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Origin of life
In the natural sciences, abiogenesis, the question of the origin of life, is the study of how life on Earth might have emerged from non-life. Scientific consensus is that abiogenesis occurred sometime between 4.4 billion years ago, when water vapor first liquefied, and 2.7 billion years ago, when the ratio of stable isotopes of carbon (12C and 13C ), iron and sulfur points to a biogenic origin of minerals and sediments and molecular biomarkers indicate photosynthesis. This topic also includes panspermia and other exogenic theories regarding possible extra-planetary or extra-terrestrial origins of life, thought to have possibly occurred sometime over the last 13.7 billion years in the evolution of the Universe since the Big Bang.
Origin of life studies is a limited field of research despite its profound impact on biology and human understanding of the natural world. Progress in this field is generally slow and sporadic, though it still draws the attention of many due to the eminence of the question being investigated. Several theories have been proposed, most notably the iron-sulfur world theory (metabolism first) and the RNA world hypothesis (genetics first).
For the observed evolution of life on earth, see the timeline of life.
History of the concept in science
Until the early Nineteenth Century people frequently believed in spontaneous generation of life from non-living matter.
Darwin and Pasteur
By the middle of the 19th century Pasteur and others had demonstrated that living organisms did not arise spontaneously from non-living matter; the question therefore arose of how life might have come about within a naturalistic framework. In a letter to Joseph Dalton Hooker on February 1, 1871, Charles Darwin made the suggestion 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." In other words, the presence of life itself makes the search for the origin of life dependent on the sterile conditions of the laboratory.
Haldane and Oparin
No real progress was made until 1924 when Aleksandr Ivanovich Oparin experimentally showed that atmospheric oxygen prevented the synthesis of the organic molecules that are the necessary building blocks for the evolution of life. In his The Origin of Life on Earth, Oparin argued that a "primeval soup" of organic molecules could be created in an oxygen-less atmosphere through the action of sunlight. These would combine in ever-more complex fashions until they dissolved into a coacervate droplet. 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, those that do not become extinct. Many modern theories of the origin of life still take Oparin's ideas as a starting point. Around the same time J. B. S. Haldane also suggested that the earth's pre-biotic oceans - very different from their modern counterparts - would have formed a "hot dilute soup" in which organic compounds, the building blocks of life, could have formed. This idea was called biopoiesis or biopoesis, the process of living matter evolving from self-replicating but nonliving molecules.
Morse and MacKenzie  have suggested that oceans may have appeared first in the Hadean era, as soon as 200 million years after the Earth was formed, in a hot (100 °C) reducing environment, and that the pH of about 5.8 rose rapidly towards neutral. This has been supported by Wilde who has pushed the date of the zircon crystals found in the metamorphosed quartzite of Mount Narryer in Western Australia, previously thought to be 4.1-4.2 billion years old, to 4.404 billion years. This means that oceans and continental crust existed within 150 million years of Earth's formation. Despite this, the Hadean environment was one highly hazardous to life. Frequent collisions with large objects, up to 500 kilometres in diameter, would have been sufficient to vaporise the ocean within a few months of impact, with hot steam mixed with rock vapour leading to high altitude clouds completely covering the planet. After a few months the height of these clouds begins to decrease but the cloud base would still be elevated probably for the next thousand years after which at low altitude it starts to rain. For another two thousand years rains slowly draw down the height of the clouds, returning the oceans to their original depth only 3,000 years after the impact event. The possible Late Heavy Bombardment possibly caused by the movements in position of the Gaseous Giant planets, that pockmarked the moon, and other inner planets (Mercury, Mars, and presumably Earth and Venus), between 3.8 and 4.1 billion years would likely have sterilised the planet if life had already evolved by that time.
Evidence of the early appearance of life comes from the Isua supercrustal belt in Western Greenland and from similar formations in nearby the Akilia Islands. Carbon entering into rock formations has a concentration of elemental δ13C of about -5.5, where because of a preferential biotic uptake of 12C, biomass has a δ13C of between -20 and -30. These isotopic fingerprints are preserved in the sediments, and Mojzis has used this technique to suggest that life existed on the planet already by 3.85 billion years ago. Lazcano and Miller (1994) suggest that the rapidity of the evolution of life is dictated by the rate of recirculating water through mid ocean submarine vents. Complete recirculation takes 10 million years, thus any organic compounds produced by then would be altered or destroyed by temperatures exceeding 300 °C. They estimate that the development of a 100 kilobase genome of a DNA/protein primitive heterotroph into a 7000 gene filamentous cyanobacterium would have required only 7 million years.
