is chemically unstable, and is only maintained as a major ingredient of the atmosphere by the action of green plants and algae—that is, by life itself. Before life, the Earth's atmosphere could not have been strongly oxidizing.

According to this theory, energy for rearranging atoms and molecules into organic forms that promoted the genesis of life came from sunlight, lightning , or geothermal heat. This model of the early environment became especially popular among scientists after a U.S. graduate student of physics named Stanley Miller (1930–), then studying at the University of Chicago, designed an experiment to test it. In 1953 Miller filled a closed glass container with a mixture of the gases that Oparin and Haldane suggested were in the ancient atmosphere. In the bottom of the container was a reservoir of boiling water , and above it an apparatus that caused electrical sparks to pass through the gas mixture. After one week of reaction, Miller found that amino acids and other organic chemicals had formed from the gases and water. In the years since Miller reported his results, other researchers have performed more sophisticated "warm little pond" experiments, and have been to synthesize additional amino acids and even nucleic acids, the molecules that organize into RNA and DNA, which in turn encode the genetic information of organisms.

Subsequent research influenced by these experiments led many scientists to believe that the concentration of organic molecules in the primordial, nutrient-laden, warm "ponds" (which may have been tidal pools, puddles, shallow lakes, or deep-sea hot springs) increased progressively over time. Eventually more complex molecules formed, such as carbohydrates, lipids, proteins, and nucleic acids. The complexity gap between simple nucleic acids and self-replicating RNA or DNA is, however, large; therefore, some scientists have theorized that assembly of more complex compounds from simpler ones may have occurred on the surface of oily drops floating on the water surface, or on the surfaces of minerals—inanimate objects whose atomic structure might have provided a template for stringing together nucleic acids and giving them a place to "live" until free-floating cells protected by lipid membranes could evolve.

However, some scientists believe that the young Earth was too inhospitable a place for life to have developed on its surface at all; lacking O2, the atmosphere would also have lacked its present-day stratospheric layer of ozone (O3), which screens large quantities of harmful ultraviolet radiation from the surface. They believe that a more likely environment for abiogenesis (life from prelife) was in the vicinity of deep-sea vents, holes in the crust under the ocean from which hot, mineral-laden water flows.

Furthermore, many scientists today believe that the prelife atmosphere may not have been as strongly reducing as the one proposed by Oparin and Haldane and used in Miller's experiment. They assert that volcanoes added carbon monoxide (CO), carbon dioxide (CO2), and nitrogen to the early atmosphere, which may even have contained traces of oxygen. Nevertheless, more recent experiments of the Miller type, run using a less reducing atmosphere, have also resulted in the synthesis of organic compounds. In fact, all 20 of the amino acids found in organisms have been created in the laboratory under experimental conditions designed to mimic what scientists believe the prelife Earth was like billions of years ago—whether using Miller's model or its less-reducing competitors.

But in the absence of life, how did these amino acids link together into more complex compounds? Living cellular chemistry links amino acids together using specific enzymes to form particular proteins. An amino acid is any compound which contains at least one amino group (NH2) and one carboxyl group (-COOH). When amino acids are linked, a hydrogen molecule and a hydroxyl group (OH) are removed from each amino acids, which then link up into a protein chain, while the hydrogen and hydroxyl link up as a water molecule (H + OH = H2O). Without enzymes, amino acids do not link up in this way—or, as a biochemist might describe it, polymerization does not proceed. How, then, could amino acids have joined to form proteins without the proteins termed enzymes to help them? One possibility is that amino acids may have joined together on hot sand , clay, or other minerals. Laboratory experiments have shown that amino acids and other organic building blocks of larger molecules, called polymers, will join together if dilute solutions of them are dripped onto warm sand, clay, or other minerals. The larger molecules formed in this way have been named proteinoids. It is easy to imagine some version of Darwin's "warm little pond"—a soup of spontaneously-formed amino acids—splashing onto hot volcanic rocks. Clay and iron pyrite have particularly favorable properties making them good "platforms" for the formation of larger molecules from smaller building blocks. One recently proposed theory of the origin of life suggests that tiny ( .01-mm diameter) hollows in iron sulfide minerals, such as are deposited in the vicinity of deep-sea hot springs, might have incubated the earliest life chemistry. Iron sulfide catalyzes the formation of organic molecules, and is used by some modern bacteria for this purpose. Sheltered in tiny iron-sulfide caverns, prebiotic chemistry might have developed at leisure, leaving this protected environment only after evolving a protective lipid membrane. This theory, however, like all theories of the origin of life, has its scientific opponents, and awaits the production of confirming or disconfirming laboratory evidence.

