Abiogenesis
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Abiogenesis (/ˌeɪbaɪ.ɵˈdʒɛnɨsɪs/ AY-by-oh-JEN-ə-siss[1]) or biopoiesis[2] is the natural process of life arising from non-living matter such as simple organic compounds.[3][4][5][6]
The Earth was formed about 4.54 billion years ago. The earliest life on Earth existed at least 3.5 billion years ago,[7][8][9] during the Eoarchean Era when sufficient crust had solidified following the molten Hadean Eon. The earliest physical evidence for life on Earth is biogenic graphite in 3.7 billion-year-old metasedimentary rocks discovered in Western Greenland[10] and microbial mat fossils found in 3.48 billion-year-old sandstone discovered in Western Australia.[11][12] Nevertheless, several studies suggest that life on Earth may have started even earlier,[13] as early as 4.25 billion years ago according to one study,[14] and 4.4 billion years ago according to another study.[15] Earth is the only place in the universe known to harbor life.[16][17] Nonetheless, the exact steps in the abiogenesis process, whether occurring on Earth or elsewhere, remain unknown.
Scientific hypotheses about the origins of life can be divided into three main stages: the geophysical, the chemical and the biological.[18] Many approaches investigate how self-replicating molecules or their components came into existence. On the assumption that life originated spontaneously on Earth, the Miller–Urey experiment and similar experiments demonstrated that most amino acids, basic chemicals of life, can be racemically synthesized in conditions which were intended to be similar to those of the early Earth. Several mechanisms have been investigated, including lightning and radiation. Other approaches ("metabolism first" hypotheses) focus on understanding how catalysis in chemical systems in the early Earth might have provided the precursor molecules necessary for self-replication.[19][20]
Early geophysical conditions
Based on recent computer model studies, the complex organic molecules necessary for life may have formed in the protoplanetary disk of dust grains surrounding the Sun before the formation of the Earth.[21] According to the computer studies, this same process may also occur around other stars that acquire planets.[21]The Hadean Earth is thought to have had a secondary atmosphere, formed through degassing of the rocks that accumulated from planetesimal impactors. At first, it was thought that the Earth's atmosphere consisted of hydrides—methane, ammonia and water vapour—and that life began under such reducing conditions, which are conducive to the formation of organic molecules. During its formation, the Earth lost a significant part of its initial mass, with a nucleus of the heavier rocky elements of the protoplanetary disk remaining.[22] However, based on today's volcanic evidence, it is now thought that the early atmosphere would have probably contained 60% hydrogen, 20% oxygen (mostly in the form of water vapour), 10% carbon dioxide, 5 to 7% hydrogen sulfide, and smaller amounts of nitrogen, carbon monoxide, free hydrogen, methane and inert gases. As Earth lacked the gravity to hold any molecular hydrogen, this component of the atmosphere would have been rapidly lost during the Hadean period, along with the bulk of the original inert gases. Solution of carbon dioxide in water is thought to have made the seas slightly acidic, with a pH of about 5.5.[23] The atmosphere at the time has been characterized as a "gigantic, productive outdoor chemical laboratory."[24] It is similar to the mixture of gases released by volcanoes, which still support some abiotic chemistry today.[24]
Oceans may have appeared first in the Hadean eon, as soon as two hundred million years (200 Ma) after the Earth was formed, in a hot 100 °C (212 °F) reducing environment, and the pH of about 5.8 rose rapidly towards neutral.[25] This has been supported by the dating of 4.404 Ga-old zircon crystals from metamorphosed quartzite of Mount Narryer in Western Australia, which are evidence that oceans and continental crust existed within 150 Ma of Earth's formation.[26] Despite the likely increased vulcanism and existence of many smaller tectonic "platelets", it has been suggested that between 4.4 and 4.3 Ga, the Earth was a water world, with little if any continental crust, an extremely turbulent atmosphere and a hydrosphere subject to high UV, from a T Tauri sun, cosmic radiation and continued bolide impact.[27]
The Hadean environment would have been highly hazardous to modern life. Frequent collisions with large objects, up to 500 kilometres (310 mi) in diameter, would have been sufficient to sterilise the planet and vaporise the ocean within a few months of impact, with hot steam mixed with rock vapour becoming high altitude clouds that would completely cover the planet. After a few months, the height of these clouds would have begun to decrease but the cloud base would still have been elevated for about the next thousand years. After that, it would have begun to rain at low altitude. For another two thousand years, rains would slowly have drawn down the height of the clouds, returning the oceans to their original depth only 3,000 years after the impact event.[28]
The earliest biological evidence for life on Earth
The earliest life on Earth existed at least 3.5 billion years ago,[7][8][9] during the Eoarchean Era when sufficient crust had solidified following the molten Hadean Eon. The earliest physical evidence for life on Earth is biogenic graphite in 3.7 billion-year-old metasedimentary rocks discovered in Western Greenland[10] and microbial mat fossils found in 3.48 billion-year-old sandstone discovered in Western Australia.[11][12] Gustaf Arrhenius of the Scripps Institute of Oceanography using a mass spectrometer has identified what appears to be, on the basis of biogenic carbon isotopes, evidence of early life, found in rocks from Akilia Island, near Isua, Greenland, dating to 3.85 billion years old.[29]Between 3.8 and 4.1 Ga, changes in the orbits of the gaseous giant planets may have caused a late heavy bombardment[30] that pockmarked the Moon and the other inner planets (Mercury, Mars, and presumably Earth and Venus). This would likely have repeatedly sterilized the planet, had life appeared before that time.[24] Geologically, the Hadean Earth would have been far more active than at any other time in its history. Studies of meteorites suggests that radioactive isotopes such as aluminium-26 with a half-life of 7.17×105 years, and potassium-40 with a half-life of 1.250×109 years, isotopes mainly produced in supernovae, were much more common.[31] Coupled with internal heating as a result of gravitational sorting between the core and the mantle there would have been a great deal of mantle convection, with the probable result of many more smaller and very active tectonic plates than in modern times.
By examining the time interval between such devastating environmental events, the time interval when life might first have come into existence can be found for different early environments. A study by Maher and Stevenson shows that if the deep marine hydrothermal setting provides a suitable site for the origin of life, abiogenesis could have happened as early as 4.0 to 4.2 Ga, whereas if it occurred at the surface of the Earth, abiogenesis could only have occurred between 3.7 and 4.0 Ga.[32]
Further evidence of the early appearance of life comes from the Isua supercrustal belt in Western Greenland and from similar formations in the nearby Akilia Island. Isotopic fingerprints typical of life, preserved in the sediments, have been used to suggest that life existed on the planet already by 3.85 billion years ago.[33] Christian de Duve argues that the determination of chemistry means that "life has to emerge quickly ... Chemical reactions happen quickly or not at all; if any reaction takes a millennium to complete then the chances are all the reagents will simply dissipate or break down in the meantime, unless they are replenished by other faster reactions."[34][35]
Conceptual history
John Desmond Bernal has identified a number of "outstanding difficulties in accounts of the origin of life". Earlier theories, he suggests, such as spontaneous generation were based upon an explanation that life was continuously created as a result of chance events.[36]Spontaneous generation
Belief in the present ongoing spontaneous generation of certain forms of life from non-living matter goes back to Aristotle and ancient Greek philosophy and continued to have support in Western scholarship until the 19th century. This belief was paired with a belief in heterogenesis, i.e., that one form of life derived from a different form (e.g. bees from flowers).[37] Classical notions of spontaneous generation, which can be considered under the modern term abiogenesis, held that certain complex, living organisms are generated by decaying organic substances. According to Aristotle, it was a readily observable truth that aphids arise from the dew which falls on plants, flies from putrid matter, mice from dirty hay, crocodiles from rotting logs at the bottom of bodies of water, and so on.[38] In the 17th century, such assumptions started to be questioned. In 1646, Sir Thomas Browne published his Pseudodoxia Epidemica (subtitled Enquiries into Very many Received Tenets, and Commonly Presumed Truths), which was an attack on false beliefs and "vulgar errors." His contemporary, Alexander Ross erroneously refuted him, stating: "To question this (i.e., spontaneous generation) is to question reason, sense and experience. If he doubts of this let him go to Egypt, and there he will find the fields swarming with mice, begot of the mud of Nylus, to the great calamity of the inhabitants."[39]In 1665, Robert Hooke published the first drawings of a microorganism. Hooke was followed in 1676 by Anton van Leeuwenhoek, who drew and described microorganisms that are now thought to have been protozoa and bacteria.[40] Many felt the existence of microorganisms was evidence in support of spontaneous generation, since microorganisms seemed too simplistic for sexual reproduction, and asexual reproduction through cell division had not yet been observed. Van Leeuwenhoek took issue with the ideas common at the time that fleas and lice could spontaneously result from putrefaction, and that frogs could likewise arise from slime. Using a broad range of experiments ranging from sealed and open meat incubation and the close study of insect reproduction, by the 1680s he became convinced that spontaneous generation was incorrect.[41]
The first experimental evidence against spontaneous generation came in 1668 when Francesco Redi showed that no maggots appeared in meat when flies were prevented from laying eggs. It was gradually shown that, at least in the case of all the higher and readily visible organisms, the previous sentiment regarding spontaneous generation was false. The alternative seemed to be biogenesis: that every living thing came from a pre-existing living thing (omne vivum ex ovo, Latin for "every living thing from an egg").
