Jump to content

User:Extremophile/Abiogenesis

From Wikipedia, the free encyclopedia
Explanation: this is a sketch and/or a source of raw material for improvement of the current abiogenesis article.

It have originally been made to the improvement of the abiogenesis' article of the Wikipedia in Portuguese, so it's not well written.

--Extremophile 04:03, 28 February 2006 (UTC)


Abiogenesis (Greek a-bio-genesis, "non biological origins") is, in its most general sense, the generation of life from non-living matter. Today the term is primarily used to refer to theories about the chemical origin of life, such as from a primordial soup. Earlier notions of abiogenesis, long discarded by science, are now more commonly known as spontaneous generation, held that living organisms are generated by decaying organic substances, e.g. that mice spontaneously appear in stored grain or maggots spontaneously appear in meat.

Earlier ideas on spontaneous generation

[edit]

Chronology

  • ADD more here
  • 1668 Redi - demonstrations with meat in sealed tubes
  • 1683 Leeuwenhoek - discovers microbes
  • 1877 Cohn - shows the existence heat-resistant endospores of bacteria
  • Complete here


First known proponents and defenders of various different ideas in the sense of spontaneous generation were Anaximander, Anaximenes, Xenophanes, Parmenides, Empedocles, Democritus and Anaxagoras

The most influent proponent of spontaneous generation in ancient times though was Aristotle, who supposed the existence of a "vital heat" within some portions of matter. In his Generation of Animals Bk 3, ch 11, he writes:

All those which do not bud off or 'spawn' are spontaneously generated. Now all things formed in this way, whether in earth or water, manifestly come into being in connexion with putrefaction and an admixture of rain-water. For as the sweet is separated off into the matter which is forming, the residue of the mixture takes such a form. Nothing comes into being by putrefying, but by concocting; putrefaction and the thing putrefied is only a residue of that which is concocted. For nothing comes into being out of the whole of anything, any more than in the products of art; if it did art would have nothing to do, but as it is in the one case art removes the useless material, in the other Nature does so. Animals and plants come into being in earth and in liquid because there is water in earth, and air in water, and in all air is vital heat so that in a sense all things are full of soul. Therefore living things form quickly whenever this air and vital heat are enclosed in anything. When they are so enclosed, the corporeal liquids being heated, there arises as it were a frothy bubble. Whether what is forming is to be more or less honourable in kind depends on the embracing of the psychical principle; this again depends on the medium in which the generation takes place and the material which is included.

These basic concepts lasted for a long time. In the 8th century, there was the popular belief that some gooses were born certain coastal trees; they've accounted that some watermelon-like fruits contained completely formed sheep inside.

In the 16th century, Paracelsus described many observations related with spontaneous generation of animals such as frogs, rats, eels, and turtles, from sources such as water, air, timber, among others.


Alexander Ross, in commenting on Sir Thomas Browne's doubt as to "whether mice may be bred by putrefaction", gives a clear statement of the common opinion on abiogenesis held until about two centuries ago. Ross wrote:

So may he (Sir Thomas Browne) doubt whether in cheese and timber worms are generated; or if beetles and wasps in cows' dung; or if butterflies, locusts, grasshoppers, shellfish, snails, eels, and such like, be procreated of putrefied matter, which is apt to receive the form of that creature to which it is by formative power disposed. To question this is to question reason, sense and experience. If he doubts 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



J. B. Van Helmont wrote a recipe to spontaneously generate mice in 21 days: it consisted in left in some place, some dirty clothes (the "vital heat", the "living principle" would be on it's sweat), and wheat seeds

Spontaneous generation was accepted by theologians such as Thomas Aquinas, and scientist such as Isaac Newton and William Harvey



17th century: Redi

[edit]

In the 17th century was still believed that small creatures, from small "worms" (which were in fact maggots) to rats could arise spontaneous from nonliving matter, even though that the ultimate origin of life was attributed to divine powers.