Proteinoids produced in laboratories can cluster together into droplets that separate, and that may protect their components from degrading influences of the surrounding environment. These droplets are like extremely simple cells, although they can not reproduce. Such droplets are called microspheres. When fats (i.e., lipids) are present, the microspheres that form are even more cell-like. If a mixture of linked amino acids called polypeptides, sugars called polysaccharides, and nucleic acids is shaken, droplets called coacervates will form. All of these kinds of droplets are called protobionts, and they may represent a stage in the genesis of cellular life.

The formation of amino acids and other organic compounds is presumed to have been a necessary step in the genesis of life; it is certain, at least, that somewhere along the line all life became dependent on DNA and RNA for reproduction. Scientists thus presume that the first self-replicating molecules were similar to the nucleic acids of modern organisms. (These early molecular systems need not have been as complex as the self-replicating systems that comprise modern cells. Researchers have recently shown, by deleting genes, that even the genetically simplest bacteria alive today can reproduce with much less than their full natural complement of DNA.) Once molecules that could self-replicate were formed, the process of evolution would account for the subsequent development of life. The particular molecules best adapted to the local environmental conditions would have duplicated themselves more efficiently than competing molecules. Eventually, primitive cells appeared; perhaps coacervates or other protobionts played a role at this stage in the genesis of life. Once cells became established, evolution by natural selection could have resulted in the development of all of the life-forms that have ever existed on Earth.

The "RNA world" and the origin of life

Most living cells today store genetic information in the long-ribbon-shaped molecules of DNA. The information stored in DNA's molecular components is transferred to another ribbon-shaped molecule, RNA, by a process termed transcription. Proteins, including enzymes, are then formed by cellular structures that translate the information on the DNA. The enzymes thus produced facilitate the biochemical cellular functions necessary to maintain life and reproduce. Many scientists believe it is unlikely that all of the components of this complex sequence of events, DNA to RNA to protein, evolved simultaneously. Some scientists propose that, in fact, RNA appeared before DNA. This view has been strengthened by the discovery that some forms of RNA, called ribozymes, can act like non-protein enzymes to catalyze biological reactions. RNA thus may have been capable of ordering amino acids into forming proteins and of replicating itself in an RNA-based arrangement termed the "RNA world."

Scientists who favor the RNA world hypothesis suggest that RNA might have been able to self-replicate even before DNA and protein enzymes had evolved. Single-stranded RNA might have been able to assume a shape that allowed it to line up amino acids in specific sequences to create specific protein molecules. RNA molecules capable of causing amino acids to link up to form a protein could have had an advantage in replication and survival, compared with other RNA molecules. At that point, molecular evolution and natural selection could have taken over in furthering the development of life. RNA that produced useful protein enzymes, for example, would have survived better than that which did not.

Critics of these ideas say that the evidence for self-replicating RNA is weak. Instead, they suggest that other organic molecules, rather than nucleic acids, were the first self-replicating chemicals capable of storing genetic information. According to this idea, these simple hereditary systems were later replaced by nucleic acids during the course of evolution. Since laboratory results in this field are hard to come by, this debate is likely to persist for a long time to come.