In 1768, Lazzaro Spallanzani demonstrated that microbes were present in the air, and could be killed by boiling. In 1861, Louis Pasteur performed a series of experiments that demonstrated that organisms such as bacteria and fungi do not spontaneously appear in sterile, nutrient-rich media, but only invade them from outside.
The origin of the terms biogenesis and abiogenesis
The term biogenesis is usually credited to either Henry Bastian or to Thomas Henry Huxley.[42] Bastian used the term (around 1869) in an unpublished exchange with John Tyndall to mean life-origination or commencement. In 1870, Huxley, as new president of the British Association for the Advancement of Science, delivered an address entitled Biogenesis and Abiogenesis.[43] In it he introduced the term biogenesis (with an opposite meaning to Bastian) and also introduced the term abiogenesis:- And thus the hypothesis that living matter always arises by the agency of pre-existing living matter, took definite shape; and had, henceforward, a right to be considered and a claim to be refuted, in each particular case, before the production of living matter in any other way could be admitted by careful reasoners. It will be necessary for me to refer to this hypothesis so frequently, that, to save circumlocution, I shall call it the hypothesis of Biogenesis; and I shall term the contrary doctrine–that living matter may be produced by not living matter–the hypothesis of Abiogenesis.[43]
- A word of explanation seems necessary with regard to the introduction of the new term archebiosis. I had originally, in unpublished writings, adopted the word biogenesis to express the same meaning—viz, life-origination or commencement.
- But in the mean time the word biogenesis has been made use of, quite independently, by a distinguished biologist [Huxley], who wished to make it bear a totally different meaning. He also introduced the term abiogenesis. I have been informed, however, on the best authority, that neither of these words can—with any regard to the language from which they are derived—be supposed to bear the meanings which have of late been publicly assigned to them. Wishing to avoid all needless confusion, I therefore renounced the use of the word biogenesis, and being, for the reason just given, unable to adopt the other term, I was compelled to introduce a new word, in order to designate the process by which living matter is supposed to come into being, independently of pre-existing living matter.[44]
Alternatives to chance: biogenesis
The belief that spontaneous self-ordering of spontaneous generation is impossible led to an alternative. By the middle of the 19th century, the theory of biogenesis had accumulated so much evidential support, due to the work of Louis Pasteur and others, that the alternative theory of spontaneous generation had been effectively disproven.Pasteur and Darwin
Pasteur himself remarked, after a definitive finding in 1864, "Never will the doctrine of spontaneous generation recover from the mortal blow struck by this simple experiment."[45][46] One alternative was that life's origins on Earth had come from somewhere else in the Universe. Periodically resurrected (see Panspermia, above) Bernal demonstrates that this approach "is equivalent in the last resort to asserting the operation of metaphysical, spiritual entities... it turns on the argument of creation by design by a creator or demiurge".[47] Such a theory, Bernal demonstrated was unscientific and a number of scientists defined life as a result of an inner "life force", which in the late 19th century was championed by Henri Bergson.
The concept of evolution proposed by Charles Darwin put an end to these metaphysical theologies. In a letter to Joseph Dalton Hooker on 1 February 1871,[48] Charles Darwin addressed the question, suggesting that the original spark of life may have begun in a "warm little pond, with all sorts of ammonia and phosphoric salts, lights, heat, electricity, etc. present, so that a protein compound was chemically formed ready to undergo still more complex changes". He went on to explain that "at the present day such matter would be instantly devoured or absorbed, which would not have been the case before living creatures were formed."[49] In other words, the presence of life itself makes the search for the spontaneous origin of life dependent on the artificial production of organic compounds in the sterile conditions of the laboratory.
"Primordial soup" hypothesis
No new notable research or theory on the subject appeared until 1924, when Alexander Oparin reasoned that atmospheric oxygen prevents the synthesis of certain organic compounds that are necessary building blocks for the evolution of life. In his book The Origin of Life,[50][51] Oparin proposed that the "spontaneous generation of life" that had been attacked by Louis Pasteur did in fact occur once, but was now impossible because the conditions found on the early Earth had changed, and preexisting organisms would immediately consume any spontaneously generated organism. Oparin argued that a "primeval soup" of organic molecules could be created in an oxygenless atmosphere through the action of sunlight. These would combine in ever more complex ways until they formed coacervate droplets. These droplets would "grow" by fusion with other droplets, and "reproduce" through fission into daughter droplets, and so have a primitive metabolism in which those factors which promote "cell integrity" survive, and those that do not become extinct. Many modern theories of the origin of life still take Oparin's ideas as a starting point.
Robert Shapiro has summarized the "primordial soup" theory of Oparin and Haldane in its "mature form" as follows:[52]
- The early Earth had a chemically reducing atmosphere.
- This atmosphere, exposed to energy in various forms, produced simple organic compounds ("monomers").
- These compounds accumulated in a "soup", which may have been concentrated at various locations (shorelines, oceanic vents etc.).
- By further transformation, more complex organic polymers – and ultimately life – developed in the soup.
One of the most important pieces of experimental support for the "soup" theory came in 1952. A graduate student, Stanley Miller, and his professor, Harold Urey, performed an experiment that demonstrated how organic molecules could have spontaneously formed from inorganic precursors, under conditions like those posited by the Oparin-Haldane Hypothesis. The now-famous "Miller–Urey experiment" used a highly reduced mixture of gases—methane, ammonia and hydrogen—to form basic organic monomers, such as amino acids.[55] This provided direct experimental support for the second point of the "soup" theory, and it is around the remaining two points of the theory that much of the debate now centers. In the Miller–Urey experiment, a mixture of water, hydrogen, methane, and ammonia was cycled through an apparatus that delivered electrical sparks to the mixture. After one week, it was found that about 10% to 15% of the carbon in the system was now in the form of a racemic mixture of organic compounds, including amino acids, which are the building blocks of proteins.
Bernal shows that based upon this and subsequent work there is no difficulty in principle in forming most of the molecules which we recognise as the basic molecules of life from their inorganic precursors. The underlying hypothesis held by Oparin, Haldane, Bernal, Miller and Urey, for instance, was that multiple conditions on the primeval Earth favored chemical reactions that synthesized the same set of complex organic compounds from such simple precursors. A 2011 reanalysis of the saved vials containing the original extracts that resulted from the Miller and Urey experiments, using current and more advanced analytical equipment and technology, has uncovered more biochemicals than originally discovered in the 1950s. One of the more important findings was 23 amino acids, far more than the five originally found.[56] However Bernal rightly shows that "it is not enough to explain the formation of such molecules, what is necessary" he says "..is a physical-chemical explanation of the origins of these molecules that suggests the presence of suitable sources and sinks for free energy".[57]
Proteinoid microspheres
In trying to uncover the intermediate stages of abiogenesis mentioned by Bernal, Sidney W. Fox in the 1950s and 1960s, studied the spontaneous formation of peptide structures under conditions that might plausibly have existed early in Earth's history. He demonstrated that amino acids could spontaneously form small chains called peptides. In one of his experiments, he allowed amino acids to dry out as if puddled in a warm, dry spot in prebiotic conditions. He found that, as they dried, the amino acids formed long, often cross-linked, thread-like, submicroscopic polypeptide molecules now named "proteinoid microspheres".[58]In another experiment using a similar method to set suitable conditions for life to form, Fox collected volcanic material from a cinder cone in Hawaii. He discovered that the temperature was over 100 °C (212 °F) just 4 inches (100 mm) beneath the surface of the cinder cone, and suggested that this might have been the environment in which life was created—molecules could have formed and then been washed through the loose volcanic ash and into the sea. He placed lumps of lava over amino acids derived from methane, ammonia and water, sterilized all materials, and baked the lava over the amino acids for a few hours in a glass oven. A brown, sticky substance formed over the surface and when the lava was drenched in sterilized water a thick, brown liquid leached out. It turned out that the amino acids had combined to form proteinoids, and the proteinoids had combined to form small globules that Fox called "microspheres". His proteinoids were not cells, although they formed clumps and chains reminiscent of cyanobacteria, but they contained no functional nucleic acids or any encoded information. Based upon such experiments, Colin S. Pittendrigh stated in December 1967 that "laboratories will be creating a living cell within ten years," a remark that reflected the typical contemporary levels of innocence of the complexity of cell structures.[59]
More recent theories
Bernal in 1967 identified three different sorts of difficulties in the abiogenetic origins of life[60]* Stage 1: he saw as the origins of organic molecules, and this is now fairly well understood. The necessity of a source and sink of energy, and the necessity of a fluid medium has been much studied (see above).
