chapter25 - BIOL 1020 CHAPTER 25 LECTURE NOTES Chapter 25:...

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BIOL 1020 – CHAPTER 25 LECTURE NOTES Chapter 25: The Origin and Evolutionary History of Life on Earth I. Chemical conditions of the early Earth that could have fostered the origin of life A. the Earth is about 4.6 billion years old (time of the first likely solid surface) 1. supported by radioisotope dating of oldest known Earth minerals (date to 4.4 billion years ago, or 4.4 bya) oldest known rocks on Earth (4.1 bya) oldest known meteorites (4.6 bya; for the age of the solar system) 2. some models go out to 6 billion years, it is hard make a rule for a definitive starting point of planet formation B. Earth’s early atmosphere (when life first appears in the fossil record) most likely consisted of CO 2 , H 2 O, CO, H 2 , N 2 , and small amounts of NH 3 , H 2 S, and CH 4 – note the lack of O 2 , which is a major constituent of today’s atmosphere C. Four requirements for the current chemical evolution model were likely met in the early Earth 1. little or no free oxygen 2. abundant energy sources (volcanism, thunderstorms, and bombardment with particles and radiation from space were all likely present as energy sources; especially important is more UV radiation than today) the sun was hotter, producing more UV light the Earth had no ozone layer to filter out most of the UV light coming in 3. chemical building blocks of water, dissolved mineral ions, and atmospheric gases 4. time (there was plenty of time before the first traces of life from 3.8 bya]) D. attempts to mimic the early Earth’s atmosphere and chemical profile have led to production of organic molecules from simpler materials after energy is added 1. 1920s – Oparin and Haldane independently proposed that organic molecules could form spontaneously from simpler raw materials when sufficient energy is supplied in a reducing (energy-rich, electron-adding) environment 2. 1950s – Miller and Urey made a “ reducing atmosphere ” of H 2 O, H 2 , NH 3 , CH 4 in a spark chamber; after sparking, they found that amino acids and other organic compounds had formed designed to mimic what was thought at the time to have been Earth’s early atmosphere later experiments with different “reducing atmospheres” that were thought to be better matches to the likely atmosphere of the early Earth produced all 20 amino acids used in proteins, various sugars and lipids, and components of DNA and RNA nucleotides current models of the Earth’s early atmosphere are that in general the atmosphere was not reducing, but that there were likely many local environments that were reducing – especially near volcanic activity 3. organic polymers can form spontaneously from monomer building blocks on some sand, clay, or rock surfaces E. there are several models for exactly where and how life as we know it on Earth began 1. prebiotic broth hypothesis – life began from an “organic soup” in the oceans 2. bubble hypothesis – a variation on the prebiotic broth, with “oily bubbles” from an organic soup interacting with
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chapter25 - BIOL 1020 CHAPTER 25 LECTURE NOTES Chapter 25:...

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