Panspermia

Radio astronomers have found that organic molecules (including amino acids), which might have played an important role in the formation of life, are present in dust clouds in outer space. Organic molecules are also known to be present in meteors that have fallen to Earth's surface. These observations provide further evidence that chemicals important for the genesis of life may have been present on the early Earth. The presence of complex organic compounds outside of our solar system suggests that the formation of compounds important for life is more likely than once thought.

The presence of organic compounds in outer space also suggests to a few scientists that life may not have actually originated on Earth. Instead, they suggest that abiogenesis may have occurred somewhere in outer space, and that organisms later arrived on Earth. Most researchers discount this "panspermia" hypothesis, because they feel that ionizing radiation and the great extremes of temperature in space would have killed any organisms before they could have reached the Earth. However, the discovery of living bacteria that can suggest intense extremes of radiation and heat has made this objection less compelling in recent years. The suggestion of an extraterrestrial origin of life suffers from the greater drawback that it merely shifts the mystery of abiogenesis from Earth to another place in the universe.

Other researchers suggest that organic precursors to life arrived on Earth aboard meteors or comets . Once here, these organic compounds arranged themselves into molecules that eventually led to the development of life. This theory simplifies the problem of explaining the origin of life by suggesting that the formation of simple organic compounds did not have to take place on Earth.

The genesis of organisms is not yet satisfactorily explained by any extant theory of the origin of life. However, given that life is the most complex chemical process in the Universe and that the chemical basis of reproduction (DNA and RNA) has been known to scientists for less than 50 years (with many details still unraveled), it would be extremely hasty to conclude that science cannot explain the origin of life. The Miller-type experiments and astronomical observations show that chemicals essential to life occur spontaneously under many conditions, and have established an essential point in the case for abiogenesis. The fact that the earliest-known life-forms (eukaryotic cells) were far simpler than later life-forms is also highly suggestive of an even earlier, simpler stage—a bridge between non-reproducing chemistry and life chemistry. Scientists still seek to understand the likely nature of that bridge.

See also Chemical evolution.

Resources

books

Crick, Francis. Life Itself, Its Origin and Nature. New York: Simon and Schuster, 1981.

Lahav, N. Biogenesis: Theories of Life's Origin. Oxford University Press, 1999.

periodicals

Mellersh, Anthony. "The Origin of Life." Natural History (6 June 1994): 10-12.

Orgel, Leslie E. "The Origin of Life on the Earth." Scientific American (October 1994): 77-83.

other

Whitfield, John. "New Theory for Origin of Life." Nature Science Update. Dec. 2, 2002. (February 6, 2003).

Dean Allen Haycock

KEY TERMS

Coacervates

—A cluster of polysaccharides, nucleic acids, and polypeptides formed when a solution of these molecules is shaken. Coacervates are a type of protobiont.

Organic compound

—A molecule containing carbon atoms.

Protobiont

—Cell-like aggregatesof organic molecules capable of maintaining a separate environment slightly different from its surroundings. Protobionts are not capable of reproduction but may have been a step toward the formation of life on Earth.

LIFE, ORIGIN OF

Two explanations dominated prescientific thinking about the origin of life: special creation and spontaneous generation. According to the former view, supernatural intervention was essential for the creation of life; according to the latter, living organisms could form spontaneously—for example, from the mud of the Nile. Not surprisingly, special creation was usually favored as an explanation of the origin of humans and the higher animals, whereas spontaneous generation seemed adequate to explain the origin of insects, frogs, and even mice.

The theory of spontaneous generation came under attack in the seventeenth century when the Italian scientist Francesco Redi showed that maggots do not arise spontaneously in rotting meat but develop from eggs laid by flies. The spontaneous generation controversy persisted for another two hundred years or so until the classic experiments of Louis Pasteur convinced almost everyone that even microorganisms appear only as the descendents of similar microorganisms. This posed the problem of the origin of life in its modern form: How were the first organisms generated from abiotic matter?

