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05-28-2003, 04:14 PM | #21 |
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On geological data for life in the Archean, and late Hadean:
Brocks, Jochen J., Gram A. Logan, Roger Buick, Roger E. Summons 1999 Archaen Molecular Fossils and the Early Rise of Eukaryotes. Science 285 (5430):1033-1036 Donald E. Canfield, Kirsten S. Habicht, and Bo Thamdrup 2000 The Archean Sulfur Cycle and the Early History of Atmospheric Oxygen Science April 28; 288: 658-661. (in Reports) Des Marais, David J. 2000 “When Did Photosynthesis Emerge on Earth?” Science 289 (5485): 1703 Iris Fry, 2000 "The Emergence of Life on Earth: A Historical and Scientific Overview" Rutgers University Press (excellent overview of OOL research) Kerr, Richard A. 2002 “Reversal Reveal Pitfalls in Spotting Ancient and E.T. Life” Science (Perspectives) 296:1384-1385 Lollar, B. Sherwood, T. D. Westgate, J. A. Ward, G. F. Slater & G. Lacrampe-Coulloume 2002 “Abiognic formation of alkanes in the Earth’s crust as a minor source for global hydrocarbon reserevoirs.” Nature (letters) Vol 416: 522- 524 Mojzsis, Stephen J., T. Mark Harrison, 2000 “Vestiges of a Beginning: Clues to the Emergent Biosphere Recorded in the Oldest Known Sedimentary Rocks” GSA Today, April *** Now considered doubtfull*** Pavlov, Alexander, James K. asting, Jeninifer L. Eigenbrode, Katherine H. Freeman 2001 “Organic haze in Earth’s early atmosphere: Source of low-13C Late Archean kerogens?” Geology v.29 no. 11:1003-1006 Rosing, T. Minik 1999 13C-Depleated Carbon Microparticles in >3700-Ma Sea-Floor Sedimentary rocks from West Greenland. Science 283 (5402): 674 Schopf, J. William 1999 "Cradle of Life:The Discovery of Earth's Earliest Fossils" , Prinston University Press (very readable book, high scool chemistry level) Sleep, N. H., K. Zahnle, P. S. Neuhoff 2001 “ Initiation of clement surface conditions on the earliest Earth” PNAS-USA v.98 no. 7: 3666-3672 VAN ZUILEN, MARK A., AIVO LEPLAND & GUSTAF ARRHENIUS 2002 Reassessing the evidence for the earliest traces of life Nature Vol 418: 6898 Whitehouse, Martin. 2000 “Time Constraints on When Life Began: The oldest Record of Life on Earth?” The Geochemical News #103, April. Xiong, Jin, William M. Fischer, Kazuhito Inoue, Masaaki Nakahara, Carl E. Bauer. 2000 “Molecular Evidence for the Early Evolution of Photosynthesis” Science 298(5485): 1724 Whitehouse, VAN ZUILEN, and Kerr are the central critics of Mojzsis. Solid evidence for cellular life, and even photosynthesis by 3.5 billion years ago. |
05-29-2003, 08:31 AM | #22 |
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abiogenesis and evolution
It is worth noting that there is not yet a consensus regarding the specific mechanism(s) of abiogenesis, let alone acceptance of a particular process producing life as a fact. On the other hand, the evolution of living things by descent with modification from a common ancestor is a fact. It is quite possible to believe that one or more supernatural beings created prokaryotic life almost four billion years ago, and that evolution went on from there. Such an origin of life would not be scientific, but natural abiogenesis is not required for evolution.
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05-29-2003, 08:40 AM | #23 |
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indeed, i'm quite aware of all that, and so is he. but we aren't strictly debating evolution.
