Chapter 22 Outline
Chapter 22
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
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