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5 Origin of Life
Pages 53-68

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From page 53... ... system that characterizes all life that we know on Earth, a system in which nucleic acids serve roles predominantly in genetics and information transfer and proteins serve roles predominantly in catalysis, may not have been characteristic of life as it first originated. Only one of these complex chemical entities may have been represented in primitive life, or perhaps none at all. Read the entire page →
From page 54... ... For example, Box 5.1 lists compounds, called "tholins," produced from relatively oxidizing environments under these conditions. BOX 5.1 Organic Compounds Identified in Tholin Mixtures Hydrogen sulfide Hexene Formamide Hydrogen cyanide Heptene Pyridine Ammonia Butadiene Styrene Ethane Benzene 2,3 Pentadiene Propane Toluene 2-Methylpyrimidine Butane Thiophene 4-Methylpyrmidine Ethene 2-Methylthiophene 3-Butenenitrile Propene Methylmercaptan Butyne Butene Ethylmercaptan Acetonitrile Pentene Propylmercaptan Carbon dioxide Carbon disulfide Methylisocyanate Acetamide SOURCE: Derived from Sagan, C., Khare, N.B., Bandurski, L.E., and Batholomew, N., 1978, Ultraviolet-photoproduced organic solids synthesized under simulated Jovian conditions: Molecular analysis, Science 199:1199-1201; Sagan, C., and Khare, N.B., 1979, Tholins: Organic chemistry of interstellar grains and gas, Nature 277:102-107; and Pietro grande, M.C., Coll, P., Sternberg, R., Szopa, C., Navarro-Gonzalez, R., Vidal-Madjar, C., and Dondi, F., 2001, Analysis of complex mixtures recovered from space missions: Statistical approach to the study of Titan atmosphere analogues tholins, J Read the entire page →
From page 55... ... The Murchison and other meteorites may also contain ribitol and ribonic acid, the reduced and oxidized forms of ribose, respectively, but ribose itself has not been found. 6 TABLE 5.2 Organic Compounds in the Murchison Meteorite Amino acids 60 ppm Purines and pyrimidines 1.3 ppm Aliphatic hydrocarbons ppm Basic N-heterocycles 7 ppm .35 Aromatic hydrocarbons 15-28 ppm Amines 8 ppm Carboxylic acids ppm Amides 55-70 ppm .300 Dicarboxylic acids ppm Alcohols 11 ppm .30 Hydroxycarboxylic acids 15 ppm Aldehydes and ketones 27 ppm SOURCE: Data from Cronin, J.R., and Pizzarello, S Read the entire page →
From page 56... ... 5.2.2 Biological-like Molecules from Planetary Processes Current research is showing the interactions between organic molecules and a wide array of minerals. These include the formation of carboxylic acids in thermal vent chemistry and the formation of reduced chemical species through photochemistry involving semiconducting minerals.8 5.2.3 The Origin of Phosphorus Phosphorus is an important component of terran life, but its synthesis in stars is not simple. Read the entire page →
From page 57... ... For organic molecules, however, the redox state is generally described with the ratio of the number of hydrogen atoms in a molecule to the number of heteroatoms. Because its carbon is bonded to two oxygen atoms and no hydrogen atoms, carbon dioxide is as oxidized as a carbon atom can be. Read the entire page →
From page 58... ... For example, organic molecules contain hydrogen atoms, which, given an appropriate catalyst or source of energy (ultraviolet light, for example) , might generate H2. Read the entire page →
From page 59... ... An example can be seen in the processes that might have generated the sugar ribose, a key component of RNA and DNA, under prebiotic conditions. A reaction called the formose reaction is known to produce ribose by converting formaldehyde in the presence of calcium hydroxide into several sugars, including ribose. Read the entire page →
From page 60... ... of a "genetic takeover," in which delicate RNA or DNA molecules arose rather late in the development of life, supplanting a hardier genetic molecule on which life was founded. 31 The reactivity of nucleophilic and electrophilic centers can convert molecules that were plausibly present on early Earth into biologically interesting products. Read the entire page →
From page 61... ... That can be illustrated with the chemical instability of ribose. One approach to stabilize ribose exploits the fact that borate forms complexes with 1,2-dihydroxy units in organic molecules. Read the entire page →
From page 62... ... Indeed, noting that such conditions may not have been present on early Earth, Kirschvink has suggested that terran life originated from ribose formed on Mars. 35 Solutions of that type are improbable. Read the entire page →
From page 63... ... 37 Physical law does not forbid such a process, but if a replicator initiated life and no natural environments can be found that make its generation favored, life may be very rare in the universe, and life that we encounter elsewhere is likely to be a result of panspermia. It is reasonable to consider the assumption that life began, somehow, among one of the mixtures of small organic molecules that are produced by abiotic processes. Read the entire page →
From page 64... ... It has been argued that the objections can be met in principle by introducing a small number of assumptions:75 • A thermodynamically favorable, irreversible "driver" reaction is coupled directly to an external source of available free energy and can occur in a plausible abiotic setting. The term coupled indicates that one process cannot occur without the other. Read the entire page →
From page 65... ... The principal initial task will be the identification of candidate driver reactions. There should be no need to specify the remainder of the system in advance once a plausible driver reaction has been found. Read the entire page →
From page 66... ... is correct -- for example, the inclusion in space missions of experiments specifically designed to detect nucleic acids. The committee recommends that missions planned for Mars should include instruments that detect lighter atoms, simple organic functional groups, and organic carbon. Read the entire page →
From page 67... ... 2000. Compositional genomes: Prebiotic information transfer in mutually catalytic noncovalent assemblies. Read the entire page →
From page 68... ... 2001. Geochemical roots of autrotrophic carbon fixation: Hydrothermal experiments in the system citric acid, H2O-(FeS) Read the entire page →

From page 53...

... system that characterizes all life that we know on Earth, a system in which nucleic acids serve roles predominantly in genetics and information transfer and proteins serve roles predominantly in catalysis, may not have been characteristic of life as it first originated. Only one of these complex chemical entities may have been represented in primitive life, or perhaps none at all.

5 Origin of Life Pages 53-68

5 Origin of Life
Pages 53-68