The Tuscan natural philosopher Francesco Redi (1626-1697) believed that the living beings came only from other living beings, what he called homogenesis in the cases which the generated offspring were of a equal kind of its progenitor, and heterogenesis when the offspring was of another kind


Developing notes/troubles

  • I've read somewhere, perhaps it's listed on the references, that although Redi disproved that worms came from putrefying meat, he believed that gall insects were spontaneously generated, which was not much later disproved by some others. But I guess that gall insects might be an example of what he called "heterogenesis", rather than spontaneous generation per se. Heterogenesis, though, was later used by others to express spontaneous generation; so the distinction between the terms and beliefs can be confused

In 1668 Redi conducted experiments in order to prove that the worms wouldn't appear in meat that was made inaccessible to flies, hindering them to lay their eggs on that. Redi said:

Even though I would feel glad by being corrected for someone wiser than me if I made erroneous claims, I shall express my conviction that the Earth, after producing the first plants and animals, on command of the Supreme and Omnipotent Creator, never again produced any kind of plant or animal, neither perfect or imperfect ones...

Redi has noted that the method of sealing the tubes with stoppers was faulty, since it would hinder not only flies but the air and the "vital principle" carried in the air. He enhanced the experiment, stopping the flasks with gauze, which allowed to air to pass, but not the flies. The result was the same; although "worms" did not raise inside the meat, many maggots appeared outside the gauze, trying to force the entrance, and they were removed by Redi.

Despite of these repetition of the test appear to be a genuine scientific approach, Redi also said that did these "in order to be sure of that I'm almost sure". In that moment of history were not made so much for methodical accuracy, but in attempting to show what would be in accord with the expectations of the court and the patricians, his potential patrons. Not rarely the Grand Duke himself would suggest the experiments to be made, besides having the power to give the approval of the experiments and strongly interfering with the consequent debates.

In this scenario, as well as in the following centuries with the experiments conducted by Spallanzani, Pasteur and others, lead to the strengthen of the concept of vitalism - the idea that only the tissues of living beings contained something like the Aristotelian "living principle", making it fundamentally different from nonliving, or "inanimate" matter, unable to do so. At this moment, both the ideas of spontaneous generation and the opposing point of view were vitalists.

18th Century: Needham and Spallanzani

[edit]

The debate over spontaneous generation was then divided between the epigeneticists - defending spontaneous generation, and as Aristotle, that the embryo was formed by a external organizing force from undifferentiated matter - and the preformationists who defended that the organization in the development was already present in a miniature of the adult organism already present in each gamete, all that happened was simple growth.

These points of view had religious and philosophical implication interfering on the acceptance. The French naturalist Comte de Buffon (1713-1781) and the English Catholic priest John Turberville Needham defended that spontaneous generation would weaken the belief in a Cartesian mechanical universe. Lazzaro Spallanzani (1729-1799) eventually joined the preformationists, even though he probably could see with his microscope that the gametes did not contain miniatures of adult organisms. Voltaire, among many others, believed that Needham's position strengthened atheism and materialism, and were a menace to the religious beliefs in a supernatural creation.

Despite of the earlier demonstrations having convinced in great extent that spontaneous generation did not occur with relatively big organisms, the invention and improvements of the microscope renewed the acceptance of the spontaneous generation of smallest organisms. In 1683 Antoni van Leeuwenhoek (1632-1723) discovered bacteria, and it was soon found that however carefully organic matter might be protected by screens, or by being placed in stoppered receptacles, putrefaction set in, and was invariably accompanied by the appearance of myriad bacteria and other low organisms. As knowledge of microscopic forms of life increased, so the apparent possibilities of abiogenesis increased, and it became a tempting hypothesis that whilst the higher forms of life arose only by generation from their kind, there was a perpetual abiogenetic fount by which the first steps in the evolution of living organisms continued to arise, under suitable conditions, from inorganic matter.

Around 1778, Comte de Buffon and Needham confronted the works of Louis Joblot (164-1723) that apparently refuted the spontaneous generation of microbes. The experiment consisted in boiling in a aqueous medium in which microbes could survive, and put half of the liquid in sealed flasks, in which microbes wouldn't appear, and the other half in open flasks, showing that the liquid was able to support microbial life despite of being boiled earlier.

Needham in 1745 repeated Joblot's experiments, but in this case, after several days, the flasks were full of microbes, even the sealed ones. Basing upon these results, Needham concluded that in each part of matter there was a "vegetative force" as predicted by Buffon's epigenetic theory for the origin of larger organisms.

In 1768 Spallanzani criticized Needham's experiments and his accepted theory as well. Opposed by doing similar experiments, but boiling the flasks with broth for more time, nearly one hour. Examining the flasks after several days, he could not found signs of microbial life, and suggested that Needham had failed by not heating enough or for long enough to kill all the microbes that already were in the mixture.