* Stage 2: he saw as the necessity to explain how organic monomers became ordered into biologically active polymers. Once again there is the necessity of sources and sinks for this process. The discovery of alkaline vents and the similarity with the "proton pump" found as the basis of biological life has begun to provide evidence for this. The second problem foreseen by Bernal was the origin of replication. The work with the RNA world is specifically intended to find answers to this problem.
* Stage 3: he saw was the most difficult. This was the discovery of methods by which biological reactions were incorporated behind cell walls. Modern work on the self organising capacities by which cell membranes self-assemble, and the work on micropores in various substrates as a half-way house towards the development of independent free-living cells is ongoing research designed to answer this problem.[61][62]
Current models
There is still no "standard model" of the origin of life. Most currently accepted models draw at least some elements from the framework laid out by Alexander Oparin (in 1924) and John Haldane (in 1925), who postulated the molecular or chemical evolution theory of life.[63] According to them, the first molecules constituting the earliest cells "were synthesized under natural conditions by a slow process of molecular evolution, and these molecules then organized into the first molecular system with properties with biological order."[63] Oparin and Haldane suggested that the atmosphere of the early Earth may have been chemically reducing in nature, composed primarily of methane (CH4), ammonia (NH3), water (H2O), hydrogen sulfide (H2S), carbon dioxide (CO2) or carbon monoxide (CO), and phosphate (PO43-), with molecular oxygen (O2) and ozone (O3) either rare or absent, however, the current scientific model is an atmosphere that contained 60% hydrogen, 20% oxygen (mostly in the form of water vapor), 10% carbon dioxide, 5 to 7% hydrogen sulfide, and smaller amounts of nitrogen, carbon monoxide, free hydrogen, methane and inert gases.[64][65] In the atmosphere proposed by Oparin and Haldane, electrical activity can catalyze the creation of certain basic small molecules (monomers) of life, such as amino acids. This was demonstrated in the Miller–Urey experiment by Stanley L. Miller and Harold C. Urey reported in 1953.John Desmond Bernal coined the term biopoiesis in 1949 to refer to the origin of life,[66] and suggested that it occurred in three "stages": 1) the origin of biological monomers; 2) the origin of biological polymers; and 3) the evolution from molecules to cells. He suggested that evolution commenced between stage 1 and 2.[67]
The chemical processes that took place on the early Earth are called chemical evolution. Both Manfred Eigen and Sol Spiegelman demonstrated that evolution, including replication, variation, and natural selection, can occur in populations of molecules as well as in organisms.[24] Spiegelman took advantage of natural selection to synthesize Spiegelman's Monster, which had a genome with just 218 bases. Eigen built on Spiegelman's work and produced a similar system with just 48 or 54 nucleotides.[68]
Chemical evolution was followed by the initiation of biological evolution, which led to the first cells.[24] No one has yet synthesized a "protocell" using basic components which would have the necessary properties of life (the so-called "bottom-up-approach"). Without such a proof-of-principle, explanations have tended to be focused on chemosynthesis of polymers. However, some researchers are working in this field, notably Steen Rasmussen and Jack Szostak. Others have argued that a "top-down approach" is more feasible. One such approach, successfully attempted by Craig Venter and others at The Institute for Genomic Research, involves engineering existing prokaryotic cells with progressively fewer genes, attempting to discern at which point the most minimal requirements for life were reached.[69][70]
Chemical origin of organic molecules
The elements, except for hydrogen, ultimately derive from stellar nucleosynthesis. Complex molecules, including organic molecules, form naturally both in space and on planets.[71] There are two possible sources of organic molecules on the early Earth:- Terrestrial origins – organic synthesis driven by impact shocks or by other energy sources (such as ultraviolet light, redox coupling, or electrical discharges) (e.g. Miller's experiments)
- Extraterrestrial origins – formation of organic molecules in interstellar dust clouds and rained down on planets.[72][73][74] (See pseudo-panspermia)
It has been estimated that the Late Heavy Bombardment may also have effectively sterilised the Earth's surface to a depth of tens of metres. If life evolved deeper than this, it would have also been shielded from the early high levels of ultraviolet radiation from the T Tauri stage of the sun's evolution. Simulations of geothermically heated oceanic crust yield far more organics than those found in the Miller-Urey experiments (see below). In the deep hydrothermal vents, Everett Shock has found "there is an enormous thermodynamic drive to form organic compounds, as seawater and hydrothermal fluids, which are far from equilibrium, mix and move towards a more stable state".[77] Shock has found that the available energy is maximised at around 100 – 150 degrees Celsius, precisely the temperatures at which the hyperthermophilic bacteria and archaea have been found, at the base of the tree of life closest to the Last Universal Common Ancestor.[78]
Chemical synthesis
While features of self-organization and self-replication are often considered the hallmark of living systems, there are many instances of abiotic molecules exhibiting such characteristics under proper conditions. Palasek showed that self-assembly of RNA molecules can occur spontaneously due to physical factors in hydrothermal vents.[79] Virus self-assembly within host cells has implications for the study of the origin of life,[80] as it lends further credence to the hypothesis that life could have started as self-assembling organic molecules.[81][82]Multiple sources of energy were available for chemical reactions on the early Earth. For example, heat (such as from geothermal processes) is a standard energy source for chemistry. Other examples include sunlight and electrical discharges (lightning), among others.[24] Unfavorable reactions can also be driven by highly favorable ones, as in the case of iron-sulfur chemistry. For example, this was probably important for carbon fixation (the conversion of carbon from its inorganic form to an organic one).[note 1] Carbon fixation via iron-sulfur chemistry is highly favorable, and occurs at neutral pH and 100 °C (212 °F). Iron-sulfur surfaces, which are abundant near hydrothermal vents, are also capable of producing small amounts of amino acids and other biological metabolites.[24]
Formamide produces all four ribonucleotides and other biological molecules when warmed in the presence of various terrestrial minerals. Formamide is ubiquitous in the universe, produced by the reaction of water and HCN (hydrogen cyanide). It has several advantages as a prebiotic precursor, including the ability to easily become concentrated through the evaporation of water.[83][84] Although HCN is poisonous, it only affects aerobic organisms (eukaryotes and aerobic bacteria). It can play roles in other chemical processes as well, such as the synthesis of the amino acid glycine.[24]
In 1961, it was shown that the nucleic acid purine base adenine can be formed by heating aqueous ammonium cyanide solutions.[85] Other pathways for synthesizing bases from inorganic materials were also reported.[86] Leslie Orgel and colleagues have shown that freezing temperatures are advantageous for the synthesis of purines, due to the concentrating effect for key precursors such as hydrogen cyanide.[87] Research by Stanley Miller and colleagues suggested that while adenine and guanine require freezing conditions for synthesis, cytosine and uracil may require boiling temperatures.[88] Research by the Miller group notes the formation of seven different amino acids and 11 types of nucleobases in ice when ammonia and cyanide were left in a freezer from 1972 to 1997.[89][90] Other work demonstrated the formation of s-triazines (alternative nucleobases), pyrimidines (including cytosine and uracil), and adenine from urea solutions subjected to freeze-thaw cycles under a reductive atmosphere (with spark discharges as an energy source).[91] The explanation given for the unusual speed of these reactions at such a low temperature is eutectic freezing. As an ice crystal forms, it stays pure: only molecules of water join the growing crystal, while impurities like salt or cyanide are excluded. These impurities become crowded in microscopic pockets of liquid within the ice, and this crowding causes the molecules to collide more often.
