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Thread: Abiogenesis: An Introduction About the Origin of Life.

  1. #1 Abiogenesis: An Introduction About the Origin of Life. 
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    Abiogenesis: An Introduction About the Origin of Life

    Abiogenesis
    is
    the term given to the process of life arising from simple organic compounds.

    A great deal of study has been dedicated to elucidate this process.
    If you are familiar with the Tree of Life concept from biology, then picture it as looking for the roots without the possibility of digging into the soil. Scientists have been searching for almost one hundred years and they have postulated numerous roots (viz. hypotheses) that try to account for the origin of life. Although we have gained new insights, there is still a great deal of uncertainty in many aspects of the field.

    With this thread, I attempt to provide a scientific overview about the origin of life.

    This thread contains models that rely heavily on biochemistry, so it is time for Biochemistry 101.

    When studying living things from a chemical viewpoint, biochemists roughly distinguish four classes of bio-molecules: amino acids, sugars, nucleotides and lipids.
    • Amino acids are the building blocks of proteins. In biological systems, there are 20 different amino acids that make up every protein (e.g. hemoglobin). A protein is a chain (or a collection of chains) of at least 20-30 amino acids, with a specific form and function. Some of these proteins have a catalytic function, i.e. increasing the rate of chemical reactions (this is the conversion of molecule A to molecule B). These proteins are called enzymes. Enzymes are also needed when DNA needs to be replicated or broken down.

    • Sugars are the carbohydrate units that make up larger structures (e.g. cellulose, starch). Carbohydrates are molecules following the pattern (CH2O)n with 2<n<8. Although they are important in biochemical pathways (e.g. photosynthesis), two of them (ribose and deoxyribose) are important because they are key components in the backbone of RNA and DNA.

    • Nucleobases form, together with (deoxy)ribose and phosphate (PO43-), the nucleotides: building blocks of the genetic information carriers DNA and RNA. As I will explain later, the formation of nucleotides under prebiotic conditions has been (and still is) a struggle for organic chemistry.

    • Lipids are the building blocks of membranes. Lipids can either shun water ('hydrophobic') or shun and like water ('amphiphilic') due to their structure (an 'hydrophobic' and an 'hydrophilic' part). An example of the former is cholesterol, whilst phospholipids (which make up i.a. cellular membranes) are an example of the latter.[1]


    Part I: Extraterrestrial:

    Panspermia is a collection of hypotheses that attempt to explain the emergence of life on Earth with the idea that it was transported from another place in the universe. It proposes that biological material has reached the Earth via impact-expelled rocks from other planets, either from the same solar system (ballistic panspermia) or from different solar systems (lithospermia).
    This may sound as far-fetched as Ancient Aliens ideas, but this is certainly not the case!

    Panspermia is based on roughly two major lines of evidence: chemical analyses of solid astronomical objects and interstellar chemistry:

    • Solid astronomical objects are e.g. meteorites (debris of asteroids) and comets (icy small bodies).
      Carbon-containing meteorites (e.g. Murchison meteorite) contain organic materials that preceded life's origin (e.g. PAHs, see below)
      [2]. Research[3] showed that the mineral surface of the Murchison meteorite was able to catalyze the formation of amino acids, nucleobases, etc. Although asteroid strikes were abundant during the Late Heavy Bombardment (4.1-3.8 billion years ago) and thus caused the abundant delivery of materials, it is not yet known if these or similar materials might have fostered or even initiated molecular evolution towards life[4].

      Comets
      have been analysed to determine their composition. Ehrenfreund, P. et al. lists several analyses, which all point out that small CHON molecules (e.g. cyanide, methanol) are present in some comets in a large 'pool' of H2O. It is possible that comets, when they collided with the young Earth, provided water and thus a medium in which life could come into being.


    • Interstellar chemistry is another line of evidence. The formation and presence of certain biological significant chemicals (e.g. aldehydes, alcohols, etc.)[5] in molecular clouds is an active field of research. A molecular cloud (MC) is, according to the Free Dictionary, a cool dense interstellar region composed of a wide variety of molecules, mainly hydrogen, plus some dust, in which stars are forming. These clouds are affected by cosmic and UV radiation which provide the energy necessary to initiate activation of many chemical reactions between different molecules. This may lead to new and possibly more complex molecules. One of the many examples are polycyclic aromatic hydrocarbons (PAHs).