The generally accepted answer to this question was provided by the theory of evolution through natural selection as proposed by Charles Darwin and Alfred Russell Wallace. Darwin in the final paragraph of the first edition of "On the Origin of Species" suggests that the whole complex world of life has evolved from one or a few simple kinds of organism that were formed on the Earth long ago. "There is a grandeur in this view of life with its several powers, having been originally breathed into a few forms or into one, and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved" (1859, chapter 14).

Darwin never published his thoughts on the origin of those earliest organisms, probably to avoid upsetting his wife, but in a much-quoted letter he speculates that life may have emerged "in some warm little pond with all sorts of ammonia and phosphoric salts, light, heat, electricity, etc., present" (1959 [1898], pp. 202–203). Thus Darwin thought that, long ago, a complex mixture of organic molecules was formed spontaneously on the Earth "in some little pond," and that they supported the appearance of the first simple living organisms. After that, the evolution of the whole biosphere was the consequence of natural selection acting on those earliest organisms and their descendants. Modern research on the origin of life is largely concerned with filling in the details of Darwin's scenario.

The Nature of the Problem

The Earth is about 4.6 billion years old. The dating of the earliest fossil microorganisms remains somewhat controversial, but it seems almost certain that organisms not unlike modern bacteria or algae were already present on the Earth about 3.5 billion years ago. During the first half-billion years of Earth's history repeated impacts of comets, asteroids, and other interplanetary objects would have sterilized the Earth's surface, so up to half a billion years was available for the evolution of complex life from abiotic origins. There is no reason to doubt that this was long enough.

DNA sequencing and the comparison of the genomes of different organisms have revolutionized human understanding of the evolutionary relationships between the varied forms of life. While many details remain to be worked out, people already have a reasonable picture of the nature of the last common ancestor of all life, and a fairly detailed outline of the sequence in which the different fossil and extant forms of life evolved from it. The many gaps in the picture are likely to be filled in during the early twenty-first century. The outstanding problem, therefore, is that of the origin of the first living, replicating microorganisms. Most scientists believe that they originated on the Earth, although the possibility that they were brought here from elsewhere in the solar system cannot be dismissed out of hand.

Early Experimental Studies

The modern era of experimental origin-of-life studies began in 1953 with the classical experiments of Harold Urey and Stanley Miller. Alexandre Ivanovich Oparin in 1924 had suggested that the organic material needed to get life started was formed in the atmosphere of the Earth when the atmosphere was still reducing. Miller, then a graduate student working with Urey, tested this hypothesis by passing an electric discharge through a "reducing atmosphere" of methane, water, and ammonia. To the surprise of his contemporaries, Miller was able to detect among the products substantial amounts of several of the amino acids that are present in proteins. This was the first successful experiment designed to demonstrate that important components of contemporary living organisms are readily formed from simple starting materials under prebiotic conditions.

In the years following Miller's experiment, most of the organic molecules that are central to molecular biology were obtained by related methods. The discovery by Juan Oro that adenine, a component both of nucleic acids and of ATP, the energy currency of the cell, could be formed from a simple solution of ammonium cyanide was particularly impressive. However, this whole approach came under attack when it was realized that the atmosphere of the Earth could never have been as strongly reducing as Miller and Urey assumed. Whether it was ever sufficiently reducing to support similar chemistry, even if less efficiently, is uncertain.

A second possible source of the organic material needed to permit the origin of life was identified in the carbonaceous chondrites, a common class of meteorite. Careful chemical analysis showed that these stones contained abundant organic material, including amino acids and the nucleic acid bases. Many scientists believe that meteorites, comets, and interplanetary dust provided much of the organic material for the origin of life.

In the late twentieth century another possible source of prebiotic organic material was identified, namely the deep-sea vents. In the vents, superheated water containing large amounts of metal sulfides comes into contact with cold seawater causing the sulfides to precipitate. Laboratory experiments suggest that metal sulfides can act as catalysts for the formation of a mixture of a variety of organic molecules from volcanic gases. Clearly, there are several possible sources of the prebiotic organic material needed for the origin of life, but it is not clear which of them was most important.

