|Chapter 7 ♦ The Origin of Life||© 1979 Robert A. Freitas Jr. All Rights Reserved|
|Two central themes of xenobiology|
Scientists today will still admit that they really don’t know how life began on our planet. Laboratory work is tricky, and nobody was present to witness events at first hand on the primitive Earth. Researchers in abiogenesis can only invent some reasonable story about how life arose, and then maximize its plausibility by theoretical and experimental investigations.20
There are two central themes that run as undercurrents throughout the whole of xenobiology. First, what is the probability that life of our kind will evolve on other worlds? By illuminating the abiogenic processes of this planet in ancient times, scientists hope to get a handle on the exact combination of conditions and events necessary for the origin of carbon-based Earthlike life anywhere in the Galaxy.
The second central theme of xenobiology, to which we shall return in later chapters, is the likelihood that life, once having emerged in a planetary environment, will constitute a form of biota more or less similar to that found on Earth. The laws of biochemistry demand that molecules combine only in certain specific ways, and usually only in a very few most probable ways. In other words, what are the physical and biochemical limits of the possible?
Speculations on the source of life have been abundant throughout recorded history. The Rig Veda mentions that biology began from the primary elements, and the Atharva Veda suggests that the oceans were the cradle of life. The Bible, with its contradictory accounts of the Creation in Genesis (did man arrive before or after the beasts?), is strictly adhered to by many fundamentalists. Philip Henry Gosse, an eminent 19th century zoologist and Christian, found it a simple task to reconcile the growing mass of paleontological evidence with the Scriptures. God, he declared, created the Earth entirely in accordance with scientific findings. The Lord fabricated geological strata, embedded fossils and the like for the sole purpose of fooling geologists. The apparently extreme age of Earth is only an illusion.
Peculiar ideas abound. Hylozoism, for instance, is the belief that matter and life are one and inseparable. From this viewpoint, life either has no origin and has always existed, or else the question may be deferred to the origin of all matter.
The theory of pyrozoa, to cite another example, was advanced by William Preyer in the last century. Preyer believed that life has existed at all times, even when our planet was still in the molten state. These first fiery living things, the pyrozoa, slowly modified and adapted themselves as the environment cooled and changed, eventually assuming the form in which life presents itself to us today.2218
Most theories on the origin of life have fallen into one of four distinct categories:
- Life has no origin — both life and matter have existed forever;
- Life is the consequence of a supernatural event, intractable and in explicable by the methods of science;
- Life originated via ordinary chemical evolution in a deterministic fashion — under similar circumstances, the same general evolutionary patterns would repeat themselves on any world; and
- Life originated elsewhere by means unknown, and was subsequently transported to Earth (panspermia).
The first two are self-explanatory, and the third closely approximates the leading modern theories. The last deserves a word of explanation.
The Greek philosopher Anaxagoras of Clazomenae (ca. 500-428 B.C.) was possibly the first to suggest that the seeds of life permeated the universe. With the downfall of spontaneous generation millennia later, panspermia enjoyed a brief revival. The theory was sponsored by many 19th-century notables, including Richter, Kelvin, Helmholtz, Arrhenius, and the great Italian chemist Avogadro.
The doctrine of lithopanspermia held that meteorites were the means by which life wandered from planet to planet throughout the cosmos. Lord Kelvin, a central proponent of this view, considered it probable that countless life-bearing "stones" existed in space, perhaps as the result of collisions between inhabited worlds. Hermann von Helmholtz, a German philosopher and a pioneer in physics, believed that the interior of a meteorite would be a safe retreat for interplanetary microbes during the long incandescent journey through thick planetary atmospheres. The presence of hydrocarbons in the carbonaceous chondrites was cited as evidence of the biological activities of the tiny organisms from space.
Modern analyses suggest that microscopic lifeforms embedded in interstellar comets are possible, but unlikely. The accumulated radiation dose from cosmic rays and natural internal radioactivity is "embarrassingly high" over the large transit times involved between worlds.22 Furthermore, it is now known that meteorites are of roughly the same age as the rest of the solar system, and that the organic molecules found in chondrites are reproducible by strictly chemical means.208,2030,2219
The famous Swedish physical chemist and Nobelist Svante Arrhenius was the loudest advocate for the theory of radiopanspermia.2304,2305,2306 He suggested that minute spores might be carried upward through planetary atmospheres by convection, where electrical forces could provide sufficient energy to expel them from the body. The pressure of sunlight would then be enough to propel these cosmozoa to other solar systems. Tramping through space, or riding piggyback on small grains of dust, these legions of microscopic interstellar emissaries thus brought the good news of life to the rest of the Galaxy.