Needham replied that by heating the liquids in such high temperatures it could be inflicting some sort of damage, making it unable to sustain life, which was answered by Spallanzani with the removal of the sealing of the flasks, which soon were populated by microbes.

Despite of the criticizing that could be done about these experiments, they were undeniable advanced for that time, raising questions that couldn't be satisfactorily answered with the technology they had, then the question remained unsolved, and spontaneous generation continued being accepted by public opinion. Many others, such as Theodor Schwann (1810-1882), Joseph-Louis Gay-Lussac (1778-1850), Franz Schultze (1815-1873), Heinrich Schroder (1810-1885) and Theodor von Dusch (1824-1890) did experiments and theories about the issue, always with conflicting results.

Spallanzani's results were never the less applied in food industry by Nicholas Appert (1750-1841), founding the industry of hygienically canned food.

19th Century

[edit]

dev notes: apparently wasn't necessary to mention anything about the germ theory. Anyway, I think that something about F. Wöhler synthesising urea abiotically in 1828 could be added somewhere

Pasteur and Pouchet

[edit]

Spontaneous generation was the basis of Jean Baptiste Lamarck's (1744-1829) theory of transmutation of species (1809), where contrarily to the idea of common ancestry defended later by Charles Darwin's book "The Origin of Species" (1859), each species was supposed to have been spontaneously generated independently, without biological relationship with other species. According with Lamarck, the nature was always creating, since de earlier ages; the water, under influence of light, certain elements, heat and electricity, entered tiny bodies of slimy mass starting a rudimentary metabolic process; would then evolve by the principles he postulated.

At that time, Félix Archimède Pouchet (1800-1872), director of the Rouen's National History Museum, defended a new version of spontaneous generation. Differently from what would be expected fro a defender of epigenesis, he defended not a materialist point of view, in which the spontaneity of the generation implied randomness, or that was uncontrolled. For Pouchet, the phenomena had divine intervention; he called this version "law of heterogenesis", which wasn't solely divinely guided, but was something that increased the "Divine Majesty".

Pouchet didn't accepted the spontaneous generation of adult animals; to him, all animals were result from the development of their eggs. The eggs, on their side, could be spontaneously generated, and didn't existed only as a consequence of sexual reproduction. Sexual reproduction was only a circumstance where the living beings induced spontaneous generation through their inner vital force.

In 1862 the French Louis Pasteur (1822-1895) won the prize offered by the French Academy of Sciences, offered to someone who could "put new light on the question of the so called spontaneous generation", with his descriptive essays of series of experiments he did in order to prove that Pouchet's experiments were flawed: the mercury he used to cooling the flasks could bring dust carrying microbes that would proliferate, creating the illusion of spontaneous generation. Besides that, he agreed that the air and the water were adequately sterilized by Pouchet. Pasteur then formulated his classic experiments with swan-necked flasks, projected to permit the air flow at the same time that the curves in the neck trapped the dust and the germs carried on it. By doing that, he was showing that the ability of the liquid to sustain life wasn't affected by boiling it, since microorganisms would proliferate there if the flask was dipped, bringing the dust and the germs on it to the liquid. The hypothesis that the absence of a "living principle" carried by the air, raised much earlier by Needham could also be discarded, as well as the idea that it lacked the necessary oxygen, as hypothesized by Theodor Schwann (1810-1882). The swan-necked flask remained in that conditions, free from germs, for about one year and a half

Dev. note: the last claim, about the flask being germ free for so long, is from an old book, and is a bit conflicting with the new information that follows, although not necessarily wrong.

The debate and the experiments continued for some time, and despite Pouchet's claims that there was tendentiousness in the evaluations, and lack of rigour in the proceedings, Pasteur turned out considered as the winner, specially in France, and that he refuted spontaneous generation was taken as a fact by most. Pouched pointed that for example, Pasteur never replicated the experiment with the swan-necked flask with hay infusions, which would contain, without the awareness of both of them at that time, heat-resistant endospores of Bacillus subtilis. That was the most convincing og Pouchet's experiments, and probably if Pasteur had conducted it, he wouldn't obtain results consistent with the results he was publishing.

It was only in the 1970s that Pasteur's notebooks were made available to the historians. Then was finally known that Pouchet's claims were justified, since only about 10% of Pasteur's experiments were according to the results published by him; conflicting results were always seen as failures, not as confirmation of spontaneous generation.

