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Unformatted text preview: Chapter 25 Notes 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 including 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 molecules into macromolecules (proteins & nucleic acids) 3) Packaging into probionts-droplets with membranes that maintained an internal chemistry different than their surroundings 4) Origin of self-replicating molecules that eventually made inheritance possible Synthesis of Organic Compounds on Early Earth Earth formed about 4.6 billion years ago, along with the rest of the solar system Earths early atmosphere likely contained water vapor and chemicals released by volcanic eruptions (nitrogen, nitrogen oxides, carbon dioxide, methane, ammonia, hydrogen, hydrogen sulfide) A. I. Oparin and J. B. S. Haldane hypothesized that the early atmosphere was a reducing environment Stanley Miller and Harold Urey conducted lab experiments that showed that the abiotic synthesis of organic molecules in a reducing atmosphere is possible Through experiments, they created organic molecules 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 submerged volcanoes and deep-sea vents Amino acids have also been found in meteorites Amino acids are the building blocks for proteins D-1 isomers or L-1 isomers Abiotic Synthesis of Macromolecules Small organic molecules polymerize when they are concentrated on hot sand, clay, or rock These polymers may have acted as catalysts for various reactions Protobionts Replication and metabolism are Protobionts-agregates of abiotically produced molecules surrounded my a membrane or membrane like structures Protobionts exhibit simple reproduction and metabolism and maintain an internal chemical environment Experiments demonstrate that protobionts could have formed spontaneously from abiotically produced organic compounds For example, small membrane-bounded droplets called liposomes can form when lipids or other organic molecules are added to water Self-Replicating RNA and the Dawn of Natural Selection The first genetic material was probably RNA not DNA RNA molecules called r ibozymes have been found to catalyze many different reactions For example, ribozymes can make complementary copies of short stretches of their own sequence or other short pieces of RNA Early protobionts with self-replicating, catalytic RNA would have been more effective at using resources and would have increased in number through natural selection The early genetic material might have formed an RNA world...
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This note was uploaded on 01/25/2011 for the course BIO 1510 taught by Professor Unknown during the Spring '07 term at Georgia Institute of Technology.
- Spring '07