Carl Sagan has done a careful analysis of the problem,20 the details of which will not be repeated here. His conclusion is that radiopanspermia is not a viable theory of the origin of life on Earth. Those microbes ejected from a stellar system by radiation pressure accumulate a dose of x-rays and UV three or four orders of magnitude higher than the maximum lethal irradiation sustain able by even the hardiest terrestrial organisms. Shielding won’t help: Life-forms large enough not to be killed aren’t ejected by radiation pressure because they are too heavy.22
The theory that life arose in the ancient swirling gas and dust clouds of interstellar space and then traversed the cosmos, seeding the Galaxy with life, may be called cosmospermia. Dr. J. Mayo Greenberg at New York State University set up a laboratory experiment a few years ago, using tiny grains of matter the size of space dust and appropriate gases. He found that many compounds of relatively high molecular weight could be formed under the influence of ultraviolet radiation. Greenberg evidently believes that a similar mechanism could lead to the production of grains of a size and composition similar to that of viruses.
Dr. Sagan has disputed such theories, noting that any hypothetical extraterrestrial organism of 10-5 cm — the size of a rabies virus or the PPLO (the smallest lifeform known) — would have a replication time on the order of two hundred million years. There could only have been fifty or so generations since the Galaxy first formed, insufficient time for natural selection and evolution to operate.141 It is hard to imagine a smaller yet viable organism; the replication time for a larger microbe would be even longer, permitting still fewer generations.
Panspermia does not address
the phenomenon of abiogenesiss
but merely displacess
the problem in space and time.
Accidental panspermia is a class of theory typified by the "Gold Garbage Theory," popularized by Dr. Thomas Gold, a leading astrophysicist at the Center for Radiophysics and Space Research at Cornell University. The Garbage Theory was first announced in a paper read before a Los Angeles meeting of space scientists in late 1958,139 and proposes that Earth may have been visited by an expedition of advanced ETs who carelessly allowed some of their native microbiota (picnic basket litter?) to escape. "While this garbage theory of the origin of life understandably lacks appeal," one xenologist notes wryly, "we should not exclude it altogether."20
A similar idea is the concept of directed panspermia, which suggests that organisms were deliberately transmitted to Earth by intelligent beings on another planet.1283 Advanced civilizations might intentionally seed sterile worlds, either as a prelude to colonization or perhaps simply to perpetuate the heritage of life on the home planet as insurance against catastrophe.
Panspermia does not address the phenomenon of abiogenesis but merely displaces the problem in space and time.* Consequently, panspermia hypotheses aren’t strictly relevant to the ultimate origin of life in the universe but simply explain how any particular world might have come to be inhabited.
* One science fiction story suggests that life on Earth may have arisen from biota left behind by a careless time traveler from our planet’s future.636 If any theory begs the question it is this one!
|The ubiquity of Life's essential compounds|
The building blocks for life are lying around everywhere.
Great clouds of organic molecules have been discovered drifting between the stars, presumably formed by various natural processes.1002,2219,2220,2221 Radioastronomers have seen relatively complex compounds hiding deep in inter stellar space, including methyl alcohol, ethyl alcohol, cyanogen, formaldehyde, formic acid and ether,1002,2217 and the search is on for amino acids.
Compounds of carbon and hydrogen, particularly cyanogen, methane and hydrocarbon radicals, are detected on the surfaces of stars.1973,2297 To find the limits of such processes, Dr. John Oró performed an experiment which simulated a hot stellar plasma. Using a graphite resistance apparatus and a plasma torch device temperatures from 1500-4000°C were obtained. Methane, ammonia and water were introduced continuously. The products were condensed at room temperature and allowed to interact for a few hours before analysis. Three amino acids appeared — alanine, glycine, and aspartic acid — along with hydrogen cyanide and a host of other organics.1072
There is no doubt that the carbon compounds essential for the development of Earthly life are ubiquitous. Organics have been detected on the Moon,2443 other planets,2037, 2046 asteroids,2037 and in comets.1973,2222
The basic constituents
necessary for the emergence
of life are universal.
The carbon chemistry of meteorites is also well-documented.702,2219
The Murcheson rock which fell in Australia on September 28, 1969 contains 2 × 10-7 moles of amino acids per gram of meteorite, which is more than many desert sands on Earth.521 These amino acids correspond rather closely to those produced in prebiotic synthesis experiments performed in the laboratory.225
The Orgueil meteorite contains approximately 7% organic matter, including hydrocarbons, fatty acids, aromatics, porphyrins, nucleic acid bases, optically active lipids, and a variety of polymeric material.1075 On the basis of the amounts of carbon compounds detected in various meteorites, researchers have concluded that these interplanetary wanderers could have brought as much as
Taken together, these studies of meteorites, comets, planets and interstellar matter strongly suggest that chemical evolution is a continuing and commonplace occurrence in all parts of the cosmos. The basic constituents necessary for the emergence of life are universal. This implies that life should be widely distributed throughout the Galaxy, wherever conditions are clement, since the required ingredients of abiogenic processes are abundantly available everywhere.
Chemical evolution refers to the period in Earth’s history during which the chemical components on the surface changed from simple forms into complex substances from which the first living organisms — protobionts — could develop. The primary investigative tool in abiogenesis research has been the prebiotic synthesis experiment. Plausible primitive Earth conditions are arranged in a closed laboratory apparatus, and the changes that take place are carefully monitored.