As in Needham and Spallanzani's time, the question had considerable influence of ideological and political positions held by many. Yet that that in Charles Darwin's theory of evolution spontaneous generation wasn't suggested as necessary, both ideas became associated by being considered as menaces to the dominant religious viewpoints. Besides that, Pasteur also had ideas that may have been more decisive in his opposition to Pouchet than the question of spontaneous generation itself. In consequence of his studies with crystals, Pasteur developed a belief that life depended on a "asymmetrical cosmic force". Since Pouchet wasn't aware of these ideas, Pasteur concluded that he ought to be wrong as he wouldn't be approaching the issue with that in mind.

With political reasons, related with the conservatism of Pasteur's most important patrons, Pasteur publically refuted Pouchet's experiments, whilst secretly conducted his own experiments attempting to spontaneous generate life: he believed, for example, that parasitic worms generated spontaneously.

The British debate

[edit]

Charles Darwin's "The Origin of Species" was publishe in the same year than Pasteur's results, yet having less acceptance on France, than in other countries. At the same time, spontaneous generation was yet thought as a plausible idea on the countries where "The Origin of Species" was also more influent, yet both things are not directly related with each other, since Darwinians disagreed among themselves about spontaneous generation.

Darwin denied that his theory of evolution required a continuous spontaneous generation of live from nonliving matter, and that it still occurred to the present day, and perceived basic impediments to that, as he mentioned in a letter to his friend, the botanist Joseph Hooker, in 1871:

It is often said that all the conditions for the first production of a living organism are now present, which could ever have been present. But if (and oh! what a big if!) we could conceive in some warm little pond, with all sorts of ammonia and phosphoric salts, lights, heat, electricity, etc. present, that a protein compound was chemically formed ready to undergo still more complex changes, at the present day such matter would be instantly devoured or absorbed, which would not have been the case before living creatures were formed..

Darwin saw the so called primitive organisms, not as evidence of recently spontaneous generated life, bur simply as organisms that remained adapted conditions that persist to today. Never the less, he wasn't so talkative about the subject in his publications, mentioning that the question of abiogenesis, if ever will be solved, will be in the future. About heterogenesis, he said that "mass of mud with matter decaying and undergoing complex chemical changes is a fine hiding-place for obscurity of ideas".

Thomas Henry Huxley, a famous defender of Darwin's theory, supposed that life had formed originally in a cellular fluid called "protoplasm", similar to to the Urschleim (original slime, in German) proposed by Ernst Haeckel. Eventually Huxley reanalyzed some samples of the Atlantic floor, and saw what he thought that could be evidence of something similar to this "protoplasm", and named it Bathybius haeckelii, in honor to Haeckel. While Haeckel believed, among other such as Peregrin Casanova, that Bathybius corresponded exactly to his theory, Huxley supposed that it was just an organic substance that covered all the oceanic floor. During the Challenger expedition, more samples from oceanic floor were collected. The samples shown no sign of the presence of Bathybius at the collecting time, but seemed to grow in the containers on the way to England, where it was bring to analyses.

Evolutionists such as Herbert Spencer and William Thistleton-Dyer opposed to the idea that even something considerably simpler like the "protoplasm" could arise spontaneously from inorganic matter, arguing that even such things were too complex. John Young Buchanan showed that Bathybius couldn't be related with the origin of life, discovering that it was just a precipitate of calcium sulfate that reacted with the alcohol used as preservative of the samples. Huxley readily admitted the mistake and the responsibility, whilst Haeckel only gave up later defending it, since it looked a great evidence of his theory.

William Thomson (later, Lord Kelvin), around 1871 theorized that the Earth was only about 100 000 000 years old (what later was shown to be wrong), and believed that it didn't left time enough for natural selection guided evolution. To avoid the problem of the origin of life, he suggested one theory of panspermia - the ida that the life was originally from space. Wilhelm Preyer, professor of University of Jena, also defended panspermia. To him, there was no logical problem with spontaneous generation happening to this day, supposing it have happened once in the distant past; he thought that if that ever happened, would abound evidences of its occurrence, since it would still occur in the present day. He objected the idea that life could have arisen only under a highly different environment in the remote past, reasoning that life wouldn't survive such radical changes from the environment that originated it. He supposed then that the universe and life were eternal.

