Notes for Lecture #9:

        Origins of Life

        Copyright 1996. May not be reproduced for commerical purposes

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        Some other references

        • W. F. Loomis, 1988. Four billion years. Sinauer.
          • A biochemical viewpoint

        • W. Day. 1984. Genesis on planet earth, 2nd ed.. Yale.
          • A more general treatment

        • R. Cowen. 1990. History of life. Blackwell.

        Origins of the Earth and life

        • Big Bang roughly 18 billion years ago

        • Formulation of Carbon and higher elements in the first generation of stars

        • Formation of the Earth and Solar system

          • Earth roughly 4.7 billion years old

          • Earth's crust becomes stable by 3.9 billion years ago

        The appearance of life

        • 3.6-3.7 billion years ago: appearance of life

          • anaerobic (methane-producing) bacteria

            • convert CO2 + H2 --> CH4

          • anaerobic (hydrogen sulfide) photosynthesis

        • 3.4 billion years ago

          • chemoautotrophs (sulfate respiration)

        • 2.5 billion years ago

          • oxygen-forming photosynthesis (cyanobacteria)

        • ~2.2 billion years ago: aerobic respiration

        • ~1.5 billion years ago: first evidence of fossil eukaryotes

        The fossil evidence for the first cells

        Microfossils at least 3.5 billion years old

        		Fossil bacteria showing evidence of cell division.

        Early prokaryotic cells arranged in a filament, Warrawoona 3.5 BYA

        Stromatolites

        Mounds of photosynthetic cyanobacteria.

        Stromatolite fossils date back at least 2.7 billion years

        Fossil from Warrawoona

        Intact stromatolites from Sharks Bay

        .

        Early thinking on the origin of life

        • Oparin, Haldane (1920's)

        • Notion of a primeval soup

        .

        Steps in the origins of life

        • Formation of complex organic molecules

        • Self-replicating systems

        • Protein synthesis

        • Compartmentalization: the first cell

        Origins of complex organic molecules

        • nucleosynthesis in stars to form complex molecules

        • molecular clouds

        • A very significant fraction of the Earth's carbon came from extensive cometary bombardment on the primitive Earth

        .

        Model systems for prebiotic evolution

        • Miller-Urey experiment

        • Fox's microspheres

        • Cech's Catalytic RNA

        The Miller-Urey experiment (1953)

        • Showed that complex organic molecules (amino acids) can be built up from very simple organic molecules (such as methane)

        Compartmentalization: Fox's microspheres

        • In the 1970's, Fox showed that by heating certain proteins, microspheres form spontaneously

        Catalytic RNAs

        • Self-cleaving rRNA (Cech)

        • RNA can both cleave itself as well as polymerase itself

        • the solution to the Òchicken versus eggÓ problem

          • don't need proteins as RNA can act as an enzyme

        The first cells may have had RNA genomes

        • DNA synthesis requires RNA primer

        • RNA, not DNA used in protein synthesis

        • Reverse transcriptase RNA --> DNA