The argument has long been made that since no geological record of the origin of life exists, the course of events leading up to the creative event is fundamentally unknowable. While most biochemists today would dispute this supposition, how close to reality are the simulated prebiotic experiments?
The alphabet of life is extremely
simple; the wide variety of life
observed today may be traced
to a mere handful of chemicals.
It is unnecessary for scientists to heat together water, methane, ammonia and hydrogen (components of the primitive atmosphere), irradiate the mess with various forms of energy, and then sit back to wait for a recognizable lifeform to reach its slimy paw over the edge of the beaker and crawl out onto the lab desktop. We won’t ever achieve this kind of completeness, because that takes evolution and the secret to evolution is time.225 (But it has been seriously suggested that a complete artificial seashore be set up to test some of the proposed mechanisms in the origin of life.1630)
From chemical equilibria we know the kinds of substances that had to be floating around in the primitive atmosphere and seas. Protein molecules ultimately consist of different combinations of only twenty different amino acids. Nucleic acids are composed of one of five bases, one of two sugars, and a single type of phosphate group. As Cyril Ponnamperuma of the NASA/Ames Exobiology Division once remarked: "The alphabet of life is extremely simple; the wide variety of life observed today may be traced to a mere handful of chemicals."85
|Abiogenesis: an unverifiable historical process|
Abiogenesis research differs markedly from most other scientific work, in that an unverifiable historical process is being reconstructed. It probably is not practical to run through an entire origin of life "from scratch," so different criteria must be used to evaluate hypotheses. For instance, postulates must at least be consistent with known astronomical, geophysical, and biochemical principles insofar as this is possible. And stepwise experiments, in which only one step of abiogenesis at a time is simulated, are reasonable if plausible and appropriate prebiotic conditions are maintained.
It is believed that the origin of life may have happened very fast, certainly less than a billion years521 and possibly less than a hundred million years.225,305,2160 Most estimates today place the creative event in the primitive seas, roughly 4.2 to 3.6 eons ago.
For many years it was known that mixtures of carbon dioxide, ammonia and water vapor would produce small amounts of simple organic chemicals if energy was supplied. But the results of these experiments were generally very discouraging and the yields miniscule under these oxidizing conditions. To originate life in such a poor, thin broth would be well-nigh impossible.
In 1953 a graduate student named Stanley Miller, working under Nobelist Harold C. Urey at the University of Chicago, constructed an apparatus to imitate the conditions of the primitive Earth (Figure 7.1). Previous investigators had always assumed the atmosphere to be oxidizing or neutral. Miller and Urey, following the suggestions of A. I. Oparin in the Soviet Union and J. B. S. Haldane in Britain during the 1920’s, took the unprecedented step of devising a reducing environment instead.2258
Miller mixed together methane, hydrogen, ammonia and water, and carefully eliminated all oxygen from the system. This gaseous concoction was then circulated past an electric spark discharge, followed by a water bath to simulate the primitive sea. After about one week of continuous operation, the "ocean" had turned a deep reddish-brown.
The experiment was halted and the contaminated water removed for analysis. Miller discovered to his amazement and delight that many amino acids had been produced in surprisingly high yields. Two percent of the total amount of carbon in the system was converted into glycine alone. Sugars, urea, and long tarlike polymers too complex to identify were also present in unusually high concentrations.
Figure 7.2 Prebiotic Chemical Evolution on the Primitive Earth
Figure 7.2 Prebiotic Chemical Evolution on the Primitive Earth
Of course, electrical energy was only one of the many sources of energy available on the primitive Earth (Figure 7.2)
In fact, ultraviolet radiation was probably the principle source: UV would have been able to penetrate to the surface be cause the protective ozone layer in the upper atmosphere did not yet exist.
A Miller-type experiment using ultraviolet rays and a reducing atmosphere was performed in 1957 by the German biochemists W. Groth and H. von Weyssenhoff at the University of Bonn.2307 Their results closely paralleled those obtained at the University of Chicago half a decade earlier.
Countless prebiotic simulations have since been achieved which confirm Miller’s original conclusions. One bibliography, current through 1974, lists more than three thousand papers on the subject.1679 An exhaustive treatment of all of them is clearly beyond the scope of this book, but the interested reader in encouraged to dive into the literature (Table 7.1).
Table 7.2 lists the sources of energy believed to be present during the first eon or so of Earth’s history. Ultraviolet radiation leads the pack. Carl Sagan and others have completed experiments with UV which seem to indicate rather high yields for prebiotic amino acids, the building blocks of proteins. Over the first billion years of chemical evolution on this world something like a hundred kilograms of amino acids per square centimeter may have been produced, resulting in a "soup" of about 1% concentration. This is the approximate consistency of chicken bouillon.
But ultraviolet radiation is a two-edged sword. While it may be the most abundant form of energy for molecule building, it is also the most destructive. Early researchers were concerned that organics would be destroyed as fast as they were created. Fortunately, the primitive oceans probably turned opaque like the brownish glop in Miller’s apparatus rather quickly. Vital chemicals newly synthesized and carried a short distance beneath the surface of the soup by convection undoubtedly escaped decomposition.