There is no truly "standard model" of the origin of life. But most currently accepted models build in one way or another upon a number of discoveries about the origin of molecular and cellular components for life, which are listed in a rough order of postulated emergence:
The origin of the basic biomolecules, while not settled, is less controversial than the significance and order of steps 2 and 3. The basic chemicals from which life was thought to have formed are:
As of 2007, 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 short on specifics. However, some researchers are working in this field, notably Steen Rasmussen at Los Alamos National Laboratory and Jack Szostak at Harvard University. Others have argued that a "top-down approach" is more feasible. One such approach, 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. The biologist John Desmond Bernal, coined the term Biopoesis for this process, and suggested that there were a number of clearly defined "stages" that could be recognised in explaining the origin of life.
Bernal suggested that Darwinian evolution may have commenced early, some time between Stage 1 and 2.
Origin of organic molecules
In 1953 a graduate student, Stanley Miller, and his professor, Harold Urey, performed an experiment that proved organic molecules could have spontaneously formed on Early Earth from inorganic precursors. 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. Whether the mixture of gases used in the Miller-Urey experiment truly reflects the atmospheric content of Early Earth is a controversial topic. Other less reducing gases produce a lower yield and variety. It was once thought that appreciable amounts of molecular oxygen were present in the prebiotic atmosphere, which would have essentially prevented the formation of organic molecules; however, the current scientific consensus is that such was not the case. See Oxygen Catastrophe.
Simple organic molecules are, of course, a long way from a fully functional self-replicating life form. But in an environment with no pre-existing life these molecules may have accumulated and provided a rich environment for chemical evolution ("soup theory"). On the other hand, the spontaneous formation of complex polymers from abiotically generated monomers under these conditions 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 experiments.
It can be argued that the most crucial challenge unanswered by this theory is how the relatively simple organic building blocks polymerise and form more complex structures, interacting in consistent ways to form a protocell. For example, in an aqueous environment hydrolysis of oligomers/polymers into their constituent monomers would be favored over the condensation of individual monomers into polymers. Also, the Miller experiment produces many substances that would undergo cross-reactions with the amino acids or terminate the peptide chain.
In the 1950s and 1960s Sidney W. Fox, 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 peptides. These amino acids and small peptides could be encouraged to form closed spherical membranes, called microspheres. Fox described these formations as protocells, protein spheres that could grow and reproduce.
In the early 1970s the problem of the origin of life was approached by Manfred Eigen and Peter Schuster of the Max Planck Institute for Biophysical Chemistry. They examined the transient stages between the molecular chaos and a self replicating hypercycle in a prebiotic soup.
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. However, these reactions are limited to self-excisions (in which a longer RNA molecule becomes shorter), and much rarer small additions that are incapable of coding for any useful protein. The hypercycle theory is further degraded since the hypothetical RNA would require the existence of complex biochemicals such as nucleotides which are not formed under the conditions proposed by the Miller-Urey experiment.
Another possible answer to this polymerization conundrum was provided in 1980s by Günter Wächtershäuser, 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 UV 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.
The experiment produced a relatively small yield of dipeptides (0.4% to 12.4%) and a smaller yield of tripeptides (0.10%) but the authors also noted that: "under these same conditions dipeptides hydrolysed rapidly." Another criticism of the result is that the experiment did not include any organic molecules that would most likely cross-react or chain-terminate.
William Martin and Michael Russell reported a modified iron-sulfur-hypothesis in 2002. According to their scenario, the first cellular life forms may have evolved inside so-called black smokers at seafloor spreading zones in the deep sea. These structures consist of microscale caverns that are coated by thin membraneous metal sulfide walls. Therefore, these structures would solve several critical points of the "pure" Wächtershäuser systems at once:
This model locates the "last universal common ancestor" (LUCA) inside a black smoker, rather than assuming the existence of a free-living form of LUCA. The last evolutionary step would be the synthesis of a lipid membrane that finally allows the organisms to leave the microcavern system of the black smokers and start their independent lives. This postulated late acquisition of lipids is consistent with the presence of completely different types of membrane lipids in archaebacteria and eubacteria (plus eukaryotes) with highly similar cellular physiology of all life forms in most other aspects.