Henry Charlton Bastian (1837-1915) was one of the Darwinists in disagreement with Charles Darwin in relation with spontaneous generation. Bastian called the ultimate origin of life "archebiosis", from where came the common ancestor of most of the lifeforms, but also believed that bacteria found in ill people were spontaneously generated - a form of spontaneous generation he called biogenesis. He conducted hundreds of experiments, similar in the idea, but with conflicting results, to those made by Pasteur.

Bastian had strong opposition with all of those in agreement with the results published by Pasteur - all the defenders of the germ theory of contagion - but mainly from other Darwinists such as Huxley, and the Irish physicist, John Tyndall (1820-1893).

Tyndal developed a clever method to microbial detection, as an attempt to refutation of Bastian's results. According to hum, a well focused light beam was a better test to detect material in suspension, such as dust carrying microbes, as was earlier shown by Pasteur. His apparatus consisted basically in a wooden chamber that permitted focus a light beam over the substance to be verified; if were particles in suspension, those would be easily seen as dust in suspension is seen in the light that enters a window in a sunny day. Flasks systems and tubes showed the causal relation of the particles in suspension and the subsequent proliferation of microbes in the substances. With this apparatus, he turned out discovering that most part of dust is composed by organic material.

His data supported the germ theory of disease, that wasn't yet well accepted by most of the British doctors. Tyndall as criticized by many who said that a physician, dealing in a biologist's area, should get to know better the facts about contagious diseases before doing such speculations. Bastian questioned the evidences raised by Tyndall in many letters sent to Times magazine. Huxley and Tyndall joined in an attempt to convince their collages that Bastian was either a fraud or incompetent in the conduct of experiments. Tyndall also believed, however, that microbes could somehow survive the boiling of the medium.

Huxley, in September of 1870, gave a speech to the British Association in Liverpool where he redefined in a simplifying manner the terms of the debate: biogenesis now meant life coming from life (the opposite of Bastian's original acceptation of the same term); and coincide abiogenesis, meaning the ultimate origin of life, which, according to him, could only have occurred in earlier conditions of the Earth, distinguishing it from the type of spontaneous generation that some supposed that could yet occur in present conditions.

by 1876 Tyndall is troublesome, Cohn Helps (develop)


20th Century: search for the origin of life

[edit]

Main article: Origin of life

The debate gained new energy with the development of biochemistry in the early 1900, after a period of considerable stagnation after the 1880s. In 1921, Ben Moore, the first biochemistry teacher of Liverpool University said that wouldn't be at the level of bacteria that the debate of the origin of life should be discussed, but a deeper level than what is possible to be observed through microscopes, below the scale of the living cells.

The knowledge acquired in this interval was enough to eliminate more and more the question of the Aristotelian concept of spontaneous generation, heterogenesis and etc. The evidences and new discoveries tended to favour progressively the side of te materialists, which, opposing to the vitalists defended that life was nothing more than specified chemical organization, without the requirement of something special such as a "vital force".

Dev. note: I recall that Julian Huxley has an interesting quote on the subject, something that a locomotive doesn't have a "force locomotiff"

It was progressively more consensual that the chemical origin of life - nowadays also called biopoiesis, and yet, biogenesis, by Teilhard de Chardin, but most commonly, abiogenesis - according to the thoughts of Darwin and others, would have occurred in a remote epoch, under very different conditions, requiring a larger time period than what is suggested by the term "spontaneous", nothing expected to happen in daily situations. Life would have had a more singular origin, from a gradual process with many stages. At these stages, "life" would probably differ much from the present lifeforms at the point of making difficult and uncertain and arbitrary its classification as "life", in a manner similar to that which the virus and prions] are found today.

Timeline The timeline of the main events related to the theorization about the chemical origin for life:

  • 1871 Charles Darwin - prebiotic pound
  • 1917 Leonard Troland - "genetic enzymes"
  • 1917 Oparin - prebiotic soup, metabolic start
  • 1929 J. B. S. Haldane - prebiotic soup, genetic approach
  • 1953 Urey/Miller - amino acid synthesis under simulated conditions of the hypothesised atmosphere
  • 1953 Watson/Crick - discover of DNA structure
  • 195? J. D. Bernal - bigger polymers being formed with the aid of clay surfaces
  • 1950-1960 Fox - microspheres
  • 1970 Eigen - hypercycles
  • 1980 A. G. Cairns-Smith - genetic takeover
  • 1982 Thomas Cech - ribozymes
  • 1986 Günter Wachtershauser - iron-sulfur world