Of the remaining energy sources, electrical discharge was the most potent. As much as 5-15% of the carbon in a mixture of methane, ammonia and water may be converted to amino acids and other organics by the energy of the discharge. Various forms of ionizing radiation give high yields as well. a particles, b particles, and g rays were common on the surface of the primitive Earth because of the presence of intense natural radioactive sources in the crust — such as
|Volcanic heat and other power sources|
Volcanic heat was another prebiotic power supply.2368,2380 It has been shown that lava-heated seawater and underwater volcanoes may be effective in producing biologically important compounds. Heat and sonic energy would have been released by infalling meteorites — certainly a significant factor in the environment of the primitive solar system.1417,2375 In fact, experiments performed recently by Bar-Nun and others have conclusively demonstrated that as much as 30% of the nitrogen in an ammonia atmosphere can be converted into amino acids in this manner.315,1664,2375 Torrential rains have even been suggested as a possible source of energy for prebiotic synthesis, and experiments have shown that a flask of formaldehyde, allowed to stand for a few days at room temperature, will produce some simple sugars.
The great lesson appears to be that the exact nature of the power supply is relatively unimportant. Amino acids, sugars, and other chemical precursors to life probably arise on any planet possessing an initially reducing atmosphere and quantities of hydrogen, carbon, nitrogen and oxygen in gaseous reduced form — regardless of the particular source, or sources, of energy available.*
* Other factors may also be important. For instance, early-type stars (F) are more likely to emit ultraviolet radiation in copious quantities than are late-type stars (K, M). The speed of chemical evolution in primitive planetary environments may actually slow as we move from class F through classes G to K stars among habitable solar systems.
The cell is the fundamental biological unit and a common denominator among all terrestrial lifeforms. Living things on this planet are made up of cells which vary in size from less than one micron to several centimeters in diameter. While the simplest organisms are unicellular, the typical human is an ambulatory assemblage of from fifty to a hundred trillion (1014) individual cells.
… no biologist today would claim that proteinoid
microspheres are alive in the sense of representing
the first protocell. And yet, to the extent that they
self-organize, accumulate information from their
surroundings, and exhibit both structure and
behavior, they are certainly near the borrline of life.
The cellular construction of Earth life is remarkably uniform: Similar water content, similar kinds of proteins, similar lipids and so forth. All have at least one membrane, perhaps no more than 100 Angstroms thick, which protects the inner workings from the harsh vagaries of the external environment. The first protobionts undoubtedly had no such complex organizational qualities.
But how can structure arise in the first place?
It has been shown by Ilya Prigogine that thermodynamic chemical systems may develop certain states wherein some of the chemical constituents have periodic, oscillating values.2230 A biologist, J. Pringle, has demonstrated that initially homogeneous systems can undergo a progressive change, leading to the appearance of "spatial heterogeneity."2231,2232 That is, structure can arise spontaneously. These two treatments of the problem of organization suggest that mechanisms may exist for collecting material into small, localized concentrations, perhaps leading to ordered structures we would recognize as cells.2368
But to build cells, we must have protein. Protein is the most fundamental construction material, used in building cell walls, enzymes, and so forth. To make proteins, there are two requirements.
- First, we need an abundance of amino acids. From our discussion above, we see that this is virtually inevitable on any normal world possessing at least small aqueous oceans and a primitive hydrogenous atmosphere.
- Second, there must be some way to hook up a long string of amino acids into a polymer of protein. Polymerization (linking together) of amino acids leads to the production of protein, which can then be used for cell-building.* It is true that even dilute primordial soups can coagulate into gelatinous masses. But such conditions are far from ideal. In all likelihood, most prebiotic syntheses probably took place in local regions of increased concentration. The efficient construction of amino acid polymers undoubtedly occurred elsewhere than in the open seas.
Numerous concentration mechanisms have been proposed which might conceivably lead to the creation of small pockets of more potent broth. The simplest method is evaporation. Primordial soup, caught in a narrow, shallow lagoon, would slowly thicken as the water that held the components in solution evaporated away.85 As suggested by Miller and Orgel, similar effects result from slowly freezing the solution in the lagoon: The solvent freezes out in the pure form first, leaving the solute concentrated in ever-smaller quantities of solvent.521
A combination of air-water and water-solid interfaces provides mechanical consolidation of suspended matter, as evidenced by the accumulation of scums and oil slicks near coastlines.1667 Another possibility is that organic compounds may have been trapped on solid surfaces such as aluminum silicate clays, quartz, and other minerals which allow polymerization reactions to proceed.1430,2381
To date, however, there is really only one proven method which yields polymers of amino acids under plausible prebiotic conditions. Dr. Sidney W. Fox at the University of Miami has obtained long-chain molecules with the following essential properties:
- They contain all amino acids common in contemporary terrestrial organisms.
- They have high molecular weights (the chains are relatively long).
- They are "active" because they interact in the catalytic or rate-enhancing sense. (This anticipates metabolic activities mediated by enzymes — which are also proteins.