From organic molecules to protocells
The question "How do simple organic molecules form a protocell?" is largely unanswered but there are many hypotheses. Some of these postulate the early appearance of nucleic acids ("genes-first") whereas others postulate the evolution of biochemical reactions and pathways first ("metabolism-first"). Recently, trends are emerging to create hybrid models that combine aspects of both.
"Genes first" models: the RNA world
The RNA world hypothesis suggests that relatively short RNA molecules could have spontaneously formed that were capable of catalyzing their own continuing replication. It is difficult to gauge the probability of this formation. A number of theories of modes of formation have been put forward. Early cell membranes could have formed spontaneously from proteinoids, protein-like molecules that are produced when amino acid solutions are heated - when present at the correct concentration in aqueous solution, these form microspheres which are observed to behave similarly to membrane-enclosed compartments. Other possibilities include systems of chemical reactions taking 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 catalyse 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 conditions approximating the early Earth. Relatively short RNA molecules which can duplicate others have been artificially produced in the lab.
A slightly different version of this hypothesis is 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 .
"Metabolism first" models: iron-sulfur world and others
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.
One of the earliest incarnations of this idea was put forward in 1924 with Aleksandr Ivanovich Oparin's notion of primitive self-replicating vesicles which predated the discovery of the structure of DNA. More recent 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.
However, the idea that a closed metabolic cycle, such as the reductive citric acid cycle, could form spontaneously (proposed by Günter Wächtershäuser) remains unsupported. According to Leslie Orgel, a leader in origin-of-life studies for the past several decades, there is reason to believe the assertion will remain so. In an article entitled "Self-Organizing Biochemical Cycles", Orgel summarizes 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." 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 four recognised ways of carbon dioxide fixation in nature today) would be even more 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.
Waves breaking on the shore create a delicate foam composed of bubbles. Winds sweeping across the ocean have a tendency to drive things to shore, much like driftwood collecting on the beach. It is possible that organic molecules were concentrated on the shorelines in much the same way. Shallow coastal waters also tend to be warmer, further concentrating the molecules through evaporation. While bubbles composed mostly of water burst quickly, water containing amphiphiles forms much more stable bubbles, lending more time to the particular bubble to perform these crucial experiments.
Amphiphiles are oily compounds containing a hydrophilic head on one or both ends of a hydrophobic molecule. Some amphiphiles have the tendency to spontaneously form membranes in water. A spherically closed membrane contains water and is a hypothetical precursor to the modern cell membrane. If a protein came along that increased the integrity of its parent bubble, then that bubble had an advantage, and was placed at the top of the natural selection waiting list. Primitive reproduction can be envisioned when the bubbles burst, releasing the results of the experiment into the surrounding medium. Once enough of the 'right stuff' was released into the medium, the development of the first prokaryotes, eukaryotes, and multicellular organisms could be achieved.
Similarly, bubbles formed entirely out of protein-like molecules, called microspheres, will form spontaneously under the right conditions. But they are not a likely precursor to the modern cell membrane, as cell membranes are composed primarily of lipid compounds rather than amino-acid compounds (for types of membrane spheres associated with abiogenesis, see protobionts, micelle, coacervate).
A recent model by Fernando and Rowe suggests that the enclosure of an autocatalytic non-enzymatic metabolism within protocells may have been one way of avoiding the side-reaction problem that is typical of metabolism first models.
It should be noted that such a theory remains untested and still assumes that a higher concentration of incomplete and toxic Miller-Urey products will *somehow* spontaneously form reproducing biochemicals. Such thoeries as the Bubble Theory are wanting in actual evidence and thick with assumption and imagination.
British ethologist Richard Dawkins wrote about autocatalysis as a potential explanation for the origin of life in his 2004 book The Ancestor's Tale. Autocatalysts are substances which catalyze the production of themselves, and therefore have the property of being a simple molecular replicator. 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 ester 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.
A model for the origin of life based on clay was forwarded by Dr A. Graham Cairns-Smith of the University of Glasgow in 1985 and adopted as a plausible illustration by several other scientists, including Richard Dawkins. Clay theory postulates that complex organic molecules arose gradually on a pre-existing, non-organic replication platform -- silicate crystals in solution. Complexity in companion molecules developed as a function of selection pressures on types of clay crystal is then exapted to serve the replication of organic molecules independently of their silicate "launch stage". It is, truly, "life from a rock."
