25_Lecture_Post

25_Lecture_Post - The History of Life on Earth Overview:...

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The History of Life on Earth Overview: Lost Worlds Past organisms were very different from those now alive The fossil record shows macroevolutionary changes over large time scales, for example: The emergence of terrestrial vertebrates The impact of mass extinctions The origin of flight in birds Cryolophosaurus skull
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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: 1. Abiotic synthesis of small organic molecules 2. Joining of these small molecules into macromolecules 3. Packaging of molecules into protocells 4. Origin of self-replicating molecules
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Synthesis of Organic Compounds on Early Earth Earth formed about 4.6 billion years ago, along with the rest of the solar system Bombardment of Earth by rocks and ice likely vaporized water and prevented seas from forming before 4.2 to 3.9 billion years ago Earth’s early atmosphere likely contained water vapor and chemicals released by volcanic eruptions (nitrogen, nitrogen oxides, carbon dioxide, methane, ammonia, hydrogen, hydrogen sulfide)
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In the 1920s, A. I. Oparin and J. B. S. Haldane hypothesized that the early atmosphere was a reducing environment In 1953, Stanley Miller and Harold Urey conducted lab experiments that showed that the abiotic synthesis of organic molecules in a reducing atmosphere is possible However, the evidence is not yet convincing that the early atmosphere was in fact reducing Instead of forming in the atmosphere, the first organic compounds may have been synthesized near volcanoes or deep- sea vents Miller-Urey type experiments demonstrate that organic molecules could have formed with various possible atmospheres
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Figure 25.2 Mass of amino acids (mg) Number of amino acids 20 10 0 1953 2008 200 100 0 1953 2008 Amino acids have also been found in meteorites
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Abiotic Synthesis of Macromolecules RNA monomers have been produced spontaneously from simple molecules Small organic molecules polymerize when they are concentrated on hot sand, clay, or rock
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Replication and metabolism are key properties of life and may have appeared together Protocells may have been fluid-filled vesicles with a membrane-like structure In water, lipids and other organic molecules can spontaneously form vesicles with a lipid bilayer Adding clay can increase the rate of vesicle formation Vesicles exhibit simple reproduction and metabolism and maintain an internal chemical environment Protocells
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Figure 25.3 (a) Self-assembly Time (minutes) Precursor molecules plus montmorillonite clay Precursor molecules only Relative turbidity, an index of vesicle number 0 20 m (b) Reproduction (c) Absorption of RNA Vesicle boundary 1 m 0 0.2 0.4 40 20 60
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Self-Replicating RNA and the Dawn of Natural Selection The first genetic material was probably RNA, not DNA RNA molecules called ribozymes have been found to catalyze many different reactions
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25_Lecture_Post - The History of Life on Earth Overview:...

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