- They are as heterogeneous as contemporary proteins.
- They yield "organized units" upon contact with water which have many properties in common with modern cells.1625
* We will not discuss here the significance of molecular optical activity. The curious reader is referred to Sagan,20 Jackson and Moore,47 Glasstone,72 Gabel and Ponnamperuma,315 Miller and Orgel,521 Ulbricht,1445 Hochstim,1446 Bonner et al,1447 Wald,1665 and Walker.2382
Dr. Fox calls his substances "proteinoids," because they greatly resemble living protein polymers. His method for producing them is quite simple. A mixture of amino acids is cooked at
Fox decided to test his method under more realistic field conditions. He secured a large piece of lava from the site of an Hawaiian volcano.1702 The temperature of the rock was raised to 170 °C, and the appropriate amino acids seated in a small depression at the top. Heating continued for several hours, after which the lava was washed off with a small spray of sterilized boiling salty water — as might have occurred naturally near a volcanic shoreline in ancient times.
Proteinoid polymers were formed, but there was more! To Dr. Fox’s surprise, billions of tiny "microspheres" appeared in the wash water: spherical, microscopic particles of uniform diameter bearing a striking resemblance to living cells (Figure 7.3).
|Scenario for microsphere protocell origin|
The Miami scientist presented a scenario for the origin of microsphere protocells in prebiotic times:
- 1. Hot lava meets soup;
- 2. water boils away, leaving sticky brown goop on lava;
- 3. contact with water (rain, sea spray, etc.) causes proteinoids to assemble themselves; and
- 4. microspheres are washed back into the soup.
These initial experiments were completed nearly two decades ago,1702 and since that time Fox and his colleagues have refined their methods and perfected their theories on the origin of cells and life. Protein-like materials are now produced with molecular weights ranging from 3000 to 10,000 under plausible primitive Earth conditions.2371 And it has been shown that a primitive “cell” with most of the attributes of life can arise spontaneously in a very brief period of time.
Detailed studies of microspheres have confirmed the researchers’ initial optimism. What makes these spherules so unique is their “active” nature. Dr. Fox has observed and recorded the following characteristic behavior of his proteinoid microspheres under various chemical and physical conditions:
- Spherical shape — 0.5 to 7.0 microns, uniformly.
- Single-walled membranes (like plants) and double-walled membranes (like animals).
- Simulation of osmosis — microspheres swell and shrink in response to changes in the chemical environment.
- Selectivity of diffusion — microspheres possess semipermeable membranes analogous to those of living cells. For instance, in one case Fox discovered that polysaccharides were selectively retained under conditions in which monosaccharides diffused freely through the microsphere walls. (Polynucleotides and other organics are also absorbed from aqueous solution.1624)
- Cleavage — a kind of binary fission of a single “cell” has been observed in acidic proteinoid microspheres.
- Motility — the microspheres, when viewed under a microscope, move non-randomly in preferred directions under certain special conditions. The addition of ATP appears to enhance the movement.
- Budding — buds appear spontaneously on proteinoid microspheres allowed to stand undisturbed in their mother liquor.
- Growth by accretion — buds which have been liberated by mild heating or electric shock will swell by diffusion to the same approximate size as the “parent” cell.
- Proliferation through budding — second generation budding has frequently been observed on buds that grew to the size of normal microspheres. The buds are apparently engaging in a kind of “reproduction.”
- Formation of junctions — microspheres are capable of approaching one another and physically attaching together in a more or less permanent fashion.
- Transmission of information — when two spheres have joined, small proteinoid microparticles within the larger sphere are observed to pass through the junctions. The whole process is highly suggestive of microbial conjugation.
- Stability — the activity of the proteinoids does not diminish with storage over a period of 5-10 years.2370
The best-known of all physical cell models prior to the discovery of proteinoids was the coacervates, thoroughly researched by the Soviet biochemist A. I. Oparin, the Dutch biochemist H. G. B. de Jong, and others. Coacervates are produced by combining solutions of oppositely charged colloids such as gelatin or histone with gum arabic. When solutions of the two substances are commingled, they interact to yield clusters of microscopic structures having the appearance of tiny liquid droplets. Coacervates have many interesting properties from the point of view of the origin of life.
For instance, after these uniform spherules have aggregated, they are able to absorb various simple organic molecules from the external medium (sugars, dyes, etc.). However, Oparin has admitted that this process quickly leads to static equilibrium, and the coacervate "protocell" then becomes a passive system, unstable and prone to break-up upon standing. Another property of coacervates is their ability to convert certain chemical monomers to polymers after diffusion through the "cell" wall, although it is generally recognized that the dynamic behavior of these droplets is fairly limited.
There is another reason why coacervates,1432 sulphobes,1625 "biphasic vesicles,"1630 and many other prospective pseudocells1211,1415 do not compare favorably with Dr. Sidney Fox’s microspheres as model protocells. Coacervate droplets are formed from polymers which themselves were synthesized by living organisms. The gum arabic used to manufacture Oparin’s droplets was not produced abiogenetically, nor is it at all clear how this might be done. The great advantage of the microspheres is that they are the direct product of single, simple amino acids — amino acids that must have been common on the shores and seas of the primitive Earth eons ago.