The RNA World

The most important recent advance in our understanding of the origin of life is the realization that there once was an RNA world. The modern biological world depends on a complex, interacting system of proteins and nucleic acids in which proteins are needed to replicate nucleic acids, but the formation of proteins depends on the prior presence of nucleic acids. It is now known that the DNA/RNA/protein world was preceded by a much simpler world in which RNA, without the help of proteins, fulfilled both a genetic and a functional role.

It is now clear from laboratory experiments that RNA molecules are capable of evolution by natural selection and are capable of catalyzing a variety of difficult chemical reactions. In particular it has been possible to evolve an RNA catalyst that carries out the most important step involved in RNA replication. It seems probable, therefore, that RNA catalysts (ribozymes) once supported a fairly complex form of life, without the help of proteins. Thus the problem of the origin of life is simplified: How were the first replicating molecules of RNA synthesized on the primitive Earth?

Attempts to demonstrate the synthesis of RNA under prebiotic conditions have met with some success, but formidable difficulties remain. The monomeric components of RNA, ribonucleotides, are complicated organic molecules made up from a sugar, a heterocyclic purine or pyrimidine base, and an inorganic phosphate group. The prebiotic syntheses of the two organic components that have been reported are relatively inefficient and nonspecific, and the combination of the three elementary components to form ribonucleotides is complicated by several troublesome side reactions. A great deal of novel chemistry needs to be discovered before a plausible prebiotic synthesis of the nucleotides can be claimed. A number of scientists are working on the problem.

The formation of long polymers from ribonucleotides is another difficult step in the synthesis of RNA. However, substantial successes have been achieved in model systems. The most extensive studies make use of an abundant clay mineral, montmorillonite, to catalyze the polymerization of an analog of the activated nucleotides that are used in the enzymatic synthesis of RNA. This work emphasizes the important role that minerals are likely to have played in the origin of life. It seems probable that many of the most difficult reactions needed to get life started occurred on mineral surfaces rather than in solution.

The replication of DNA or RNA is dependent on specific base-pairing between adenylic acid and uridylic or thymidylic acid and between guanylic acid and cytidylic acid. Base pairing is an intrinsic property of the nucleotide bases, so that a preformed strand of RNA (DNA) will align the complementary mononucleotides in the correct sequence even in the absence of a protein enzyme. If the nucleotides are presented in an activated form suitable for incorporation into polymers, a preformed RNA (DNA) strand, therefore, will bring about the nonenzymatic synthesis of a new complementary strand. This process is known as template-directed synthesis.

Template-directed synthesis is a central theme in many scenarios for the origin of the RNA world. It has been shown, for example, that a great variety of RNA sequences can be "copied," that is a great variety of sequences will catalyze the synthesis of their complements, converting single-stranded RNA to double-stranded RNA. Thus mineral catalysis of the formation of long single-stranded RNA molecules followed by template-directed copying could, in principle, have assembled a complex mixture (library) of double-stranded RNA on the primitive Earth, but only if a supply of ribonucleotides was available.

It is possible to propose a scenario for the origin of the RNA world by optimistic extrapolation of the available experimental evidence. First nucleotides were formed abiotically; they condensed together on mineral surfaces to give single-stranded RNA that was then copied by template-directed synthesis to give a "library" of double-stranded RNA molecules. Among these was one that included an RNA polymerase that was able to get efficient RNA replication started.

The serious obstacles to the prebiotic synthesis of RNA have led many researchers to propose a different kind of solution to the problem of the origin of the RNA world. They believe that one or more much simpler biochemical worlds preceded the RNA world and "invented" RNA. The search for such simple worlds is just beginning, but there are already a number of RNA-like polymers that, although they are somewhat simpler than RNA, look as though they could have functioned as genetic systems. The search for even simple systems is an active field.