Cairns-Smith is a staunch critic of other models of chemical evolution. However, he admits, that like many models of the origin of life, his own also has its shortcomings (Horgan 1991).
In 2007, Kahr and colleagues reported their experiments to examine 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 crystal system and found that the imperfections in the mother crystals were indeed reproduced in the daughters. The daughter crystals had many additional imperfections. For a gene-like behavior the additional imperfections should be much less than the parent ones, thus Kahr concludes that the crystals "were not faithful enough to store and transfer information form one generation to the next".
"Deep-hot biosphere" model of Gold
The discovery of nanobes (filamental structures that are smaller than bacteria, but that may contain DNA) in deep rocks, led to a controversial theory put forward by Thomas Gold in the 1990s that life first developed not on the surface of the Earth, but several kilometers below the surface. It is now known that microbial life is plentiful up to five kilometers below the earth's surface in the form of archaea, which are generally considered to have originated either before or around the same time as eubacteria, most of which live on the surface including the oceans. 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. He also noted 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.
"Primitive" extraterrestrial life
An alternative to Earthly abiogenesis is the hypothesis that primitive life may have originally formed extraterrestrially, either in space or on a nearby planet (Mars). (Note that exogenesis is related to, but not the same as, the notion of panspermia). A supporter of this theory is Francis Crick.
Organic compounds are relatively common in space, especially in the outer solar system where volatiles are not evaporated by solar heating. Comets are encrusted by 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 irradiation by ultraviolet light. It is supposed that a rain of material from comets could have brought significant quantities of such complex organic molecules to Earth.
An alternative but related hypothesis, proposed to explain the presence of life on Earth so soon after the planet had cooled down, with apparently very little time for prebiotic evolution, is that life formed first on early Mars. Due to its smaller size Mars cooled before Earth (a difference of hundreds of millions of years), allowing prebiotic processes there while Earth was still too hot. Life was then transported to the cooled Earth when crustal material was blasted off Mars by asteroid and comet impacts. Mars continued to cool faster and eventually became hostile to the continued evolution or even existence of life (it lost its atmosphere due to low volcanism), Earth is following the same fate as Mars, but at a slower rate.
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 plausibility of the concept is scant, but it finds support in recent study of Martian meteorites found in Antarctica and in studies of extremophile microbes. Additional support comes from a recent discovery of a bacterial ecosytem whose energy source is radioactivity.
The Lipid World
There is a theory that ascribes the first self-replicating object to be lipid-like. It is known that phospholipids spontaneously form bilayers in water - the same structure as in cell membranes. These molecules were not present on early earth, however 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.
The Polyphosphate model
The problem with 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. Polyphosphates are formed by polymerization of ordinary monophosphate ions PO4-3 by ultraviolet light. Polyphosphates cause polymerization of amino acids into peptides. Ample ultraviolet light must have existed in the early oceans. The 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 free calcium ions from solution. Possibly, the answer may be in some stable, non-reactive complex such as calcium citrate.
The Ecopoesis model
The Ecopoesis model proposes that the geochemical cycles of biogenic elements, driven by an early oxygen-rich atmosphere, were the basis of a planetary metabolism that preceded and conditioned the gradual evolution of organismal life.
PAH world hypothesis
Other sources of complex molecules have been postulated, including extra-terrestrial 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 polycyclic aromatic hydrocarbons (PAH's) in a nebula. Those are the most complex molecules so far found in space. The use of PAH's has also been proposed as a precursor to the RNA world in the PAH world hypothesis.
Different forms of life may have appeared quasi-simultaneously in the early history of Earth . 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 analoguous to organisms of the current life tree.
The modern concept of abiogenesis has been criticized by scientists throughout the years. Astronomer Sir Fred Hoyle did so based on the probability of abiogenesis randomly occurring. Physicist Hubert Yockey did so by saying that it is closer to theology than science.
Other scientists have proposed counterpoints to abiogenesis, such as, Harold Urey, Stanley Miller, Francis Crick (a molecular biologist), and Leslie Orgel (through the Directed Panspermia hypothesis).
Beyond making the trivial observation that life exists, it is difficult to prove or falsify abiogenesis; therefore, the hypothesis has many such critics, both in the scientific and nonscientific communities. Nonetheless, research and hypothesizing continue in the hope of developing a satisfactory theoretical mechanism of abiogenesis.