    Organisms have yet to be found in extraterrestrial material, but organic molecules are abundant in meteorites, on planets (e.g. Mars), moons (e.g. Titan) and in MCs. This argues in favor of
    pseudo-panspermia, the delivery of organic compounds from space, to give the prebiotic soup some starter ingredients.

    It might be possible that MCs are places where abiogenesis might have occurred, since they are rich in water and organic compounds and are vast biochemical reactors. However, more investigations are required!
    NASA has planned missions (
    TiME, JUICE) to collect data that might favor the (pseudo-)panspermia hypothesis.
    They also conduct research on these organic molecules on Earth (in the
    Astrophysics and Astrochemistry Laboratory at the Ames Research Center).


    Last edited by Cogito Ergo Sum; May 6th, 2014 at 03:26 PM. Reason: Hyperlinks added.
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    ~ Arthur Schopenhauer, The Art of Being Right: 38 Ways to Win an Argument (1831), Stratagem XXXVIII.
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    Part II: Terrestrial:

    History:
    We must return to the 1920s. Before the structure of DNA and RNA were elucidated, the ideas about the origin of life were merely describing mixtures that could form cell-like structures (coacervates) in a primordial soup. This was known as the Oparin-Haldane hypothesis, named after biochemists Aleksandr Oparin and John Haldane. In 1953, abiogenesis was put to test by Stanley Miller and Harold Urey; they conducted the famous Miller-Urey experiment, which would be replicated multiple times under more representative conditions[6]. That was the start of an experimental approach to the origin of life!

    Nowadays, scientists are trying to figure out how and where genetic material and proteins were synthesized. Some pathways are already described, although the big picture remains uncertain. The search goes on in three main trends: the RNA World ('genes first'), the metabolism-first models and the surface hypotheses.

    RNA and Deep Sea Ideas:
    What came first, DNA or proteins? DNA contains the information that is translated to the build-up of proteins, whilst proteins are necessary for i.a. DNA replication, transcription (copying a DNA sequence to RNA, which contains instructions for protein production) and a whole lot more.

    It was proposed at least two times
    [7][8] that RNA was a likely candidate to look out for, because RNA could carry information like DNA and carry out biochemistry the way proteins do. This idea, called the "RNA World", can thus be summarized as follows:
    “RNAs were the original biological catalysts in precellular times - the so-called RNA world.”[9]

    We are now looking for biochemical reactors, where RNA could have been constructed.
    Hydrothermal vents (chimneys at the oceans floor) are proposed to be RNA-making reactors[10] where PCR-like reactions could occur[11], in other words, a 'vessel' where the amount of a certain bio-molecule could be amplified. It must also be noted that they possess an H+ gradient[12], which can be used as an energy source. In summary, these vents are possible locations for the replication of genetic material and the synthesis of its precursors. Now we must focus on the question how RNA and proteins were formed.

    RNA could have been synthesized from pyrimidine ribonucleotides (T, U, C), which in turn are synthesized by simpler precursors[13] (this solves the issue with cytosine[14]; namely that is broken down faster than it is made). Yet, RNA is a very unstable molecule, so other researchers have proposed RNA-like precursors: simpler molecules that mimic RNA. Examples are P(eptide)NA[15], T(hreofuranosyl)NA[16] and G(lycol)NA[17]. PNA has gained attention because it bonds with DNA[18], has biological significance in current cells[19] and its precursor is detected in microorganisms[20].

    The RNA-World could then be pushed aside by the appearance of DNA (in primitive lipid cells that were much simpler and smaller).
    How this transition occurred, is not yet known, although it has been proposed that viruses might have played a role in this[
    21].

    Yet, more work needs to be done. Leslie Orgel gives a succinct yet excellent summary:
    One must recognize that, despite considerable progress, the problem of the origin of the RNA World is far from being solved.”[22]


    There are voices in the field of abiogenesis stating that RNA could not have been the first molecule, because chemistry has failed to produce RNA under laboratory conditions[23] (remember the struggle of organic chemistry). Moreover, new evidence has indicated that amino acids are also likely candidates for the first molecule, instead of RNA. Amino acids are much more stable and easier to form under prebiotic conditions.
    A study
    [24] showed that it is plausible that they formed the first enzyme that was capable of replicating RNA in a controlled manner. It is also postulated that peptides (tiny proteins) and RNA could have developed together[25] (Peptide-RNA World scenario).