Of course, no biologist today would claim that proteinoid microspheres are alive in the sense of representing the first protocell. And yet, to the extent that they self-organize, accumulate information from their surroundings, and exhibit both structure and behavior, they are certainly near the borderline of life.
… the development of molecular self-replication
In the previous section it was mentioned that there are two requirements for the production of proteins. First, there must be amino acids, and second, there must be a way to hook them together to form polymers.
DNA — the primary information-carrying
There is, however, a third requirement for the origin of living systems on Earth. It will be recalled from the discussion of the definition of life that "it is the business of life to accumulate information and complexity." Let us consider this mandate in view of the problem of building proteins.
To abiogenetically produce a living system, that system must be capable of accumulating information and order from its environment. The proteins constructed by a cell must have the proper architecture for whatever job needs to be done. So our third requirement may be stated: There must be a way to hook the amino acids together in the correct sequence. Any old proteins will not do — they must be the right ones.
There exist simple chemical techniques to achieve this kind of ordering. One common example is called "autocatalysis" by chemists. Autocatalysis is a way for a process to catalyze its own production. Once a tiny bit of it has been produced, that bit catalyses the rate of reaction to yield still more, and faster.
Aside from this simple selective feedback effect, the development of molecular self-replication was probably the most critical single event in the origin of life on Earth. The origin of replication and the genetic code, as opposed to the origin of proteins and cells, allowed natural selection to begin to operate on stored information. And once evolution begins, selective advantages of superior membranes and of multicellular colonies can be expressed in the form of increased organismal complexity.
DNA — the primary information-carrying molecule used by all lifeforms on this planet — is a polymeric nucleic acid (Figure 7.4). We’ve already seen how easy it is to get amino acids and their polymers. But what about nucleic acids? Can they be demonstrated in prebiotic synthesis experiments, along with their polymers?
Prebiotic assembly of purines and
pyrimidines into full-fledged nucleotides
has proven more difficult, … The main
obstacle to success seems to be the
formidable complexity of the nucleotide
In 1963, Dr. Cyril Ponnamperuma managed to synthesize adenine (one of the two most important nucleic acid purine bases) under simulated primitive Earth conditions. The NASA scientist and his three colleagues used a Miller-type apparatus, and began their synthesis with nothing more than methane, ammonia and water in the system. The mixture was bombarded with energetic electrons, and about 0.01% of the carbon in the methane was converted into adenine.304 This is highly significant because adenine is useful, not only for making DNA, but also RNA, ATP, ADP, FAD, and a host of other critical life-molecules.
In a related experiment two years later, Dr. John Oró of the University of Houston and A. P. Kimball produced adenine is a closed reaction system which included ammonia, water, and hydrogen cyanide. Heat was supplied as the energy source, and this time the production of the purine base rose to 0.5% of the available carbon.303 This value was observed over a wide range of chemical conditions, indicating the relative ease with which this complex molecule must arise in a plausible prebiotic environment. The synthesis of the other important purine, guanine, has also been convincingly demonstrated.
There have been various attempts to fabricate the three major varieties of pyrimidine bases which are also necessary in the production of nucleic acids. However, the appearance of these substances under conditions similar to the primitive Earth has not been investigated as thoroughly as the purines.
One experiment that yields a hefty 20% of cytosine requires a three-step process involving methane and nitrogen initially to create a cyanoacetylene intermediate, which then goes on to produce the pyrimidine when combined with cyanate ion. Uracil, another pyrimidine, is obtained in very good yield by the direct hydrolysis of cytosine — a prebiotically reasonable reaction. All the pyrimidines have been synthesized in environments at least arguably analogous to that of the early Earth.
Prebiotic assembly of purines and pyrimidines into full-fledged nucleotides has proven more difficult, and intensive investigations are now underway to determine and eliminate the problem. The main obstacle to success seems to be the formidable complexity of the nucleotide molecules themselves. While bases and sugars are relatively easy to produce, combining them together is a much harder task.
Nevertheless, demonstrations of nucleotide synthesis under geologically plausible constraints have been made. One such technique involves the use of a mediating mineral called apatite, which contains phosphates and oxalate ion, in an "evaporating pond" scenario.
Purines and pyrimidines are comparatively
simple to manufacture abiogenetically.
The assembly of nucleotides has also met
with some imited success, but to date
it has proven difficult to synthesize more
than six-unit polymeric chains in
a prebiotically plausible way.
We are not quite home yet. Just as amino acids needed polymerization to become protein, so must nucleotides by polymerized into DNA. What progress has been made in the prebiotic synthesis of polynucleotides?