At the time of the Miller–Urey experiment, scientific consensus was that the early Earth had a reducing atmosphere with compounds relatively rich in hydrogen and poor in oxygen (e.g., CH
4 and NH
3 as opposed to CO
2 and NO
2). However, current scientific consensus describes the primitive atmosphere as either weakly reducing or neutral[92][93]. Such an atmosphere would diminish both the amount and variety of amino acids that could be produced, although studies that include iron and carbonate minerals (thought to be present in early oceans) in the experimental conditions have again produced a diverse array of amino acids.[92] Other scientific research has focused on two other potential reducing environments: outer space and deep-sea thermal vents.[94][95][96]
The spontaneous formation of complex polymers from abiotically generated monomers under the conditions posited by the "soup" theory is not at all a straightforward process. Besides the necessary basic organic monomers, compounds that would have prohibited the formation of polymers were formed in high concentration during the Miller–Urey and Oró experiments.[97] The Miller–Urey experiment, for example, produces many substances that would react with the amino acids or terminate their coupling into peptide chains.[98]
Autocatalysis
Autocatalysts are substances that catalyze the production of themselves, and therefore are simple molecular replicators. The simplest self-replicating chemical systems are autocatalytic, and typically contain three components: two precursors that join together to form a product molecule, and the product molecule itself. The product molecule catalyzes the reaction by providing a complementary template which binds to the precursors, thus bringing them together. Such systems have been demonstrated both in biological macromolecules and in small organic molecules.[99][100] Systems that do not proceed by template mechanisms, such as the self-reproduction of micelles and vesicles, have also been observed.[100]In 1993, Stuart Kauffman proposed that life initially arose as autocatalytic chemical networks.[101] British ethologist Richard Dawkins wrote about autocatalysis as a potential explanation for the origin of life in his 2004 book The Ancestor's Tale.[102] In his book, Dawkins cites experiments performed by Julius Rebek and his colleagues at the Scripps Research Institute in California in which they combined amino adenosine and pentafluorophenyl esters with the autocatalyst amino adenosine triacid ester (AATE). One system from the experiment contained variants of AATE which catalysed the synthesis of themselves. This experiment demonstrated the possibility that autocatalysts could exhibit competition within a population of entities with heredity, which could be interpreted as a rudimentary form of natural selection.[citation needed]
In the early 1970s, Manfred Eigen and Peter Schuster examined the transient stages between the molecular chaos and a self-replicating hypercycle in a prebiotic soup.[103] In a hypercycle, the information storing system (possibly RNA) produces an enzyme, which catalyzes the formation of another information system, in sequence until the product of the last aids in the formation of the first information system. Mathematically treated, hypercycles could create quasispecies, which through natural selection entered into a form of Darwinian evolution. A boost to hypercycle theory was the discovery that RNA, in certain circumstances, forms itself into ribozymes, capable of catalyzing their own chemical reactions.[104] The hypercycle theory requires the existence of complex biochemicals such as nucleotides which are not formed under the conditions proposed by the Miller–Urey experiment.
Geoffrey W. Hoffmann, a student of Eigen, contributed to the concept of life involving both replication and metabolism emerging from catalytic noise. His contributions included showing that an early sloppy translation machinery can be stable against an error catastrophe of the type that had been envisaged as problematical by Leslie Orgel ("Orgel's paradox")[105][106] and calculations regarding the occurrence of a set of required catalytic activities together with the exclusion of catalytic activities that would be disruptive.[107]
Homochirality
Homochirality refers to the geometric property of some materials that are composed of chiral units. Chiral refers to nonsuperimposable 3D forms that are mirror images of one another, as are left and right hands. Living organisms use molecules that have the same chirality ("handedness"): with some exceptions, amino acids are left-handed while nucleotides and sugars are right-handed. Chiral molecules can be synthesized, but in the absence of a chiral source or a chiral catalyst, they are formed in a 50/50 mixture of both enantiomers. This is called a racemic mixture. Known mechanisms for the production of non-racemic mixtures from racemic starting materials include: asymmetric physical laws, such as the electroweak interaction; asymmetric environments, such as those caused by circularly polarized light, quartz crystals, or the Earth's rotation; and statistical fluctuations during racemic synthesis.[108]Once established, chirality would be selected for.[109] A small enantiomeric excess can be amplified into a large one by asymmetric autocatalysis, such as in the Soai reaction.[110] In asymmetric autocatalysis, the catalyst is a chiral molecule, which means that a chiral molecule is catalysing its own production. An initial enantiomeric excess, such as can be produced by polarized light, then allows the more abundant enantiomer to outcompete the other.[111]
Clark has suggested that homochirality may have started in outer space, as the studies of the amino acids on the Murchison meteorite showed L-alanine to be more than twice as frequent as its D form, and L-glutamic acid was more than three times prevalent than its D counterpart. Various chiral crystal surfaces can also act as sites for possible concentration and assembly of chiral monomer units into macromolecules.[112] Compounds found on meteorites suggest that the chirality of life derives from abiogenic synthesis, since amino acids from meteorites show a left-handed bias, whereas sugars show a predominantly right-handed bias, the same as found in living organisms.[113]
Reproduction, Duplication and the RNA world
The RNA world hypothesis describes an early Earth with self-replicating and catalytic RNA but no DNA or proteins. It is generally accepted that current life on Earth descends from an RNA world,[115] although RNA-based life may not have been the first life to exist.[116][117] This conclusion is drawn from many independent lines of evidence, such as the observations that RNA is central to the translation process and that small RNAs can catalyze all of the chemical groups and information transfers required for life.[117][118] The structure of the ribosome has been called the "smoking gun," as it showed that the ribosome is a ribozyme, with a central core of RNA and no amino acid side chains within 18 angstroms of the active site where peptide bond formation is catalyzed.[116] The concept of the RNA world was first proposed in the 1960s by Francis Crick, Leslie Orgel, and Carl Woese, and the term was coined by Walter Gilbert in 1986.[117][119]
Possible precursors for the evolution of protein synthesis include a mechanism to synthesize short peptide cofactors or from a mechanism for the duplication of RNA. It is likely that the ancestral ribosome was composed entirely of RNA, although some roles have since been taken over by proteins. Major remaining questions on this topic include identifying the selective force for the evolution of the ribosome and determining how the genetic code arose.[120]
Eugene Koonin said, "Despite considerable experimental and theoretical effort, no compelling scenarios currently exist for the origin of replication and translation, the key processes that together comprise the core of biological systems and the apparent pre-requisite of biological evolution. The RNA World concept might offer the best chance for the resolution of this conundrum but so far cannot adequately account for the emergence of an efficient RNA replicase or the translation system. The MWO (Ed.: "many worlds in one"[121]) version of the cosmological model of eternal inflation could suggest a way out of this conundrum because, in an infinite multiverse with a finite number of distinct macroscopic histories (each repeated an infinite number of times), emergence of even highly complex systems by chance is not just possible but inevitable."[121]
RNA synthesis and replication
The RNA world has spurred scientists to try to determine if RNA molecules could have spontaneously formed that were capable of catalyzing their own replication.[122][123][124] Evidences suggest chemical conditions (including the presence of boron, molybdenum and oxygen) for initially producing RNA molecules may have been better on the planet Mars than those on the planet Earth.[122][123] If so, life-suitable molecules, originating on Mars, may have later migrated to Earth via meteor ejections.[122][123]A number of hypotheses of modes of formation have been put forward. As of 1994, there were difficulties in the abiotic synthesis of the nucleotides cytosine and uracil.[125] Subsequent research has shown possible routes of synthesis; for example, formamide produces all four ribonucleotides and other biological molecules when warmed in the presence of various terrestrial minerals.[83][84] Early cell membranes could have formed spontaneously from proteinoids, which are protein-like molecules produced when amino acid solutions are heated while in the correct concentration in aqueous solution. These are seen to form micro-spheres which are observed to behave similarly to membrane-enclosed compartments. Other possibilities include systems of chemical reactions that take place within clay substrates or on the surface of pyrite rocks.