    Last edited by Cogito Ergo Sum; September 2nd, 2014 at 04:28 AM.
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    Metabolism models:
    The third hypothesis is the idea that genes came after metabolism (the collection of (bio)chemical reactions required for an organism to stay alive).

    Since there were no organisms at first, they are referring to chemical reactions that result into certain bio-molecules, e.g. amino acids
    [26]. One of the ideas is the FeS-hypothesis, which proposes that early life may have formed on the surface of iron sulfide minerals via shuttle reactions[27]. Computer models have indicated that are also problems with metabolism-first models[28].

    Other research ventures into the rise of metabolism starting from sugars (as sugar metabolism drive reactions in all modern cells).
    For example, a recent study
    [29] found that many precellular reaction sequences were similar to the modern reaction sequences, hinting that they "could have been constrained by the iron‐rich oceanic environment of the early Archean [4000 - 2500 million years ago]. The origin of metabolism could thus date back to the prebiotic world".

    Surface ideas:
    The previous hypotheses are either focused on the deep sea or outer space. Yet, there are other ideas.

    The surface hypotheses cover all the explanations that try to explain the emergence of life by seeking at the Earths surface.
    Some scientists have left this trail due to the fact that the circumstances on the early Earth were very hostile:
    dramatic temperature fluctuations due to meteor impacts, dust clouds, storms, and intense UV radiation[30]. It is unlikely that life or bio-molecules could thrive under such harsh conditions. For example, a study[31] shows that the formation of RNA nucleotides is troublesome at high temperatures.

    However, recent evidence[32] shows that, in order to account for the intracellular concentrations of ions, cells might have arisen at geothermal fields (e.g. Yellowstone Park). For example, potassium outnumbers sodium in living cells, yet in oceans and lakes, sodium dominates. Other ions, like zinc, magnesium and phosphate are also present in much higher concentrations in modern cells than they are in oceans of past and present. These findings make an ocean-like origin unlikely.

    Another researcher has proposed that radioactive beaches are also likely places
    [33]. In essence, the beaches would have functioned as nuclear power plants, where the energy might have assisted the synthesis of bio-molecules.


    Part III: Summary, Sources and Recommendations:


    In the end, there are also models that do not fit into the previous discussion.
    I left some hypotheses out for two reasons: I want to remain brief and I want to stimulate your curiosity. If this introduction does not satisfy you, I recommend you to go the Wikipedia article about abiogenesis.

    It is clear that organic chemistry, geology, astrochemistry, biochemistry and other scientific fields are slowly filling the gaps of our knowledge about our origin(s).
    Although we will arrive to one (or more) plausible model(s), one cannot replicate what really happened (remember the analogy of the Tree of Life). However, that does not hold us back from doing research! There is nothing in science that challenges us more to find answers than the simple "we do not know".


    "
    The road ahead is hard and long but without doubt it leads to the ultimate knowledge of the nature of life."
    [34]