The experimental record is admittedly spotty. When adenine nucleotides were heated in the presence of polyphosphate for 18 hours at 55 °C, adenine polynucleotide polymers were obtained ranging from 20-30 nucleotides per chain. However, in the words of the experimenter, "the concentration of the reactants had to be as high as possible when the formation of high polymeric material was desired."1625
That is, unless quite artificial conditions were contrived, the adenine nucleotides could not be forged into very long chains. In another experiment, solutions of adenine nucleotide were irradiated with UV light. Long chains were again obtained, but only when extraordinarily high concentrations of polyphosphate were maintained.1628 Under similarly unrealistic conditions, uracil polynucleotides with chain lengths ranging from 10-50 units have been found.1626,1627
One good experiment has been performed by John Oró and E. Stephen-Sherwood, using a plausible "evaporating lakebed" scenario and temperatures from 60-80 °C. Uracil two-unit chains were formed with a yield of 23%, and three-unit segments with a 12% yield. Cytosine polynucleotide chains were obtained by these experimenters with up to six nucleotides in straight-line linkages. Thymine polynucleotides 2-12 units long were produced when an unreasonable chemical environment was used; with more closely matched prebiotic conditions, five-unit chains were obtained in yields of 1% or less.1429
The polymerization of some nucleotides has proven unexpectedly difficult, partly because of the inevitable formation of unnatural side chains and partly because the reaction just doesn’t seem to want to go. Various solutions to these problems have been suggested. For instance, there are enzymes — ordinary proteins — that are capable of catalyzing these polymerization reactions with ease. These enzymes, or enzymes like them, could have arisen by nonbiological means. If this is the case, claims one researcher, "such catalysts may have been responsible for the first polymerization of nucleotides on the primitive Earth."72
So at present, here is where we stand. Purines and pyrimidines are comparatively simple to manufacture abiogenetically. The assembly of nucleotides has also met with some limited success, but to date it has proven difficult to synthesize more than six-unit polymeric chains in a prebiotically plausible way.2370
Can these short strands alone make a stab at primitive replication? Dr. Leslie Orgel at the Salk Institute in San Diego, California, mixed up a solution of nucleic acids that might be considered prebiotically reasonable. He then placed some of the six-nucleotide polymers in his specially-enriched "soup." The short-chain DNA polymers correctly replicated themselves once out of every ten tries.
Q. How did Life arise?
A. No one knows.
Thus far we have concentrated on the parallel development of polymeric amino acids (proteins) and polymeric nucleotides (DNA). We’ve seen that Dr. Sidney Fox’s proteinoid microspheres exhibit many properties which are strikingly similar to those displayed by contemporary living cells. We’ve also seen that Dr. Leslie Orgel has succeeded in demonstrating accurate, if erratic, replication in primitive polynucleotides. And yet, despite these remarkable achievements, the great final question remains untackled: How and when did the first living organism arise?*
It has been fairly clearly demonstrated
that life as we know it could not have
arisen if either one or the other
[proteins or nucleic acids] was wholly absent.
The answer is as unsatisfying as it is precise: No one knows. The arguments on this score smack of the "chicken-or-the-egg" controversy. It is unknown at present if proteins and protocells came first, to be followed later by replicative nucleic acids, or whether the nucleic acids were first, and from them the cells later spawned.
It has been fairly clearly demonstrated that life as we know it could not have arisen if either one or the other was wholly absent.521 Organisms lacking nucleic acids would have no means of achieving genetic continuity and evolutionary progression, while organisms without proteins would find themselves severely limited in their ability to utilize the chemicals in their environment. Some manner of coevolution seems to be indicated.
One theory holds that nucleic acids evolved some kind of boundary layer, a proteinous skin to protect themselves from their surroundings — the so-called "naked gene" theory. When this invention inhibited or prevented reproduction, the parent nucleic acid molecule became extinct. When the new boundary layer served to protect the DNA without interfering with replication, these were the "protobionts" which survived.
There is some experimental evidence to support the view that polynucleotides might be able to influence protein synthesis directly.1431,2255 To do this, they must cause a selective linear organization of amino acids, and must facilitate amino acid polymerization.1444 Unfortunately, other studies have shown that the interaction between polynucleotides with individual (monomeric) amino acids is relatively weak.1248
Most valid evolutionary sequence:
From the simple to the more complex
More convincing, perhaps, is the idea that cells were first. Self-assembly in molecular structures has been known for many decades, and experimental evidence to date favors the easy synthesis of proteins in comparison to polynucleotides.1634 Sidney Fox has remarked that the sequence:
- protoprotein → protocell → nucleic-acid-coded contemporary cell
is the most valid evolutionary sequence because it proceeds from the simple to the more complex.1625
The primitive protocell, as modeled by the proteinoid microspheres, could have exhibited many of the properties customarily regarded as belonging only to "living" things. Under Fox’s theory, the cell would have developed nucleic acids to serve its ends, rather than the other way around.
One final piece of evidence seems to argue for the primacy of cells. In 1974, Dr. Fox and his colleagues published some experimental findings on micro-spheres which seem to imply that the proteinoid protocell can do everything optimistically predicted for it. The abstract of the paper reads, in part:
roteinoid microspheres of appropriate sorts promote the conversion of ATP to adenine dinucleotide and adenine trinucleotide. When viewed in a context with the origin and properties of proteinoid microspheres, these results model the origin from a protocell of a more contemporary type of cell able to synthesize its own polyamino acids and polynucleotides.1435 (emphasis added)
[Life is] an obligatory result
of the general growth
of the universe.