At the beginning of the 20th Century, was thought that viruses were proteins, most specifically, enzymes, due to their catalytic properties - the ability of inciting chemical reactions without being chemically altered during the reactions. In 1914 the Harvard biochemist Leonard Thompson Troland, influenced by the geneticist T. H. Morgan, hypothesized that the first life form would have formed randomly in the primordial oceans, and would be a simple protein with the ability of catalyzing its own replication. Since it arises, it would make a chain reaction originating a enormous population of its own copies. Troland, as earlier as 1917, has suggested that the nucleic acids, and not the proteins, that carried the genetic material, something that was confirmed in the following decades.

To Felix D'Hérelle, viruses, especially the bacteriophages, were a trouble for the cell theory - the theory that the minimal unity of life is the cell - because they definitely were not cells, but at the same time they behave as living beings while parasiting bacteria. For him, the first living being would also had been a virus, of proteic composition in the form of micella - a composition of a hypothetical molecular aggregate, structural part of the "protoplasm".

Around 1900, the Mendelian genetics had been rediscovered by scientists such as Hugo de Vries, Carl Correns and Erich von Tschermak, after a period of obscurity. The American geneticist Hermann Joseph Muller, in 1921, put viruses as being almost the same thing, little more than genes 

Wendell Stanley



Oparin-Haldane hypothesis

[edit]

Yet on the decade of 1920 were developed the theories that became vulgarly known as theories of a "primordial soup", independently, by Aleksandr Ivanovitch Oparin, on Russia in 1924 and by J. B. S. Haldane in Great Britain, in 1929 hypothesised independently (since Oparin's book was not translated to English until 1936) that a series of reactions involving the supposed atmospheric chemistry of the primordial Earth would culminate in the origin of life.

Oparin's theory starts with hypothesis to the origin of the solar system and of the planets, as determinants of the Earth's atmosphere. In the primitive Earth, volcanic eruptions would bring hydrogen and ammonia that would be transformed by atmospheric events in larger compounds that would accumulate in the oceans, eventually forming colloid aggregates, or coacervates. According with studies by H. H. Bungenburg de Jong in 1932, coacervates were shown as capable of selective absorption of substances, analogously to those made by cell's membranes. Oparin thought that therefore, they could "feed" roughly of other organic compounds and even of other coacervates, and that this abilities would eventually lead to the catalysis of and maintenance of a primitive metabolism.

The coacervates wouldn't be living organisms already, but they would be forming in enormous quantities, growing and eventually fragmenting. The fragments would be of similar to equal chemical composition of its originating coacervate, thus probably maintaining all its properties and inner reactions. That would be a imperfect process of reproduction; the imperfection of the reproduction generates the variation that would have been enough to a rudimentary natural selection occur. From the process of selection of these coacervates would eventually arise a extremely primitive lifeform.

Haldane supposed that the primordial oceans were analog to a immense chemical lab, powered by sunlight. In the atmosphere, the gases and the UV radiation would originate the organic compounds, and over the sea would form a "warm soup" with enormous quantities of monomers and polymers. Groups of these molecules would acquire lipid membranes, and further evolution would eventually lead to the first living cells.

Oparin-Haldane theory came to be widely accepted by the 1950s, due to the support given by names such as Norman H. Horowitz, John Desmond Bernal, among others. By that time, with the confirmation that was DNA that really carried the genes, Oparin sought to improve the hypothesis, adding the appearance of genes in this scenario of a pre-existing metabolism.

The hypothesis received experimental support when in 1953 Stanley Miller conducted the Miller-Urey experiment, as a test of a theory put forward by his professor, the Nobel literate Harold C. Urey. Urey proposed that the reductive atmosphere of the primitive Earth would be propitious for the origin of simple organic compounds. The experiment was initially proposed without any of them being aware that it had much to do with Oparin's theory, which only were analysed by them at the time of the preparatives for the experiment, along with Urey's own ideas about the origin of the solar system. The experiment resulted in the origin of many amino acids, basic bricks of life. Later, in 1961, Joan Oró conducted similar experiments obtaining nucleotides.

Dev. note: need something about how the earlier atmosphere wasn't how they thought it was, but more experiments were reasonably successful (yet that less successful) obtaining the compounds.