Factors supportive of an important role for RNA in early life include its ability to act both to store information and to catalyze chemical reactions (as a ribozyme); its many important roles as an intermediate in the expression and maintenance of the genetic information (in the form of DNA) in modern organisms; and the ease of chemical synthesis of at least the components of the molecule under the conditions that approximated the early Earth. Relatively short RNA molecules have been artificially produced in labs, which are capable of replication.[126] Such replicase RNA, which functions as both code and catalyst provides its own template upon which copying can occur. Jack Szostak has shown that certain catalytic RNAs can, indeed, join smaller RNA sequences together, creating the potential, in the right conditions for self-replication. If these conditions were present, Darwinian selection would favour the proliferation of such self-catalysing structures, to which further functionalities could be added.[127] Lincoln and Joyce have identified RNA systems capable of self-sustained replication.[128] The systems, which include two ribozymes that catalyze each other's synthesis, replicated with doubling time of about one hour, and were subject to natural selection.[129] In evolutionary competition experiments, this led to the emergence of new systems which replicated more efficiently.[116] This was the first demonstration of evolutionary adaptation occurring in a molecular genetic system.[129]
Life can be considered to have emerged when RNA chains began to express the basic conditions necessary for natural selection to operate as conceived by Darwin: heritability, variation of type, and competition for limited resources. Fitness of an RNA replicator (its per capita rate of increase) would likely be a function of adaptive capacities that were intrinsic (in the sense that they were determined by the nucleotide sequence) and the availability of resources.[130][131] The three primary adaptive capacities may have been (1) the capacity to replicate with moderate fidelity (giving rise to both heritability and variation of type), (2) the capacity to avoid decay, and (3) the capacity to acquire and process resources.[130][131] These capacities would have been determined initially by the folded configurations of the RNA replicators that, in turn, would be encoded in their individual nucleotide sequences. Competitive success among different replicators would have depended on the relative values of these adaptive capacities.
Pre-RNA world
It is possible that a different type of nucleic acid, such as PNA, TNA or GNA, was the first one to emerge as a self-reproducing molecule, to be replaced by RNA only later.[132][133] Larralde et al., say that "the generally accepted prebiotic synthesis of ribose, the formose reaction, yields numerous sugars without any selectivity."[134] and they conclude that their "results suggest that the backbone of the first genetic material could not have contained ribose or other sugars because of their instability." The ester linkage of ribose and phosphoric acid in RNA is known to be prone to hydrolysis.[135]Pyrimidine ribonucleosides and their respective nucleotides have been prebiotically synthesised by a sequence of reactions which by-pass the free sugars, and are assembled in a stepwise fashion by using nitrogenous or oxygenous chemistries. John Sutherland has demonstrated high yielding routes to cytidine and uridine ribonucleotides built from small 2 and 3 carbon fragments such as glycolaldehyde, glyceraldehyde or glyceraldehyde-3-phosphate, cyanamide and cyanoacetylene. One of the steps in this sequence allows the isolation of enantiopure ribose aminooxazoline if the enantiomeric excess of glyceraldehyde is 60% or greater.[136] This can be viewed as a prebiotic purification step, where the said compound spontaneously crystallised out from a mixture of the other pentose aminooxazolines. Ribose aminooxazoline can then react with cyanoacetylene in a mild and highly efficient manner to give the alpha cytidine ribonucleotide. Photoanomerization with UV light allows for inversion about the 1' anomeric centre to give the correct beta stereochemistry.[137] In 2009 they showed that the same simple building blocks allow access, via phosphate controlled nucleobase elaboration, to 2',3'-cyclic pyrimidine nucleotides directly, which are known to be able to polymerise into RNA. This paper also highlights the possibility for the photo-sanitization of the pyrimidine-2',3'-cyclic phosphates.[138] James Ferris's studies have shown that clay minerals of montmorillonite will catalyze the formation of RNA in aqueous solution, by joining activated mono RNA nucleotides to join together to form longer chains.[139] Although these chains have random sequences, the possibility that one sequence began to non-randomly increase its frequency by increasing the speed of its catalysis is possible to "kick start" biochemical evolution.
Protocells
A protocell is self-organized, endogenously ordered, spherical collection of lipids proposed as a stepping-stone to the origin of life.[140] A central question in evolution is how simple protocells first arose and began the competitive process that drove the evolution of life. Although a functional protocell has not yet been achieved in a laboratory setting, the goal appears well within reach.[141][142][143]
Self-assembled vesicles are essential components of primitive cells.[140] The second law of thermodynamics requires that the universe move in a direction in which disorder (or entropy) increases, yet life is distinguished by its great degree of organization. Therefore, a boundary is needed to separate life processes from non-living matter.[144] Researchers Irene A. Chen and Jack W. Szostak (Nobel Prize in Physiology or Medicine 2009) amongst others, demonstrated that simple physicochemical properties of elementary protocells can give rise to essential cellular behaviors, including primitive forms of Darwinian competition and energy storage. Such cooperative interactions between the membrane and encapsulated contents could greatly simplify the transition from replicating molecules to true cells.[142] Furthermore, competition for membrane molecules would favor stabilized membranes, suggesting a selective advantage for the evolution of cross-linked fatty acids and even the phospholipids of today.[142] This micro-encapsulation allowed for metabolism within the membrane, exchange of small molecules and prevention of passage of large substances across it.[145] The main advantages of encapsulation include increased solubility of the cargo and storing energy in the form of a chemical gradient.
A 2012 study led by Armen Mulkidjanian of Germany's University of Osnabrück, suggests that inland pools of condensed and cooled geothermal vapour have the ideal characteristics for the origin of life.[146] Scientists discovered in 2002 that by adding a montmorillonite clay to a solution of fatty acid micelles (lipid spheres), the clay sped up the rate of vesicles formation 100-fold.[143] So this one mineral can get precursors (nucleotides) to spontaneously assemble into RNA and membrane precursors to assemble into membrane.
Another protocell model is the Jeewanu. First synthesized in 1963 from simple minerals and basic organics while exposed to sunlight, it is still reported to have some metabolic capabilities, the presence of semipermeable membrane, amino acids, phospholipids, carbohydrates and RNA-like molecules.[147][148] However, the nature and properties of the Jeewanu remains to be clarified.
Origin of biological metabolism
Laboratory research suggests that metabolism-like reactions could have occurred naturally in early oceans, before the first organisms evolved.[19][20] The findings suggests that metabolism predates the origin of life and evolved through the chemical conditions that prevailed in the worlds earliest oceans. Reconstructions in laboratories show that some of these reactions can produce RNA, and some others resemble two essential reaction cascades of metabolism: glycolysis and the pentose phosphate pathway, that provide essential precursors for nucleic acids, amino acids and lipids.[19] Following are some observed discoveries and related hypotheses.Iron-sulfur world
Another possible answer to the polymerization conundrum was provided in the 1980s by Günter Wächtershäuser, encouraged and supported by Karl R. Popper,[149][150][151] in his iron–sulfur world theory. In this theory, he postulated the evolution of (bio)chemical pathways as fundamentals of the evolution of life. Moreover, he presented a consistent system of tracing today's biochemistry back to ancestral reactions that provide alternative pathways to the synthesis of organic building blocks from simple gaseous compounds.In contrast to the classical Miller experiments, which depend on external sources of energy (such as simulated lightning or ultraviolet irradiation), "Wächtershäuser systems" come with a built-in source of energy, sulfides of iron and other minerals (e.g. pyrite). The energy released from redox reactions of these metal sulfides is not only available for the synthesis of organic molecules, but also for the formation of oligomers and polymers. It is therefore hypothesized that such systems may be able to evolve into autocatalytic sets of self-replicating, metabolically active entities that would predate the life forms known today.[19][20] The experiment produced a relatively small yield of dipeptides (0.4% to 12.4%) and a smaller yield of tripeptides (0.10%) although under the same conditions, dipeptides were quickly broken down.[152]
Several models reject the idea of the self-replication of a "naked-gene" and postulate the emergence of a primitive metabolism which could provide an environment for the later emergence of RNA replication. The centrality of the Krebs cycle to energy production in aerobic organisms, and in drawing in carbon dioxide and hydrogen ions in biosynthesis of complex organic chemicals, including amino acids and nucleotides, suggests that it was one of the first parts of the metabolism to evolve.[153] Somewhat in agreement with these notions, Mike Russell has proposed that "the purpose of life is to hydrogenate carbon dioxide" (as part of a "metabolism-first", rather than a "genetics-first", scenario).[154][155] Physicist Jeremy England of MIT has proposed that thermodynamically, life was bound to eventually arrive, as based on established physics, he mathematically indicates "that when a group of atoms is driven by an external source of energy (like the sun or chemical fuel) and surrounded by a heat bath (like the ocean or atmosphere), it will often gradually restructure itself in order to dissipate increasingly more energy. This could mean that under certain conditions, matter inexorably acquires the key physical attribute associated with life.".[156][157]
One of the earliest incarnations of this idea was put forward in 1924 with Alexander Oparin's notion of primitive self-replicating vesicles which predated the discovery of the structure of DNA. Variants in the 1980s and 1990s include Günter Wächtershäuser's iron-sulfur world theory and models introduced by Christian de Duve based on the chemistry of thioesters. More abstract and theoretical arguments for the plausibility of the emergence of metabolism without the presence of genes include a mathematical model introduced by Freeman Dyson in the early 1980s and Stuart Kauffman's notion of collectively autocatalytic sets, discussed later in that decade.