We’ve seen that scientists have discovered a relatively smooth chain of synthesis from the stuff of stars to the stuff of life. On the basis of pre biotic experiments performed to date, it is probable that most of the organic molecules of life with a molecular weight less than 1000 spontaneously appeared in significant quantities during the early years of our world. While a number of problems remain, most indications are that the origin and development of life on Earth had a certain inevitability about it.
From the simplest compounds present when our planet first congealed about 4.6 eons ago, to the first viable protobiont some half a billion years later, the patterns of development and the upward march of complexity seem unavoidable. Only the most general conditions must be needed for carbon-based life to arise: A body of water, a primitive reducing atmosphere, some source of energy, and lots of time. Life, in Soviet Academician Oparin’s own words, is "an obligatory result of the general growth of the universe."2297
Even now we humans just begin to suspect the truth: The universe is not ours alone to keep.
There are countless side issues that cry out to be discussed at this point, but which unfortunately can be given only a passing nod. First of all, there is the absolutely fascinating question of the genetic code. As is well-known, genetic information is written on the DNA strand in short, three-nucleotide "words" called codons. By properly reading these encoded blueprints, a cell can construct exactly the right protein molecules.
|The series of adenine nucleotides in a codon|
For instance, a series of three adenine nucleotides in a codon tells the cellular machinery to use one molecule of an amino acid called lysine at that location. Three guanines in a row means that a molecule of the amino acid glycine should be used. One by one, the codons tell which amino acid to use and in what order, and proteins are built up in precisely the right way.
What is the origin of this marvelous code?1064,1444,2383 Is a three-nucleotide codon somehow optimal,1777 or would four have been more evolutionarily efficient?1064,1066 Why not the simplicity of only two? And what determined the rules of the coding itself? Three guanines mean glycine to a virus, a dandelion, or a human. Is the code somehow efficiency-maximized or error-minimized?1065 (It appears to be!1066,2378)
|Origin of chromosomes / purpose of genes / methods for storage of genetic information<|
What is the origin of chromosomes,2301 and the true purpose of genes?2322 These are important questions for xenobiologists to be asking, because the universality of our genetic mechanisms will determine the limits of variation that can be expected in alien biochemistries.
For xenologists, of course, there are far more fundamental issues that must be raised. For example, why must genetic information be stored digitally in a linear sequence of monomer units? Could not some form of analog system serve? What of the possibility of genetic systems whose information was stored, replicated and transcribed in a planar fashion rather than linearly?1777
… on Earth, DNA is used for "replication" and
proteins are used solely for "expression" or "action."
Dr. Francis Crick has pointed out that on Earth, DNA is used for "replication" and proteins are used solely for "expression" or "action." Is it possible, he asks, "to devise a system in which one molecule does both jobs, or are there perhaps strong arguments, from systems analysis, which might suggest that to divide the job into two gives a great advantage?"22 Others have echoed this idea.521
|Necessity for genotype distinct from a phenotype?|
Similarly, Michael Arbib of the University of Massachusetts at Amherst questions "whether it is necessary for any lifeform to have a genotype distinct from a phenotype; in other words, whether we have to have a program to direct growth and change, or whether in fact the organism might be able to reproduce itself as a whole."85 Crick seems to agree,, suggesting that it might be possible to "design a system which was based on the inheritance of acquired characteristics."22 (At least one science fiction story has been written along these lines.2216) Arbib also wonders: "One might imagine some planet whose beings reproduce by xerography with no gene required!"85 The possibility of inheritance without genes has been suggested before,1178 although in a different context. (A general review of replication was published in 2004 by Freitas amd Merkle.)
|Nucleic acid chauvinism|
And we must take care not to be guilty of "nucleic acid chauvinism." We are familiar with only one molecular replicating system, but there is no reason why others should not be possible. Gordon Allen writes: "Life on other planets need not be based on nucleic acids or proteins if their catalytic functions can be otherwise provided."1591
Dr. Alexander Rich at MIT also suspects that the functions of Earthly nucleic acid are not unique. Rich believes that "other molecules could be used to form other polymers which could be used as information carriers for living systems." Later, he elaborates:
I think it would be amusing to make a chemical system of complementary polymers based on monomers that are not nucleic acid derivatives, simply to demonstrate that it can be done. In about ten years’ time, I think we will have a well-developed field of synthetic polymeric information carriers that will give us a great deal of insight into our own terrestrial system. That another system is possible might have relevance, if not to biology on this planet then perhaps to another.1587,1632
Clearly, a search should be made for non-nucleic acid self-replicating molecules. Exotic systems based on silicon, boron, or nitrogen-phosphorus chemistries are possible: Specialists in these fields expect an abundance of compounds comparable to that of carbon chemistry.1777 But we must not anticipate the subject matter of the next chapter.