First signs of life - Firstsignsoflife ( The...

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First signs of life                Precambrian Oxygen levels were very low (there are pyrite grains in ancient  sandstones – you can erode and transport pyrite if there is oxygen in the atmosphere)              The  Early  (secondary)  atmosphere   had  little  free   oxygen, it let through   UV  radiation  and  was  hostile  to life. Meteorite  bombardment was  probably  also  a more  important process then.             The   3.8 B.Y . old Greenland rocks have carbon that is isotopically similar to that  produced organically            Stromatolites (fossil filamentous algae)  are the first fossils found in sedimentary  rocks. They date to  3.5 B.Y. old.  These plants are              Algae, very hardy and could have contributed to a buildup of oxygen.               3.5 Billion year old rock from South Africa  reveals the remains of microscopic  singlecelled marine organisms dividing, as does the 2 B.Y. old 
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