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Unformatted text preview: Chapter 25 History of Life on Earth- Overview o Past organisms much different from those right now o Early Earth also different o Fossil records show evolutionary changes over large time scales Emergence of terrestrial beings Origin of photosynthesis Long-term impacts of mass extinctions Conditions on early Earth made the origin of life possible- Chemical and physical processes on early Earth may have produced very simple cells through a sequence of stages o Abiotic synthesis of small organic molecules o Joining of these small molecules into macromolecules o Packaging of molecules into protocells o Origin of self-replicating molecules- Synthesis of organic compounds on early Earth o Earth formed 4.6 billion years ago, bombardment ended 3.9 billion years ago o Early atmosphere contains water vapor and chemicals released by volcanic eruptions (N2, NO2, CO2, CH4, NH3, H2, H2S) o 1920s: A.I. Oparin + J.B.S. Haldane independently theorize that Earths atmosphere added electrons to the environment, thus permitting formation of simple organic molecules Energy for this couldve come from lightning and intense UV radiation o 1953: Miller + Urey test the hypothesis creating conditions Resulted in formation of amino acids found in organisms today, along with others; confirms how organic compounds have been key to origin of life and can be generated from abiotic synthesis However, still uncertain whether enough methane/ammonia was present to permit reduction; Early atmosphere made of mainly N 2 , CO 2 ; was neither reducing or oxidizing May have been synthesized near submerged volcanoes and deep-sea vents 1969: amino acids in meteorites confirm amino acid presents; we use L isomer o Abiotic synthesis of Polymers, Protobionts Amino acid polymers formed by dripping on hot hard surface; polymers formed spontaneously without help of enzymes/ribosomes But ofc, more complex mix of linked/cross-linked amino acids; probably acted as weak catalysts Protobionists: aggregates of abiotically produced molecules surrounded by a membrane(-like) structure Key properties of life: replication and metabolism Exhibit simple reproduction/metabolism and maintain internal chemical environment; from spontaneous abiotically produced org cmpds i.e.: liposomes (membrane-bound) form when lipids added to H 2 O o RNA World + Dawn of Natl Selection First genetic material: RNA; Cech finds that RNA can do lots of enzyme- like catalytic functions (ribozymes) Can make complementary copies of short pieces of RNA, self- splicing introns, act on different molecules (tRNA) Usually slow, but protein speeds up reaction rates RNA (SS) takes on various 3D shapes, has phenotype/genotype early protocells with self-replicating, catalytic RNA would be more effective using resources and would increase in numbers thru natl selection, surviving in diff environments Forms the RNA world - Fossil record documents history of life...
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