Leslie Orgel summarized his analysis of the proposal by stating, "There is at present no reason to expect that multistep cycles such as the reductive citric acid cycle will self-organize on the surface of FeS/FeS2 or some other mineral."[158] It is possible that another type of metabolic pathway was used at the beginning of life. For example, instead of the reductive citric acid cycle, the "open" acetyl-CoA pathway (another one of the five recognised ways of carbon dioxide fixation in nature today) would be compatible with the idea of self-organisation on a metal sulfide surface. The key enzyme of this pathway, carbon monoxide dehydrogenase/acetyl-CoA synthase harbours mixed nickel-iron-sulfur clusters in its reaction centers and catalyses the formation of acetyl-CoA (which may be regarded as a modern form of acetyl-thiol) in a single step.
Zn-World hypothesis
The Zn-World (zinc world) theory of Armen Mulkidjanian[159] is an extension of Wächtershäuser's pyrite hypothesis. Wächtershäuser based his theory of the initial chemical processes leading to informational molecules (i.e. RNA, peptides) on a regular mesh of electric charges at the surface of pyrite that may have made the primeval polymerization thermodynamically more favourable by attracting reactants and arranging them appropriately relative to each other.[160] The Zn-World theory specifies and differentiates further.[159][161] Hydrothermal fluids rich in H2S interacting with cold primordial ocean (or "Darwin pond") water leads to the precipitation of metal sulfide particles. Oceanic vent systems and other hydrothermal systems have a zonal structure reflected in ancient volcanogenic massive sulfide deposits (VMS) of hydrothermal origin. They reach many kilometers in diameter and date back to the Archean eon. Most abundant are pyrite (FeS2), chalcopyrite (CuFeS2), and sphalerite (ZnS), with additions of galena (PbS) and alabandite (MnS). ZnS and MnS have a unique ability to store radiation energy, e.g. provided by UV light. Since during the relevant time window of the origins of replicating molecules the primordial atmospheric pressure was high enough (>100 bar) to precipitate near the Earth's surface and UV irradiation was 10 to 100 times more intense than now, the unique photosynthetic properties mediated by ZnS provided just the right energy conditions to energize the synthesis of informational and metabolic molecules and the selection of photostable nucleobases.The Zn-World theory has been further filled out with experimental and theoretical evidence for the ionic constitution of the interior of the first proto-cells before Archea, Eubacteria and Proto-Eukarya evolved. Archibald Maccallum noted the resemblance of organismal fluids such as blood, lymph to seawater;[162] however, the inorganic composition of all cells differ from that of modern sea water, which led Mulkidjanian and colleagues to reconstruct the "hatcheries" of the first cells combining geochemical analysis with phylogenomic scrutiny of the inorganic ion requirements of universal components of modern cells. The authors conclude that ubiquitous, and by inference primordial, proteins and functional systems show affinity to and functional requirement for K+, Zn2+, Mn2+, and phosphate. Geochemical reconstruction shows that the ionic composition conducive to the origin of cells could not have existed in what we today call marine settings but is compatible with emissions of vapor-dominated zones of what we today call inland geothermal systems. Under the anoxic, CO2-dominated primordial atmosphere, the chemistry of water condensates and exhalations near geothermal fields would resemble the internal milieu of modern cells. Therefore, the precellular stages of evolution may have taken place in shallow "Darwin-ponds" lined with porous silicate minerals mixed with metal sulfides and enriched in K+, Zn2+, and phosphorus compounds.[163][164]
Deep sea vent hypothesis
The deep sea vent, or alkaline hydrothermal vent, theory for the origin of life on Earth posits that life may have begun at submarine hydrothermal vents,[165] where hydrogen-rich fluids emerge from below the sea floor, as a result of serpentinization of ultra-mafic olivine with sea water and a pH interface with carbon dioxide-rich ocean water. Sustained chemical energy in such systems is derived from redox reactions, in which electron donors, such as molecular hydrogen, react with electron acceptors, such as carbon dioxide (see iron-sulfur world theory). These are highly exothermic reactions.[note 2]
Michael Russell demonstrated that alkaline vents created an abiogenic proton-motive force chemiosmotic gradient,[166] in which conditions are ideal for an abiogenic hatchery for life. Their microscopic compartments "provide a natural means of concentrating organic molecules", composed of iron-sulfur minerals such as mackinawite, endowed these mineral cells with the catalytic properties envisaged by Günter Wächtershäuser.[153] This movement of ions across the membrane depends on a combination of two factors:
- Diffusion force caused by concentration gradient – all particles including ions tend to diffuse from higher concentration to lower.
- Electrostatic force caused by electrical potential gradient – cations like protons H+ tend to diffuse down the electrical potential, anions in the opposite direction.
Nobel laureate Szostak suggested that geothermal activity provides greater opportunities for the origination of life in open lakes where there is a buildup of minerals. In 2010, based on spectral analysis of sea and hot mineral water as well as cactus juice, Ignat Ignatov and Oleg Mosin demonstrated that life may have predominantly originated in hot mineral water. The hot mineral water that contains bicarbonate and calcium ions has the most optimal range.[167] This is similar case as the origin of life in hydrothermal vents, but with bicarbonate and calcium ions in hot water. This water has a pH of 9–11 and is possible to have the reactions in sea water. According to Nobel winner Melvin Calvin, certain reactions of condensation-dehydration of amino acids and nucleotides in individual blocks of peptides and nucleic acids can take place in the primary hydrosphere with pH 9-11 at a later evolutionary stage.[168] Some of these compounds like hydrocyanic acid (HCN) have been proven in the experiments of Miller. This is the environment in which the stromatolites have been created. David Ward described the formation of stromatolites in hot mineral water at the Yellowstone National Park. Stromatolites have lived in hot mineral water and in proximity to areas with volcanic activity.[169] Processes have evolved in the sea near geysers of hot mineral water. In 2011 Tadashi Sugawara created a protocell in hot water.[170]
Thermosynthesis
Today's bioenergetic process of fermentation is carried out by either the aforementioned citric acid cycle or the Acetyl-CoA pathway, both of which have been connected to the primordial iron-sulfur world. In a different approach, the thermosynthesis hypothesis considers the bioenergetic process of chemiosmosis, which plays an essential role in cellular respiration and photosynthesis, more basal than fermentation: the ATP synthase enzyme, which sustains chemiosmosis, is proposed as the currently extant enzyme most closely related to the first metabolic process.[171][172]First, life needed an energy source to bring about the condensation reaction that yielded the peptide bonds of proteins and the phosphodiester bonds of RNA. In a generalization and thermal variation of the binding change mechanism of today's ATP synthase, the "first protein" would have bound substrates (peptides, phosphate, nucleosides, RNA 'monomers') and condensed them to a reaction product that remained bound until after a temperature change it was released by thermal unfolding.
The energy source under the thermosynthesis hypothesis was thermal cycling, the result of suspension of protocells in a convection current, as is plausible in a volcanic hot spring; the convection accounts for the self-organization and dissipative structure required in any origin of life model. The still ubiquitous role of thermal cycling in germination and cell division is considered a relic of primordial thermosynthesis.
By phosphorylating cell membrane lipids, this "first protein" gave a selective advantage to the lipid protocell that contained the protein. This protein also synthesized a library of many proteins, of which only a minute fraction had thermosynthesis capabilities. As proposed by Dyson,[173] it propagated functionally: it made daughters with similar capabilities, but it did not copy itself. Functioning daughters consisted of different amino acid sequences.