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05-31-2003, 04:30 PM | #24 | |
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Re: amino acids
Quote:
George Johnson On Science : One of the most important science discoveries last year sheds light on origin of life By George Johnson In the past year, science has made important progress on one of the truly big questions in biology: the origin of life. Most scientists accept - tentatively - the theory that life arose spontaneously here on earth from inanimate matter, as associations of molecules became more and more complex. The logical way to test the spontaneous origin theory is to repeat the process. In 1953, Stanley Miller and Harold Urey assembled in a glass flask an atmosphere similar to what the early earth's atmosphere is thought to have been like. It was a smelly sort of atmosphere, composed of hydrogen-rich molecules like hydrogen sulfide ("rotten eggs"), methane ("swamp gas"), and ammonia ("smelling salts"). Bombarding the mixture with lightning in the form of sparks, Miller and Urey found that within a week 15 percent of the carbon originally present as methane gas had been converted into other carbon compounds, including amino acids (the building blocks of proteins) and nucleotides (the building blocks of nucleic acids like DNA and RNA). They concluded that the basic building blocks used in the construction of living organisms could indeed have arisen spontaneously. The next step predicted by the spontaneous origin theory has not been so easy to test - that these building blocks spontaneously linked together to form the large molecules of which cells are made. Somehow, the individual amino acids produced in the Miller-Urey experiment must have linked together in chains to form proteins, like pearls coming together to form a necklace. This linking-together presents what at first seems an insurmountable problem: Theory tells us that it is chemically impossible for amino acids to aggregate spontaneously in water. The basic problem is that water will tend to push the chemical reaction backward, toward breaking up proteins rather than forming them. One way out of this quandary is to imagine that life first arose away from water, say within a clay or mineral. While a little zany, this idea has attracted serious consideration, at least to some degree because it is hard to think of other water-free alternatives. Another way out of the quandary is to step back and look at the problem from a different perspective. There is lots of water in each cell of your body, and yet proteins get assembled in your cells with no difficulty. Water presents no problem there, because in cells the process of protein assembly does not actually occur out in the water. Instead, it is carried out within great contraptions called ribosomes, and there is no water inside a ribosome. Ribosomes are very complex cellular machines. Each ribosome is made of more than 50 different proteins, as well as three chains of RNA composed of about 3,000 nucleotides. RNA is a nucleic acid similar to DNA, but single-stranded. It has been traditionally assumed that the proteins act as enzymes to facilitate the amino-acid assembly process, with RNA providing a scaffold to position the proteins properly. But if proteins in ribosomes bring about the linking together of amino acids to form new proteins, then where did the ribosome proteins come from? Last year we learned the answer to that chicken-or-egg question, and a surprising answer it turned out to be. Over the past 12 months, several research groups have used powerful X-ray diffraction to determine the complete detailed structure of a ribosome at atomic resolution. The researchers found that the many proteins of a ribosome are scattered over its surface like decorations on a Christmas tree. The role of these proteins seems to be limited to stabilizing the many bends and twists of the RNA chains. The proteins act like spot-welds between the RNA strands they touch. Importantly, there are no proteins on the inside of the ribosome where the chemistry of protein synthesis takes place - just twists of RNA. This was totally unexpected. The ribosome's RNA, not its protein, must bring about the joining of amino acids. Science, the most widely read scientific journal in the United States, chose this revelation as the second-most-important scientific discovery of 2000 (runner-up to completion of the Human Genome Project). This discovery that RNA facilitates protein synthesis dissolves the quandary of spontaneous protein assembly. RNA nucleotides produced in Miller-Urey experiments can link together to form chains, and these chains can act as enzymes to bring about the linking together of amino acids to form proteins. There is a great deal we don't know, but the theory of spontaneous origin seems to have passed another hurdle. George Johnson is a biology professor at Washington University. I am also including a reprint of my take on certain aspects: The weight of evidence for abiogenesis is mounting. The assertion that a bright line of demarcation exists between "living" and "non-living" matter is the subject of my topic "General Theorem of Existence" (May 31, Existence of God forum), wherein I advance the premise that no line of demarcation exists (other than in human imagination). Your critique is invited. I, therefore, disagree with the view that abiogenesis can be separated from the evolutionary process. Evolution is a process by which relatively maladaptive systems are "selected out". In that process can be found a high degree of correlation between adaptive change and molecular complexity, including encoding of information. It seems to me that, in our planet's youth, certain molecular arrangements happened to be more adaptive to their environment and survived. Additional random structural intricacy led, in some cases, to greater adaptive modality and survival, and so forth. Selection doesn't connote simply breeding. "Death" (non-survival) exerts by far the greatest selective pressure in the evolutionary process. In short, I believe that this "miracle" of chemistry was subject to the same forces of nature that have given form to all of the intelligent systems that have ever dwelled on earth. In this view, the forces of evolution are the forces of universal law. Hence, evolution had no "beginning" on this planet. It is simply the way of the universe. Abiogenesis is a consequence of that evolutionary law. |
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