    My gratitude goes foremost to member Strange, who encouraged me to write this thread. I am also indebted to members mat5592 and PhDemon, who were so kind to take up the role as reviewer. My appreciation goes also towards member Zwirko, for providing useful sources for my Further Reading section, and towards member John Galt, who gave me vital information about Panspermia.
    Last edited by Cogito Ergo Sum; April 27th, 2014 at 11:34 AM.
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    Sources:
    [1] Lodish, H., Kaiser, C.A., Berk, A. et al. (2012), "Molecular Cell Biology, 7th Edition", MacMillan, pp. 33-40
    [2] Wing, M.R. et al. (1991), Orig. Life Evol. Biosph. 21(5-6), pp. 375-383
    [3] Saladino, R. et al. (2011),
    Orig. Life Evol. Biosph. 41(5), pp. 437-451
    [4] Pizzarello, S. et al. (2010), Cold Spring Harb. Perspect. Biol. 2(3)
    [5] Ehrenfreund, P. et al. (2000), Annu. Rev. Astron. Astrophys. 38, pp. 427-483
    [6] Cleaves, H.J. et al. (2008),
    Orig. Life Evol. Biosph. 38(2), pp. 105-115
    [7] Crick, F.H.C. (1968), J. Mol. Bio. 38, pp. 367-379
    [8] Gilbert, W. (1986), Nature 319(6055), p. 618
    [9] Voet, J.G., Voet D., Pratt, C.W. (2012), "Principles of Biochemistry, 4th Edition", Wiley, p. 844
    [10] Koonin, E.V. (2007), PNAS 104(22), pp. 9105-9106
    [11] Baaske, P. et al. (2007), PNAS 104(22), pp. 9346-9351
    [12] Martin, W. et al. (2008), Nature Reviews 6, pp. 805-814
    [13] Powner, M.W. et al. (2009), Nature 459, pp. 239-242
    [14] Shapiro, R. (1999), PNAS 96, pp. 4396-4401
    [15] Nelson, K.E. et al. (2000), PNAS 97(8), pp. 3868-3871
    [16] Schöning, K.U. et al. (2000), Science 290(5495), pp. 1347-1351
    [17] Zhang, L. et al. (2005), J. Am. Chem. Soc. 127, pp. 4174-4175
    [18] Egholm, M. et al. (1993), Nature 365, pp. 566-568
    [19] Ray, A. et al. (2000), FASEBJ. 14(9), pp. 1041-1060
    [20] Banack, S.A. et al. (2012), PLoS ONE 7(11), e49043
    [21] Koonin, E.V. et al. (2006), Biol. Direct. 1(29)
    [22] Orgel, L.E. (2004),
    Crit. Rev. Bioch. and Mol. Biol. 39, pp. 99-123
    [23] Gutiérrez-Preciado, A., et al. (2010), Nature Education 3(9):29
    [24] Liam, M. L., et al. (2013), PNAS 113(6), pp. 2135-2139
    [25] Li, L. et al. (2013), Journal of Biol. Chem. 288, pp. 26856-26863
    [26] Huber, C. et al. (2003),
    Science 301(5635), pp. 938-940
    [27] Wächtenshäuser, G. (1988), Microbiol. Rev. 52(4), pp. 452-484
    [28] Vasas, V. et al. (2010), PNAS 107(4), pp. 1470-1475
    [29] Keller, M.A. et al. (2014), Mol. Sys. Biol. 10(4), p. 725-737
    [30] Madigan, M., Martinko, J., Stahl, D. et al. (2012), "
    Brock Biology of Microorganisms, 13th Edition", Pearson, p. 477
    [31] Levy, M. et al. (1998), PNAS 95(14), pp. 7933-7938
    [32] Mulkidjanian, A.Y. et al. (2012), PNAS 109(14), pp. 821-830
    [33] Zachary, A. (2007), Astrobiology 7(6), pp. 852-872
    [34] Oparin, A.I. (1938), "Origin of Life", MacMillan, p. 252


    Further reading:

    Education paper:
    Billings, L. et al. (2006), "The Astrobiology Primer: An Outline of General Knowledge—Version 1, 2006", Astrobiology 6(5), pp. 735-813

    Talk Origins
    articles:

    Musgrave, I. (1998), "Lies, Damned Lies, Statistics, and Probability of Abiogenesis Calculations"
    Moritz, A., (2010), “The Origin of Life”

    Scientific articles:
    Robinson, R. (2005), “Jump-Starting a Cellular World”,
    PLoS Biol. 3
    (11)
    Koonin, E.V. et al. (2005), "On the origin of genomes and cells", Trends in Genetics 21(12), pp. 647-654
    Forterre, P. (2005), "A Story of Viruses and Cells", Biochimie 87, pp. 793-803
    Joyce, G.F. (2010), “Bit by Bit: The Darwinian Basis of Life”, PLoS Biol. 10(5)
    "The Origins of Life", Cold Spring. Harb. Persp. in Biol., 2
    "RNA Worlds", Cold Spring Harb. Persp. in Biol., 2-4

    News articles:
    How Did Life Begin? RNA That Replicates Itself Indefinitely Developed For First Time - ScienceDaily
    Biochemists resurrect 'Molecular Fossils' - ScienceDaily
    RNA controls splicing during gene expression, further evidence for 'RNA World' origin of life - ScienceDaily
    Interplanetary dust particles could deliver water and organics to jump-start life on Earth - ScienceDaily
    Study tests theory that life originated at deep sea vents - ScienceDaily
    New study outlines 'water world' theory of life's origins - ScienceDaily
    Last edited by Cogito Ergo Sum; April 27th, 2014 at 11:31 AM.
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    Excellent synopsis, this should be a sticky!
    I was some of the mud that got to sit up and look around.
    Lucky me. Lucky mud.
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    Quote Originally Posted by GiantEvil View Post
    Excellent synopsis, this should be a sticky!

