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Ch 25 supplement - CHAPTER 25 THE HISTORY OF LIFE ON EARTH...

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Unformatted text preview: CHAPTER 25: 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 ¡ Figure 25.1 What does fossil evidence say about where these dinosaurs lived? Concept 25.1: 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 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 ¡ ¡ĂƌƚŚ͛Ɛ ĞĂƌůLJ ĂƚŵŽƐƉŚĞƌĞ ůŝŬĞůLJ ĐŽŶƚĂŝŶĞĚ ǁĂƚĞƌ ǀĂƉŽƌ ĂŶĚ ĐŚĞŵŝĐĂůƐ ƌĞůĞĂƐĞĚ ďLJ ǀŽůĐĂŶŝĐ ĞƌƵƉƚŝŽŶƐ ;ŶŝƚƌŽŐĞŶ͕ nitrogen oxides, carbon dioxide, methane, ammonia, hydrogen, hydrogen sulfide) ¡ 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 ¡ Amino acids have also been found in meteorites ¡ Figure 25.2 Amino acid synthesis in a simulated volcanic eruption. 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 Protocells ¡ 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 ¡ Figure 25.3 Features of abiotically produced vesicles....
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