Sir Fred Hoyle, with Chandra Wickramasinghe, was a critic of abiogenesis. Specifically, Hoyle rejected chemical evolution in explaining the naturalistic origin of life. His argument was mainly based on the improbability of what were thought to be the necessary components coming together for chemical evolution. Though modern theories address his argument, Hoyle never saw chemical evolution as a reasonable explanation, preferring panspermia as an alternative natural explanation to the origin of life on Earth.
Information theorist Hubert Yockey argued that chemical evolutionary research faces the following problem:
Research on the origin of life seems to be unique in that the conclusion has already been authoritatively accepted…. What remains to be done is to find the scenarios which describe the detailed mechanisms and processes by which this happened. One must conclude that, contrary to the established and current wisdom a scenario describing the genesis of life on earth by chance and natural causes which can be accepted on the basis of fact and not faith has not yet been written.
In a book he wrote 15 years later, Yockey argued that the idea of abiogenesis from a primordial soup is a failed paradigm:
Although at the beginning the paradigm was worth consideration, now the entire effort in the primeval soup paradigm is self-deception on the ideology of its champions.… The history of science shows that a paradigm, once it has achieved the status of acceptance (and is incorporated in textbooks) and regardless of its failures, is declared invalid only when a new paradigm is available to replace it. Nevertheless, in order to make progress in science, it is necessary to clear the decks, so to speak, of failed paradigms. This must be done even if this leaves the decks entirely clear and no paradigms survive. It is a characteristic of the true believer in religion, philosophy and ideology that he must have a set of beliefs, come what may (Hoffer, 1951). Belief in a primeval soup on the grounds that no other paradigm is available is an example of the logical fallacy of the false alternative. In science it is a virtue to acknowledge ignorance. This has been universally the case in the history of science as Kuhn (1970) has discussed in detail. There is no reason that this should be different in the research on the origin of life.
Yockey's publications have become favorites to quote among creationists, though he is not a creationist himself (as noted in this 1995 email).
Abiogenic synthesis of key chemicals
A number of problems with the RNA world hypothesis remain. There are no known chemical pathways for the abiogenic synthesis of nucleotides from pyrimidine nucleobases cytosine and uracil under prebiotic conditions. Other problems are the difficulty of nucleoside synthesis (from ribose and nucleobase), ligating nucleosides with phosphate to form the RNA backbone, and the short lifetime of the nucleoside molecules, especially cytosine which is prone to hydrolysis. Recent experiments also suggest that the original estimates of the size of an RNA molecule capable of self-replication were most probably vast underestimates. More-modern forms of the RNA World theory propose that a simpler molecule was capable of self-replication (that other "World" then evolved over time to produce the RNA World). At this time however, the various hypotheses have incomplete evidence supporting them. Many of them can be simulated and tested in the lab, but a lack of undisturbed sedimentary rock from that early in Earth's history leaves few opportunities to test this hypothesis robustly.
Another unsolved issue in chemical evolution is the origin of homochirality, i.e. all building blocks in living organisms having the same "handedness" (amino acids being left-handed, nucleic acid sugars (ribose and deoxyribose) being right-handed, and chiral phosphoglycerides). Chiral molecules can be synthesized, but in the absence of a chiral source or a chiral catalyst are formed in a 50/50 mixture of both enantiomers. This is called a racemic mixture. However, homochirality is essential for the formation of functional ribozymes and proteins. Proper formation is impeded by the very presence of right-handed amino acids and/or left-handed sugars in that they create malformed structures.
Clark has suggested that homochirality may have started in space, as the studies of the amino acids on the Murchison meteorite showed L-analine to be more than twice as frequent as its D form, and L-glutamic acid was more than 3 times prevalent than its D counterpart. It is suggested that polarised light has the power to destroy one enantiomer within the proto-planetary disk. Once established, chirality would be selected for.
Work performed in 2003 by scientists at Purdue identified the amino acid serine as being a probable root cause of the organic molecules' homochirality. Serine forms particularly strong bonds with amino acids of the same chirality, resulting in a cluster of eight molecules that must be all right-handed or left-handed. This property stands in contrast with other amino acids which are able to form weak bonds with amino acids of opposite chirality. Although the mystery of why left-handed serine became dominant is still unsolved, this result suggests an answer to the question of chiral transmission: how organic molecules of one chirality maintain dominance once asymmetry is established.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Origin_of_life". A list of authors is available in Wikipedia.|