    Thank you for the kind words!
    I sincerely hope that the staff members also consider the possibility of making this thread sticky.
    "The only safe rule is to dispute only with those of your acquaintance of whom you know that they possess sufficient intelligence and self-respect not to advance absurdities; to appeal to reason and not to authority, and to listen to reason and yield to it; and, finally, to be willing to accept reason even from an opponent, and to be just enough to bear being proved to be in the wrong."

    ~ Arthur Schopenhauer, The Art of Being Right: 38 Ways to Win an Argument (1831), Stratagem XXXVIII.
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    Sticky! Good job, thanks!
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    Disclaimer: I do not declare myself to be an expert on ANY subject. If I state something as fact that is obviously wrong, please don't hesitate to correct me. I welcome such corrections in an attempt to be as truthful and accurate as possible.

    "Gullibility kills" - Carl Sagan
    "All people know the same truth. Our lives consist of how we chose to distort it." - Harry Block
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    Awesome read, even for the uneducated (Like me) Thanks for posting this.

    ballistic panspermia

    Sounds like a rather violent way for planets to reproduce.
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    Quote Originally Posted by Raziell View Post
    Awesome read, even for the uneducated (Like me) Thanks for posting this.

    I am glad you enjoyed it. I have put a lot of effort in simplifying things so that laymen could understand it.

    Quote Originally Posted by Raziell View Post
    ballistic panspermia

    Sounds like a rather violent way for planets to reproduce.

    Some like it rough.
    "The only safe rule is to dispute only with those of your acquaintance of whom you know that they possess sufficient intelligence and self-respect not to advance absurdities; to appeal to reason and not to authority, and to listen to reason and yield to it; and, finally, to be willing to accept reason even from an opponent, and to be just enough to bear being proved to be in the wrong."

    ~ Arthur Schopenhauer, The Art of Being Right: 38 Ways to Win an Argument (1831), Stratagem XXXVIII.
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    Quote Originally Posted by Cogito Ergo Sum View Post
    In the end, there are also models that do not fit into the previous discussion.
    One of the missing models that, for me, holds more promise than any other is that of Stuart Kauffman. The wikipedia article on him states "In 1971, Kauffman proposed the self-organized emergence of collectively autocatalytic sets of polymers, specifically peptides, for the origin of molecular reproduction. .......He is best known for arguing that the complexity of biological systems and organisms might result as much from self-organization and far-from-equilibrium dynamics as from Darwinian natural selection."

    He has published several books on the subject - dense, but fascinating reading. I can recommend these two:

    Stuart Kauffman 'At Home in the Universe' Penguin Books Ltd 1995 ISBN:0-670-86997-X

    Stuart Kauffman 'Investigations' Oxford University Press 2000 ISBN:0-19-512104-X
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    Quote Originally Posted by John Galt View Post
    One of the missing models that, for me, holds more promise than any other is that of Stuart Kauffman. The wikipedia article on him states "In 1971, Kauffman proposed the self-organized emergence of collectively autocatalytic sets of polymers, specifically peptides, for the origin of molecular reproduction. .......He is best known for arguing that the complexity of biological systems and organisms might result as much from self-organization and far-from-equilibrium dynamics as from Darwinian natural selection."

    He has published several books on the subject - dense, but fascinating reading. I can recommend these two:

    Stuart Kauffman 'At Home in the Universe' Penguin Books Ltd 1995 ISBN:0-670-86997-X

    Stuart Kauffman 'Investigations' Oxford University Press 2000 ISBN:0-19-512104-X

    When I did my research, I found a few papers about autocatalytic cycles. Yet, it was difficult to fit this idea into my text.
    However, it is possible that I add a short paragraph about this idea in my thread in the future.

    I am certain that the books you have cited, are worth the read. However, before I consult the library, I am going to start here:
    http://www.each.usp.br/camiloneto/sc...Adaptation.pdf
    "The only safe rule is to dispute only with those of your acquaintance of whom you know that they possess sufficient intelligence and self-respect not to advance absurdities; to appeal to reason and not to authority, and to listen to reason and yield to it; and, finally, to be willing to accept reason even from an opponent, and to be just enough to bear being proved to be in the wrong."

