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Origin of Life Introduction |
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The Origin of Life Introduction
The planet earth came into being about 4.5 billion years ago with a primeval atmosphere that was hostile to life. There were no continents as we see them today and the earth was one mass of archipelagoes of volcanic rock.
Between the sun and the surface f the rocks, the ozone shield as we have in the present day atmosphere was lacing in the oxygen devoid primeval atmosphere and hence lethal U-V rays of the sun bathed the barren rocks and left them sterile. The composition of the primeval atmosphere has been debated but the primitive sea seems to have had plenty of dissolved ammonia, carbon dioxide, and abundant deposits of sulphates and iron.
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Prior to the origin of life, it looks as if the precursors to living single celled microorganisms may have arisen either in a ball of ice caps of the ocean prevented the lethal U-V rays from entering the ocean and primitive organic chemicals arose in pockets of water. Interplanetary debris, hydrothermal vents and atmospheric reactions could have provided such compounds as formaldehyde, cyanide and ammonia.
These then could have combined in water within a lattice of ice resulting in the formation of simple amino acids like lysine. A huge meteorite impact could have ultimately thawed the frozen world. Glaciers, volcanoes, geysers, individually or in combination with interplanetary debris may have been collected in a pond or small body of water.
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This elemental water attract certain molecules and serve as a catalysts in the subsequent reactions. Two aldehyde phosphate molecules may have combined to form a sugar phosphate and possibly ribonucleic acid (RNA). Before DNA and proteins evolved, life may have consisted solely of RNA molecules floating in water replicating, mutating and undergoing natural selection of their own.
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When the planet earth was still sizzling, gases and vital compounds were released from the molten magma. It is not unlikely that resting on a stabilizing surface of pyrite, carbon monoxide and a methyl group may have combined towards the formation of activated acetic acid, a crucial chemical for synthesizing other organic compounds. Asteroids, comets, meteorites and even specks of interplanetary dust may have helped in the formation of a ‘primordial soup’, the organic base of lie. In fact, modern experiments under conditions simulating the primeval atmosphere have borne ample testimony to support the primodial soup hypothesis.
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Since organic matter was virtually absent, it has been conjectured that chemosynthetic microorganisms were the pioneer colonizers of the primitive earth. Logically, sulphate reducing and iron oxidizing bacteria which utilized the sulphate and iron deposits of the primitive sea might have been the first organisms to get enriched and multiply. Fossil evidences seem to support such a contention. The earliest evidence can be traced as far back as Precambrian period in what are known as ‘stromatolites’ which can still be seen in nutrient poor environments such as Lake Clifton along Australia’s southwestern coast. These precambrain stones have a spongy outer layer made of oxygen loving cyanobacterial filaments and inner layer populated by bacteria that thrive in the absence of oxygen deriving energy by anaerobic metabolism of iron and sulphur.
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Rod-shaped bacteria-like organisms have been encountered as fossils in the border region between the Republic of South Africa and Swaziland which date back to 3.2 billion years, Similar bacterial deposits resembling iron bacteria have been encountered from the ‘Gunflint cherts’ from Ontario which are estimated to be two billion years old. The Gunflint flora also show fossils of algae which might have been both photosynthetic and nitrogen fixing.
Thus, mineral decomposing organisms may have given rise to photosynthetic organisms (bacteria and algae), which n turn, provided the basic organic materials for diverse groups of heterotrophic bacteria and filamentous fungi to grow and evolve. At this juncture, the primitive atmosphere became increasingly oxygenic due to photosynthetic microorganisms and the earth’s ozone shield stared emerging gradually. The environment thus became some what hospitable to more microbial life.
At first the primitive microorganisms did not multiply systematically in the absence of mitosis or possess specialized structures such as mitochondria. Su h prokaryotic cells gave rise to eukaryotic cells (about 1.2 to 1.4 billion years ago) which were capable of not only systematic division (mitosis) but also possessed enzyme activity by virtue of having well-developed mitochondria.
Gradually, passive existence among microorganism gave rise to symbiosis between microalgae and filamentous fungi resulting in the formation of lichens, which as we know have stood the test of time as pioneer colonizers of rock in the process of soil formation.
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