Whereas the iron-sulfur world identifies a circular pathway as the most simple—and therefore assumes the existence of enzymes—the thermosynthesis hypothesis does not even invoke a pathway, and does not assume the existence of regular enzymes: ATP synthase's binding change mechanism resembles a physical adsorption process that yields free energy,[174] rather than a regular enzyme's mechanism, which decreases the free energy. The RNA world also implies the existence of several enzymes. It has been claimed that the emergence of cyclic systems of protein catalysts is implausible.[175]
Other models of abiogenesis
Clay hypothesis
A model for the origin of life based on clay was forwarded by A. Graham Cairns-Smith in 1985 and explored as a plausible illustration by several scientists.[176] The Clay hypothesis postulates that complex organic molecules arose gradually on a pre-existing, non-organic replication platform of silicate crystals in solution.Cairns-Smith is a trenchant critic of other models of chemical evolution.[177] However, he admits that like many models of the origin of life, his own also has its shortcomings.
In 2007, Kahr and colleagues reported their experiments that tested the idea that crystals can act as a source of transferable information, using crystals of potassium hydrogen phthalate. "Mother" crystals with imperfections were cleaved and used as seeds to grow "daughter" crystals from solution. They then examined the distribution of imperfections in the new crystals and found that the imperfections in the mother crystals were reproduced in the daughters, but the daughter crystals also had many additional imperfections. For gene-like behavior to be observed, the quantity of inheritance of these imperfections should have exceeded that of the mutations in the successive generations, but it did not. Thus Kahr concluded that the crystals, "were not faithful enough to store and transfer information from one generation to the next".[178][179]
Gold's "deep-hot biosphere" model
In the 1970s, Thomas Gold proposed the theory that life first developed not on the surface of the Earth, but several kilometers below the surface. The discovery in the late 1990s of nanobes (filamental structures that are smaller than bacteria, but that may contain DNA) in deep rocks[180] might be seen as lending support to Gold's theory.It is now reasonably well established that microbial life is plentiful at shallow depths in the Earth, up to 5 kilometres (3.1 mi) below the surface,[180] in the form of extremophile archaea, rather than the better-known eubacteria (which live in more accessible conditions). It is claimed that discovery of microbial life below the surface of another body in our solar system would lend significant credence to this theory. Thomas Gold also asserted that a trickle of food from a deep, unreachable, source is needed for survival because life arising in a puddle of organic material is likely to consume all of its food and become extinct. Gold's theory is that the flow of such food is due to out-gassing of primordial methane from the Earth's mantle; more conventional explanations of the food supply of deep microbes (away from sedimentary carbon compounds) is that the organisms subsist on hydrogen released by an interaction between water and (reduced) iron compounds in rocks.
Primitive extraterrestrial life
Exogenesis is related to, but not the same as, the notion of panspermia. Neither hypothesis actually answers the question of how life first originated, but merely shifts it to another planet or a comet. However, the advantage of an extraterrestrial origin of primitive life is that life is not required to have evolved on each planet it occurs on, but rather in a single location, and then spread about the galaxy to other star systems via cometary and/or meteorite impact. Evidence to support the hypothesis is scant, but it finds support in studies of Martian meteorites found in Antarctica and in studies of extremophile microbes' survival in outer space.[181][182][183][184][185][186][187]On 24 January 2014, NASA reported that current studies on the planet Mars by the Curiosity and Opportunity rovers will now be searching for evidence of ancient life, including a biosphere based on autotrophic, chemotrophic and/or chemolithoautotrophic microorganisms, as well as ancient water, including fluvio-lacustrine environments (plains related to ancient rivers or lakes) that may have been habitable.[188][189][190][191] The search for evidence of habitability, taphonomy (related to fossils), and organic carbon on the planet Mars is now a primary NASA objective.[188]
Extraterrestrial organic molecules
An organic compound is any member of a large class of gaseous, liquid, or solid chemicals whose molecules contain carbon. Carbon is the fourth most abundant element in the universe by mass after hydrogen, helium, and oxygen.[192] Carbon is abundant in the Sun, stars, comets, and in the atmospheres of most planets.[193] Organic compounds are relatively common in space, formed by "factories of complex molecular synthesis" which occur in molecular clouds and circumstellar envelopes, and chemically evolve after reactions are initiated mostly by ionizing radiation.[71][194]
Based on computer model studies, the complex organic molecules necessary for life may have formed on dust grains in the protoplanetary disk surrounding the Sun before the formation of the Earth.[21] According to the computer studies, this same process may also occur around other stars that acquire planets.[21]
Observations suggest that the majority of organic compounds introduced on Earth by interstellar dust particles are considered principal agents in the formation of complex molecules, thanks to their peculiar surface-catalytic activities.[195][196] Studies reported in 2008, based on 12C/13C isotopic ratios of organic compounds found in the Murchison meteorite, suggested that the RNA component uracil and related molecules, including xanthine, were formed extraterrestrially.[197][198] On 8 August 2011, a report based on NASA studies of meteorites found on Earth was published suggesting DNA components (adenine, guanine and related organic molecules) were made in outer space.[195][199][200][201] Scientists also found that the cosmic dust permeating the universe contains complex organics ("amorphous organic solids with a mixed aromatic-aliphatic structure") that could be created naturally, and rapidly, by stars.[202][203][204] A scientist who suggested that these compounds may have been related to the development of life on Earth said that "If this is the case, life on Earth may have had an easier time getting started as these organics can serve as basic ingredients for life."[202]
Glycolaldehyde, the first example of an interstellar sugar molecule, was detected in the star-forming region near the center of our galaxy. It was discovered in 2000 by Jes Jørgensen and Jan M. Hollis.[205] Then, on 29 August 2012, the same team reported the detection of glycolaldehyde in a distant star system. The molecule was found around the protostellar binary IRAS 16293-2422 400 light years from Earth.[206][207][208] Glycolaldehyde is needed to form ribonucleic acid (RNA), which is similar in function to DNA. These findings suggest that complex organic molecules may form in stellar systems prior to the formation of planets, eventually arriving on young planets early in their formation.[209]
Because sugars are associated with both metabolism and the genetic code, two of the most basic aspects of life, it is thought the discovery of extraterrestrial sugar increases the likelihood that life may exist elsewhere in our galaxy.[205]
NASA announced in 2009 that scientists had identified another fundamental chemical building block of life in a comet for the first time, glycine, an amino acid, which was detected in material ejected from Comet Wild-2 in 2004 and grabbed by NASA's Stardust probe. Glycine has been detected in meteorites before. Carl Pilcher, who leads NASA's Astrobiology Institute commented that "The discovery of glycine in a comet supports the idea that the fundamental building blocks of life are prevalent in space, and strengthens the argument that life in the universe may be common rather than rare."[210] Comets are encrusted with outer layers of dark material, thought to be a tar-like substance composed of complex organic material formed from simple carbon compounds after reactions initiated mostly by ionizing radiation. It is possible that a rain of material from comets could have brought significant quantities of such complex organic molecules to Earth.[211][212] Amino acids which were formed extraterrestrially may also have arrived on Earth via comets.[24] It is estimated that during the Late Heavy Bombardment, meteorites may have delivered up to five million tons of biogenic elements to Earth per year.[24]
Polycyclic aromatic hydrocarbons (PAH) are the most common and abundant of the known polyatomic molecules in the visible universe, and are considered a likely constituent of the primordial sea.[213][214][215] PAHs, along with fullerenes (or "buckyballs"), have been recently detected in nebulae.[216][217]
On 3 April 2013, NASA reported that complex organic chemicals could arise on Titan, a moon of Saturn, based on studies simulating the atmosphere of Titan.[218]
Lipid world
The lipid world theory postulates that the first self-replicating object was lipid-like.[219][220] It is known that phospholipids form lipid bilayers in water while under agitation – the same structure as in cell membranes. These molecules were not present on early Earth, but other amphiphilic long chain molecules also form membranes. Furthermore, these bodies may expand (by insertion of additional lipids), and under excessive expansion may undergo spontaneous splitting which preserves the same size and composition of lipids in the two progenies. The main idea in this theory is that the molecular composition of the lipid bodies is the preliminary way for information storage, and evolution led to the appearance of polymer entities such as RNA or DNA that may store information favorably. Studies on vesicles from potentially prebiotic amphiphiles have so far been limited to systems containing one or two types of amphiphiles. This in contrast to the output of simulated prebiotic chemical reactions, which typically produce very heterogeneous mixtures of compounds.[221] Within the hypothesis of a lipid bilayer membrane composed of a mixture of various distinct amphiphilic compounds there is the opportunity of a huge number of theoretically possible combinations in the arrangements of these amphiphiles in the membrane. Among all these potential combinations, a specific local arrangement of the membrane would have favored the constitution of an hypercycle,[222][223] according to the terminology by Manfred Eigen, actually a positive feedback composed of two mutual catalysts represented by a membrane site and a specific compound trapped in the vesicle.Such site/compound pairs are transmissible to the daughter vesicles leading to the emergence of distinct lineages of vesicles which would have allowed Darwinian natural selection.[224]