    ~ Arthur Schopenhauer, The Art of Being Right: 38 Ways to Win an Argument (1831), Stratagem XXXVIII.
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    If you have a spare year or two, a worthwhile resource to peruse is the "Perspectives in Biology" archive of the famous Cold Spring Harbor Laboratory. Two sections of relevance and interest here are the Origins of Life collection and the RNA Worlds collection. There are 43 in-depth review articles of a high quality to choose from.


    Links:
    Origins of Life - 19 articles
    RNA Worlds - 24 articles
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    Was there any life forms before cynobacteria?Does anyone know?
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    ALmost certainly. It is thought that the first organisms were chemotrophs, deriving their energy from convenient chemical reactions. The sparsity of evidence and the great distance in time mean that much of our thinking for this period is speculative. However, I am not aware of any serious speculation that would place cyanobacteria as the first life forms. (If anyone knows of such speculation I would be fascinated to learn about it.)
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    Quote Originally Posted by Zwirko View Post
    If you have a spare year or two, a worthwhile resource to peruse is the "Perspectives in Biology" archive of the famous Cold Spring Harbor Laboratory. Two sections of relevance and interest here are the Origins of Life collection and the RNA Worlds collection. There are 43 in-depth review articles of a high quality to choose from.


    Links:
    Origins of Life - 19 articles
    RNA Worlds - 24 articles

    Thank you!
    I have used one article from the 2010 issue (Pizzarello, S. et al.), but I did not know that they offered so much information about the subject.
    I will add the links to the Further Reading section so that people can learn more about the subject if they are interested.
    Last edited by Cogito Ergo Sum; November 15th, 2013 at 02:03 PM.
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    ~ Arthur Schopenhauer, The Art of Being Right: 38 Ways to Win an Argument (1831), Stratagem XXXVIII.
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    Administrator KALSTER's Avatar
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    Quote Originally Posted by WeinerWistle View Post
    Was there any life forms before cynobacteria?Does anyone know?
    Like John says, almost certainly. Cyanobacteria use photosynthesis, which is very complicated process. In fact, it is likely that the chloroplasts found in plants and algae was an organism on its own and an ancestor of cyanobacteria, before evolving into a close symbiosis with them.
    Last edited by KALSTER; September 18th, 2013 at 04:43 AM. Reason: fixed incorrect information
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    Moderator Moderator Cogito Ergo Sum's Avatar
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    Quote Originally Posted by WeinerWistle View Post
    Was there any life forms before cynobacteria?Does anyone know?

    Yes, there were earlier life forms on Earth.
    Those life forms had (and still) have anoxic metabolism (such as photosynthesis via S), such as purple sulfur bacteria.
    Anoxic metabolisms were logical, since O2 did not appear on Earth in large quantities until the Great Oxygenation Event.
    Last edited by Cogito Ergo Sum; November 14th, 2013 at 03:18 PM.
    "The only safe rule is to dispute only with those of your acquaintance of whom you know that they possess sufficient intelligence and self-respect not to advance absurdities; to appeal to reason and not to authority, and to listen to reason and yield to it; and, finally, to be willing to accept reason even from an opponent, and to be just enough to bear being proved to be in the wrong."

    ~ Arthur Schopenhauer, The Art of Being Right: 38 Ways to Win an Argument (1831), Stratagem XXXVIII.
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  19. #18  
    ...matter and pixie dust wegs's Avatar
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    Thank you for directing me here, Cogito! This is a extremely worthwhile and incredibly informative read.
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  20. #19  
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    Great posts. Something I haven't come across a whole lot in my studies so far.

    I'm sure there's a noble prize in it for whoever cracks it. Wish I could have something more to contribute to the discussion because it is a fascinating topic
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  21. #20  
    Forum Professor Zwirko's Avatar
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    Nice, but technical, review of the current stat of play in metabolism-first theories:

    Sousa, F.L., Thiergart, T., Landan, G., Nelson-Sathi, S., Pereira, I. A., Allen, J.F., Lane, N. and Martin, W.F. (2013)
    Early bioenergetic evolution.
    Philosophical Transactions of the Royal Society B: Biological Sciences 368:20130088
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