Summary

It is generally accepted that once a replicating genetic polymer appeared on the early Earth, evolution through natural selection could account for the appearance of ever more complex organisms, and finally of the familiar biosphere. It is known that one such evolving world, the RNA world, preceded the world of DNA, RNA, and proteins. Scientists do not know how the RNA world came into existence. There are several theories, but none is as yet supported by strong experimental evidence. Ongoing research should provide an answer sometime in the early twenty-first century.

See also Darwinism.

Bibliography

Botta, O., and J. L. Bada. "Extraterrestrial Organic Compounds in Meteorites." Surveys in Geophysics 23 (2002): 411–467.

Darwin, C. R. "Letter to J. D. Hooker, [1 February] 1871. In The Life and Letters of Charles Darwin. Vol. II, ed. by F. Darwin [1898]. New York: Basic Books, 1959, pp. 202–203.

Darwin, Charles. On the Origin of Species. London: John Murray, 1859.

Farley, J. The Spontaneous Generation Controversy from Descartes to Oparin. Baltimore: Johns Hopkins University Press, 1977.

Gilbert, W. "The RNA World." Nature 319 (1986): 618.

Johnston, W. K., P. J. Unrau, M. S. Lawrence, M. E. Glasner, and D. P. Bartel. "RNA-Catalyzed RNA Polymerization: Accurate and General RNA-Templated Primer Extension." Science 292 (2001): 1319–1325.

Joyce, G. F. "Nonenzymatic Template-Directed Synthesis of Informational Macromolecules." Cold Spring Harb Symp Quant Biol 52 (1987): 41–51.

Kasting, J. F., and L. L. Brown. "The Early Atmosphere as a Source of Biogenic Compounds." In The Molecular Origins of Life, edited by A. Brack, 35–56. New York: Cambridge University Press, 1998.

Miller, S. L. "A Production of Amino Acids Under Possible Primitive Earth Conditions." Science 117 (1953): 528–529.

Oparin, A. I. The Origin of Life. Translated by S. Morgulis. New York: Macmillan, 1938

Orgel, L. E. "The Origin of Life on the Earth." Scientific American 271 (1938): 52–61.

Oro, J., and A. Kimball. "Synthesis of Adenine from Ammonium Cyanide." Biochem Biophys Res Commun 2 (1960): 407–412.

Steitz, T. A., and P. B. Moore. "RNA, the First Macromolecular Catalyst: The Ribosome is a Ribozyme." Trends Biochem Sci 28 (2003): 411–418.

Leslie E. Orgel (2005)

Origin of Life

How did life begin on Earth? The fact is that no one knows the answer yet, and it remains one of the primary unsolved questions of biology. We may never know with certainty because life began on Earth nearly four billion years ago. The events that initiated life no longer occur, and even the conditions of that the early Earth are not known with any certainty.

We do know one thing with reasonable certainty: Even bacteria, the simplest forms of life today, are so complex that they could not have appeared spontaneously on the early earth. More likely there were even simpler forms of life that required several hundred million years to evolve into bacterial life, complete with deoxyribonucleic acid (DNA) genes , metabolic pathways, ribonucleic acid (RNA) machinery, and protein catalysts .

The first life probably appeared several hundred million years after Earth was formed as a planet in the early solar system 4.5 billion years ago. There are many lines of evidence that support this statement, but the simplest to understand is the fossil record. Even bacteria leave fossils, and such micro-fossils were discovered in Australian rocks that are about 3.5 billion years old.

Something else scientists know with certainty is that Earth was very different when life first began. For example, the large number of craters on the moon's surface were produced by giant impacts of comets and asteroidsized objects that were part of the accretion process by which the moon formed. The collisions continued until about 3.9 billion years ago. During that time Earth was also being hit by objects many kilometers in diameter, and the first life could not have begun until the violent bombardment ceased. Therefore, scientists estimate that the simplest form of life probably was present about 3.8 billion years ago, and over a few hundred million years evolved into the bacteria that left the Australian microfossils.

