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Chapter 22 Outline

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The Origin and History of Life on Earth

* The universe began with a cosmic explosion called the Big Bang about 13.7 billion years ago (bya). * Our solar system began about 4.6 bya after one or more local supernova explosions. * The Earth is estimated to be 4.55 billion years old.

* The period between 4.0 and 3.5 billion years ago marked the emergence of life on our planet.

* Section 22.1: Origin of Life on Earth:

* Four Overlapping Stages of the Origin of Life:
* Stage 1: Nucleotides and amino acids were produced prior to the existence of cells. * Stage 2: Nucleotides became polymerized to form RNA and/or DNA, and amino acids become polymerized to form proteins. * Stage 3: Polymers became enclosed in membranes.

* Stage 4: Polymers enclosed in membranes acquired cellular properties. * Stage 1: Organic Molecules Formed Prior to the Existence of Cells: * The conditions on early Earth may have been more conducive to the spontaneous formation of organic molecules. * Organic molecules, and eventually macromolecules, formed spontaneously. * Termed prebiotic (before life) or abiotic (without life) synthesis. * These slowly forming organic molecules accumulated because there was little free oxygen gas, so they were not spontaneously oxidized, and there were as yet no living organisms, so they were also not metabolized. * The slow accumulation of these molecules in the early oceans over a long period of time formed the prebiotic soup. * Reducing Atmosphere Hypothesis: Based on geological data, many scientists in the 1950s proposed that the atmosphere on early earth was rich in water vapor (H2O), hydrogen gas (H2), methane (CH4), and ammonia (NH3). * These components, along with a lack of atmospheric oxygen (O2), produce a reducing atmosphere because methane and ammonia readily give up electrons to other molecules, thereby reducing them. * Such oxidation-reduction reactions, or redox reactions, are required for the formation of complex organic molecules from simple inorganic molecules. * In 1953, Stanley Miller, an American Chemist, was the first scientist to use experimentation to test whether the prebiotic synthesis of organic molecules is possible. * His experimental apparatus was intended to simulate the conditions on early Earth that were postulated in the 1950s. * Extraterrestrial Hypothesis: Many scientists have argued that sufficient organic molecules may have been present in the materials from asteroids and comets that reached the surface of early Earth in the form of meteorites. * Opponents argue that most of this would be destroyed in the intense heating and collision that accompanies the passage of large bodies through the atmosphere and collision with the surface of the Earth. * Deep-Sea Vent Hypothesis: Biologically important molecules may have been formed in the temperature gradient between extremely hot vent water and cold ocean water. * Supported by experiments.

* Complex biological communities are found here that derive energy from chemicals in the vent (not the sun). * Stage 2: Organic Polymers May Have Formed on the Surface of Clay: * Scientists hypothesize that the second stage in the origin of life was a period in which simple organic molecules polymerized to form more complex organic polymers such as DNA, RNA, or proteins. * Because hydrolysis competes with polymerization, many scientists have speculated that he synthesis of polymers did not occur in a watery prebiotic soup, but instead took place on a solid surface (such as clay) or in evaporating tidal pools. * Stage 3: Cell-Like Structures May Have Originated When Polymers Were Enclosed by a Boundary: * The third stage in the origin of living cells is hypothesized to be the... Show More

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