Polyphosphates
A problem in most scenarios of abiogenesis is that the thermodynamic equilibrium of amino acid versus peptides is in the direction of separate amino acids. What has been missing is some force that drives polymerization. The resolution of this problem may well be in the properties of polyphosphates.[225][226] Polyphosphates are formed by polymerization of ordinary monophosphate ions PO4−3. Several mechanisms for such polymerization have been suggested. Polyphosphates cause polymerization of amino acids into peptides. They are also logical precursors in the synthesis of such key biochemical compounds as ATP. A key issue seems to be that calcium reacts with soluble phosphate to form insoluble calcium phosphate (apatite), so some plausible mechanism must be found to keep calcium ions from causing precipitation of phosphate. There has been much work on this topic over the years, but an interesting new idea is that meteorites may have introduced reactive phosphorus species on the early Earth.[227]PAH world hypothesis
Polycyclic aromatic hydrocarbons (PAHs) are known to be abundant in the universe,[213][214][215] including in the interstellar medium, in comets, and in meteorites, and are some of the most complex molecules so far found in space.[193]Other sources of complex molecules have been postulated, including extraterrestrial stellar or interstellar origin. For example, from spectral analyses, organic molecules are known to be present in comets and meteorites. In 2004, a team detected traces of PAHs in a nebula.[228] In 2010, another team also detected PAHs, along with fullerenes (or "buckyballs"), in nebulae.[229] The use of PAHs has also been proposed as a precursor to the RNA world in the PAH world hypothesis.[citation needed]
The Spitzer Space Telescope has detected a star, HH 46-IR, which is forming by a process similar to that by which the sun formed. In the disk of material surrounding the star, there is a very large range of molecules, including cyanide compounds, hydrocarbons, and carbon monoxide. In September 2012, NASA scientists reported that PAHs, subjected to interstellar medium (ISM) conditions, are transformed, through hydrogenation, oxygenation and hydroxylation, to more complex organics – "a step along the path toward amino acids and nucleotides, the raw materials of proteins and DNA, respectively".[230][231] Further, as a result of these transformations, the PAHs lose their spectroscopic signature which could be one of the reasons "for the lack of PAH detection in interstellar ice grains, particularly the outer regions of cold, dense clouds or the upper molecular layers of protoplanetary disks."[230][231]
On 21 February 2014, NASA announced a greatly upgraded database[193] for tracking PAHs in the universe. According to scientists, more than 20% of the carbon in the universe may be associated with PAHs, possible starting materials for the formation of life. PAHs seem to have been formed shortly after the Big Bang, are widespread throughout the universe,[213][214][215] and are associated with new stars and exoplanets.[193]
Radioactive beach hypothesis
Zachary Adam claims that tidal processes that occurred during a time when the moon was much closer may have concentrated grains of uranium and other radioactive elements at the high-water mark on primordial beaches, where they may have been responsible for generating life's building blocks.[232] According to computer models reported in Astrobiology,[233] a deposit of such radioactive materials could show the same self-sustaining nuclear reaction as that found in the Oklo uranium ore seam in Gabon. Such radioactive beach sand might have provided sufficient energy to generate organic molecules, such as amino acids and sugars from acetonitrile in water. Radioactive monazite material also has released soluble phosphate into the regions between sand-grains, making it biologically "accessible". Thus amino acids, sugars, and soluble phosphates might have been produced simultaneously, according to Adam. Radioactive actinides, left behind in some concentration by the reaction, might have formed part of organo-metallic complexes. These complexes could have been important early catalysts to living processes.John Parnell has suggested that such a process could provide part of the "crucible of life" in the early stages of any early wet rocky planet, so long as the planet is large enough to have generated a system of plate tectonics which brings radioactive minerals to the surface. As the early Earth is thought to have had many smaller plates, it might have provided a suitable environment for such processes.[234]
Ultraviolet and temperature-assisted replication model
From a thermodynamic perspective of the origin of life springs the ultraviolet and temperature-assisted replication (UVTAR) model. Karo Michaelian points out that any model for the origin of life must take into account the fact that life is an irreversible thermodynamic process which arises and persists because it produces entropy. Entropy production is not incidental to the process of life, but rather the fundamental reason for its existence. Present day life augments the entropy production of Earth by catalysing the water cycle through evapotranspiration.[235][236] Michaelian argues that if the thermodynamic function of life today is to produce entropy through coupling with the water cycle, then this probably was its function at its very beginnings. It turns out that both RNA and DNA when in water solution are very strong absorbers and extremely rapid dissipaters of ultraviolet light within the 200–300 nm wavelength range, which is that part of the sun's spectrum that could have penetrated the dense prebiotic atmosphere.[237] have shown that the amount of ultraviolet (UV) light reaching the Earth's surface in the Archean eon could have been up to 31 orders of magnitude greater than it is today at 260 nm where RNA and DNA absorb most strongly. Absorption and dissipation of UV light by the organic molecules at the Archean ocean surface would have significantly increased the temperature of the surface and led to enhanced evaporation and thus to have augmented the primitive water cycle. Since absorption and dissipation of high energy photons is an entropy producing process, argues that non-equilbrium abiogenic synthesis of RNA and DNA utilizing UV light would have been thermodynamically favored.[138]A simple mechanism that could explain the replication of RNA and DNA without resort to the use of enzymes could also be provided within the same thermodynamic framework by assuming that life arose when the temperature of the primitive seas had cooled to somewhat below the denaturing temperature of RNA or DNA (based on the ratio of 18O/16O found in cherts of the Barberton greenstone belt of South Africa of about 3.5 to 3.2 Ga., surface temperatures are predicted to have been around 70±15 °C,[238] close to RNA or DNA denaturing (uncoiling and separation) temperatures. During the night, the surface water temperature would drop below the denaturing temperature and single strand RNA/DNA could act as a template for the formation of double strand RNA/DNA. During the daylight hours, RNA and DNA would absorb UV light and convert this directly to heat the ocean surface, thereby raising the local temperature enough to allow for denaturing of RNA and DNA. The copying process would have been repeated with each diurnal cycle.[239][240] Such a temperature assisted mechanism of replication bears similarity to polymerase chain reaction (PCR), a routine laboratory procedure employed to multiply DNA segments. Michaelian suggests that the traditional origin of life research, that expects to describe the emergence of life from near-equilibrium conditions, is erroneous and that non-equilibrium conditions must be considered, in particular, the importance of entropy production to the emergence of life.
Since denaturation would be most probable in the late afternoon when the Archean sea surface temperature would be highest, and since late afternoon submarine sunlight is somewhat circularly polarized, the homochirality of the organic molecules of life can also be explained within the proposed thermodynamic framework.[241][242]
Multiple genesis
Different forms of life with variable origin processes may have appeared quasi-simultaneously in the early history of Earth.[243] The other forms may be extinct, leaving distinctive fossils through their different biochemistry (e.g., using arsenic instead of phosphorus), survive as extremophiles, or simply be unnoticed through their being analogous to organisms of the current life tree. Hartman[244] for example combines a number of theories together, by proposing that:The first organisms were self-replicating iron-rich clays which fixed carbon dioxide into oxalic and other dicarboxylic acids. This system of replicating clays and their metabolic phenotype then evolved into the sulfide rich region of the hotspring acquiring the ability to fix nitrogen. Finally phosphate was incorporated into the evolving system which allowed the synthesis of nucleotides and phospholipids. If biosynthesis recapitulates biopoiesis, then the synthesis of amino acids preceded the synthesis of the purine and pyrimidine bases. Furthermore the polymerization of the amino acid thioesters into polypeptides preceded the directed polymerization of amino acid esters by polynucleotides.Lynn Margulis's endosymbiotic theory suggests that multiple forms of archea entered into symbiotic relationship to form the eukaryotic cell. The horizontal transfer of genetic material between archea promotes such symbiotic relationships, and thus many separate organisms may have contributed to building what has been recognised as the Last Universal Common Ancestor (LUCA) of modern organisms.