What sorts of chemical and physical processes might have produced the first forms of life? A brief summary of the properties of life today can tell scientists a lot about how life began. All life is cellular, from single-celled bacteria to multicellular human beings. Cells have anywhere from a thousand or so genes (bacteria) to thirty thousand genes (human beings) and each gene carries the information to synthesize a specific protein. The synthesis of proteins requires energy and occurs on ribosomes . RNA carries genetic information transcribed from DNA to the ribosomes where it is used to direct the synthesis of proteins.

Properties of Life

The most basic activity of life is a process called polymerization. During this process organized systems of molecules use energy and nutrients to grow by linking smaller molecules into larger molecules. The chemical reactions involving energy and nutrients are collectively called metabolism , and the individual reactions of metabolism are catalyzed, meaning their rates are increased in a controlled way by specific molecules (proteins, in the case of all living organisms). Second, a living organism has the potential to reproduce itself at some point in its life cycle. Third, because mutations can lead to variations among individuals, populations of living organisms can evolve over time from generation to generation, responding to changes in their environment through natural selection. When one talks about the origin of life, one therefore must think about how organized systems of organic molecules could have appeared on the early Earth, and how they could take on the basic properties of the living state defined above.

Early Proposals

Louis Pasteur was the first scientist to think about how life can begin. In 1865, Pasteur showed that bacteria do not occur spontaneously in sterile culture media, and concluded that life can only appear from preexisting microorganisms. This view was accepted by the scientific community for more than fifty years, until a young Russian scientist named Alexander Oparin realized that preexisting organisms could not have been present on the early earth, which must have been sterilized by the heat of its formation. And yet, somehow life began. Oparin suggested that organic molecules could spontaneously aggregate into larger structures he called coacervates, one of which could have happened to have the basic properties of life. In general, Oparin's proposal about aggregation remains a viable hypothesis for the origin of life, but his coacervates are no longer considered to be plausible models of the first forms of life.

The next advance came with a better understanding of chemistry and biochemistry. Life as it is known in the twenty-first century requires organic compounds containing carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur, and these are present in four kinds of biochemical compounds and their polymers: amino acids and proteins, nucleotides and nucleic acids, simple sugars and polysaccharides like starch and cellulose , and lipids , which self-assemble into cell membranes. Were such compounds available for the origin of life? The answer seems to be yes. Even today, certain meteorites fall to Earth that contain thousands of different organic compounds, including amino acids, synthesized by nonbiological processes. Scientists also know from the early experiments of Stanley Miller and Harold Urey that organic compounds can be synthesized under simulated prebiotic conditions, so it seems reasonable to assume that simple organic compounds were present on the early Earth.

Self-Assembly

Now one can think about the actual process by which a living organism could appear on the early Earth. An important point to understand is that some organic molecules have properties that allow them to spontaneously organize into larger structures. A common example is the self-assembly of soap molecules into soap bubbles. A living cell resembles a microscopic bubble, and the same forces that produce a soap bubble also stabilize the membrane that surrounds all living cells like a skin and separates the cytoplasm from the outside world. It is easy to imagine that such microscopic bubbles were present on the early Earth, and it has been shown that some of the organic compounds in meteorites can in fact produce bubblelike structures.

Although the assembly of microscopic membranes from soaplike molecules is interesting, two other self-assembly processes are equally important. The first is that the long strings of polymerized amino acids called proteins can fold up into tightly packed balls that represent the functional proteins, such as enzymes . This folding process occurs in all cells as proteins are synthesized from amino acids on ribosomes. If proteinlike molecules were somehow produced on the early Earth, they would also have the capacity to fold into a variety of structures, some of which might act as catalysts.

The second self-assembly process is that long strings of polymerized nucleotides called nucleic acids can wind together into double stranded structures. The famous DNA double helix is an example, and this is the only way that scientists know that a molecule can reproduce itself. That is, one strand of DNA acts as a template , and a second strand is produced on the template when nucleotides bind to it and are then linked together. All life today depends on this process, which is called replication, and the earliest forms of life must have had a primitive version incorporated into their system of molecules.