Other ideas and discoveries

[edit]

It was hypothesised by some scientists that the DNA molecule stored the genetic information, what was accepted by 1940. Later, in 1953, Francis Crick and James Watson discovered the structure of the molecule, increasing the knowledge about the pieces of the known forms of life, and that maybe were related with abiogenesis. The considerably large size of these polymers was to many a trouble for their formation solely by the mechanisms proposed by Oparin and Haldane.

Benal suggested, and was later more elaborated by others sch as Miller and A. G. Cairns-Smith, that tidal lagoons and ponds could have been a place where those molecules could be formed by help of a clay surface, whose chemical properties would aid the building of lager molecules from smaller molecules concentrating there, at the same time that it protected those from UV rays, which could be a menace to their integrity at this stage.

Sidney W. Fox, from the 1950s to 1960, conducted experiments in which amino acids united forming proteinoids - polypeptides similar to proteins - by means of heating, as was supposed to eventually occur in the primeval Earth. In the following work, was shown that these amino acids and small polypeptides could form closed spherical membranes, called then microspheres. Fox described these as formations of protocells, believing that they were an important intermediate step in the way to the origin of life. The microspheres had inside its membrane a aqueous mean, that had shown movement similar to cyclosis. They were able to absorb other molecules present in the environment; could form larger structures by fusing with each other, and in certain situations, protuberances could emerge and detach, and then grow individually.

Thomas R. Cech, in the 1980s, discovered the ribozymes, RNA molecules capable of enzymatic activity, besides storage and transmission of genetic information. With all those abilities, the RNA could have had the ability of self-replication. This was the basis of the RNA world hypothesis, elaborated by the Harvard biologist Walter Gilbert in 1986, based on the suggestion made by Crick as early as 1968, about RNA being the first molecule of life. According to the theory, the primitive life would have been composed initially of RNA molecules, and would eventually evolve, ceding the functions of long-term genetic storage to DNA, and of enzymatic activity to proteins. It's similar to 1914's Troland hypothesis, except that now having knowledge of a molecule with the necessary properties. Additionally, the random origin of auto-catalysis could also be largely discarded due to the mathematical work of Manfred Eigen and Peter Schuster, on their 1979's description of a theory of chemical hypercycles, a explanation for self organization of chemical substances in prebiotic systems, on the way to reproduction.

In the decade of 1980, Günter Wächtershäuser also developed the influent iron-sulfur world theory, where the primitive chemistry of life would not have occurred in the oceanic surface, but in mineral surfaces, such as pyrites, next to deep submarine vents, where the earlier cells would have been lipid bubbles on the mineral surface. In this autotrophic origin, the earlier metabolism would antecede the genetic, and once it was established would permit the synthesis of molecules of increasing complexity.

Yet on the 1950s, Oparin and others started to see a strong relationship between abiogenesis and astrobiology. The knowledge of how could have originated on Earth, is the key to know in which other planets could exist life. This relationship strengthened with the hypothesis of outer space origin of organic molecules, which was confirmed later. The increasing complexity of molecules found in space could be seen as a source of organic material to an terrestrial abiogenesis, but also reinforced the possibility of extraterrestrial life, and even of panspermia to some such Fred Hoyle and Chandra Wickramasinghe.

dev note: I think that also worth develop something about the works of Sol Spiegelman and Thomas Gold. Spiegalman had made experiments with RNA, most importantly one in which a incredibly small RNA virus evolved in a extremely hospitable environment, by natural selection of minimal viruses; and other in which a RNA virus was randomly created, by throwing non-viral pieces of RNA in a propitious mixture: some pieces just assembled together without replication abilities, but eventually one happened to has this ability. Thomas Gold had interesting ideas of subterranean abiogenesis.


References

[edit]

This article may incorporate parts of other Wikipedia articles, licenced under GNUFDL:

  • Abiogenesis. Wikipedia, The Free Encyclopedia. Artigo de 20 de fevereiro de 2006.
  • History of genetics. Artigo de 27 de fevereiro de 2006.
  • Bathybius. Wikipedia, The Free Encyclopedia. Artigo de 26 de fevereiro de 2006.

eigen RNA world Iron-sulfur world

Other references:

Retrieved from "https://en.wikipedia.org/w/index.php?title=User:Extremophile/Abiogenesis&oldid=482979878"