Defining How Life Began

Given all this, scientists can hypothesize how life began on Earth. There is little doubt that mixtures of organic compounds became organized into complex systems by self-assembly processes, because the same thing happens in the organic compounds of meteorites, which are as old as the solar system. These self-assembled systems can be thought of as countless natural experiments that occurred all over Earth for hundreds of millions of years.

The next step occurred when a few of the microscopic systems had the particular set of molecules and properties that allowed them to capture energy and nutrients from the environment, and use them to produce larger polymeric molecules. In the next step toward life, one of the growing systems contained molecules that could be used as templates to direct further growth, so that a second polymeric molecule was in a sense a replica of the first molecule. DNA synthesis in cells is a primary example of molecular growth by polymerization, and also demonstrates how the information in one molecule can be reproduced in a second molecule. Because these processes can be reproduced under laboratory conditions, one can be reasonably certain that they are plausible reactions on the early Earth, even though scientists don't know yet how the first long polymers were produced.

The last step in the origin of life is that one or more of the growing, replicating systems happened to find a way to use the sequence of monomers in one molecule, such as a nucleic acid, to direct the sequence of monomers in another kind of molecule such as a protein. This was the origin of the genetic code and the beginning of life. It also marked the beginning of evolution, because molecular systems composed of two different interacting molecules like nucleic acids and proteins have the potential to undergo mutational change followed by selection.

It is amazing to think that this complex set of events occurred spontaneously on the early Earth, and that life was up and running only a few hundred million years after Earth had cooled sufficiently for liquid water to exist. And yet, this seems to be what happened, and if it happened on Earth it could also happen elsewhere, since the laws of chemistry and physics are believed to be universal. This larger understanding of life has led to a new scientific discipline called astrobiology, which is defined as the study of life in the universe.

Could Life Have Begun Elsewhere?

Could life have begun elsewhere? The simplest place to look is in the solar system and compare other planets with Earth. Scientists now have a better understanding of where life exists on Earth, and it is much more widely distributed than we might have guessed. Bacterial life exists over a remarkable temperature range, from near 0°C (32°F) on melting snow to over 115°C (239°F) in submarine hydrothermal vents. It exists in acidic environments as strong as battery acid or as alkaline as household ammonia. Bacterial life exists in the dark, in the absence of oxygen, and has even been found growing in the radioactive water of nuclear reactors. In fact, the only constant is that microbial life requires liquid water, and if liquid water exists elsewhere we might expect that life could have started as it did on Earth, and may even still be flourishing.

Europa was discovered by Galileo Galilei in 1610. This moon of Jupiter is the sixth largest moon in our solar system.

Where in the solar system might one find liquid water? There are only two places that scientists know of: Mars and Europa. Mars certainly has water, but in the form of ice. Liquid water cannot exist for long on the surface of Mars, due to the cold temperature and low atmospheric pressure, but it could be locked up in ice beneath the surface, just as water is present in the permafrost of Arctic tundras. Recent images from the Mars Global Surveyor clearly show that liquid water occasionally breaks through the ice and pours down steep slopes on the edges of craters. Europa, a moon of Jupiter about the size of Earth's moon, also has water in the form of a thick sheet of ice, and beneath the ice is a global ocean of liquid water. On both Mars and Europa there is a distinct possibility that life similar to bacteria could be present, and future space missions may finally answer the age-old question: Does life exist elsewhere?

see also Cell Evolution; Evolution; Evolution, Evidence for; Life, What Is

David W. Deamer

Bibliography

Malin, M. C., and K. S. Edgett. "Evidence for Recent Groundwater Seepage and Surface Runoff on Mars." Science 288 (2000): 2330–2335.

Miller, S. L. "Production of Amino Acids Under Possible Primitive Earth Conditions." Science 117 (1953): 528–529.

Miller, S. L., and Urey, H. C. "Organic Compound Synthesis on the Primitive Earth." Science 130 (1959): 245–251.