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Chapter 20 Life’s Origin and Early Evolution AP Biology Spring 2011 Chapter 20.1 In the Beginning Big Bang Big Bang: describes the instantaneous distribution of all mater and energy throughout the universe Big Bang Gaseous particles condensed into stars, where nuclear reactions began giving off light Dying stars left behind clouds of dust and gas, which began to cool and form our solar system As particles began clustering together and gaining mass, pre-planetary objects started to form increasing gravitational pull About 4.6 billion years ago Earth and other planets had formed Conditions of Early Earth Volcanic eruptions Led to early atmosphere of water vapor, carbon dioxide, nitrogen, and hydrogen Free oxygen levels were low until about 2.2 billion years ago When the earth’s crust cooled, water condensed, rain began and carried minerals from rocks in runoff Runoff carried nutrients into the seas where life began Origins of the Building Blocks of Life Early experimentation demonstrated non-biological mechanisms for producing organic molecules Stanly Miller: used lab apparatus to demonstrate synthesis of amino acids from hydrogen, methane, ammonia, and water under abiotic conditions Other possibilities: arrival of organic compounds from outer space and synthesis of biological molecules near hydrothermal vents in deep seas Assembly of amino acids has been detected in carbon rich meteorites Origins of the Building Blocks of Life Still many questions about early earth conditions! Chapter 20.2 How Did Cells Emerge Origins of Proteins and Metabolism Metabolism: group of interacting molecules that copy themselves repeatedly and use different molecules to supply the reaction Origins of Proteins and Metabolism Hypothesis #1: Proteins formed on clay tidal flats Clay would have attracted amino acids with an electrical charge Sunlight could have provided energy to form bonds Origins of Proteins and Metabolism Hypothesis #2: Metabolic pathways evolved at hydrothermal vents Heat, minerals, and pressure combined at the vents to create conditions sufficient to induce organic compound synthesis Experimental evidence supports part of each claim Origin of the First Plasma Membranes Proto-cells were simple membrane bound sacs containing nucleic acids that served as templates for proteins Membrane bound sacs can form spontaneously, incorporating proteins and fatty acids in conditions similar to the clay tidal flats Why do we need plasma membranes? Origin of Genetic Material From accumulated organic compounds emerged replicating systems consisting of DNA, RNA, and proteins An “RNA world” may have preceded DNA’s dominance as the main informational molecule Evidence supports this Origin of Organic Material RNA still serves as enzymes in cells rRNA catalyzes protein formation DNA sequencing of rRNA shows very small variation between prokaryotes and eukaryotes Self-replicating ribosomes have been synthesized in the lab Origin of Organic Material How DNA entered the picture is not yet clear DNA may have provided several advantages over RNA DNA is more stable and less susceptible to damage DNA enabled storage of more information DNA prevented viral attacks on RNA (until viruses adapted new enzymes that worked on DNA) Chapter 20.3 The First Cells The Golden Age of Prokaryotes Common ancestor about 4.3 billion years ago Primitive fossils date back 3.2-3.5 billion years ago Genomic analysis of living prokaryotes indicate an important divergence 3.46 billion years ago that may have produced a branch leading to bacteria and another lineage leading to archea and eukaryotes The Golden Age of Prokaryotes Evolution of the cyclic pathway of photosynthesis in bacteria tapped renewable source of energy Sunlight Large accumulations of these cells are seen today as fossils (stromatolites) The Golden Age of Prokaryotes Proterozoic eon (2.5 billion year ago) Noncyclic pathway had evolved among cyanobacteria that were producing oxygen Led to an oxygen rich environment with three important consequences: An oxygen rich atmosphere stopped the nonliving synthesis of organic matter Free oxygen permitted aerobic respiration, which now became the dominant energy-releasing pathway Oxygen enriched atmosphere and helped to screen UV rays allowing life to move onto land The Rise of Eukarotes The oldest complete eukaryotic fossils are about 2.1 billion years old and had organelles Red Algae: earliest known organism 750 million years ago algal diversity increased 570 million years ago: earliest animal fossils Animals diversity increased during an adaptive radiation 543 million years ago Led to all major animal lineages Chapter 20.4 Where Did Organelles Come From? Origin of the Nucleus, ER, and Golgi Is possible that the infoldings of the plasma membrane seen in prokaryotic cells could have led to the narrow channels that permit the separation of tasks and materials seen in the ER of eukaryotic cells Origin of the Nucleus, ER, and Golgi Importance of infoldings: Increases surface area for membrane bound enzymatic/metabolic activity Could have surrounded DNA to become nuclear envelope Protects cells from foreign DNA Bacteria take up DNA from their surroundings and viruses Origin of Mitochondria and Chloroplasts According to theory of endosymbiosis: One species becomes a resident inside another cell to benefit both Origin of Mitochondria and Chloroplasts Aerobic bacteria could have become the mitochondria Mitochondria are similar in size to bacteria, have their own DNA, divide independent of the cell, and the mitochondrial inner membrane resembles a bacterial cell’s plasma membrane Origin of Mitochondria and Chloroplasts Cyanobacteria could have become chloroplasts Chloroplasts resemble cyanobacteria in metabolism and have DNA that is self-replicating Glaucophytes are protists with unique photosynthetic organelles resembling cyanobacteria with separate cell walls Evidence of Endosymbiosis Lab culture of Amoeba became infected with a bacterium Some cells died but others thrived and became dependent on the invaders to make an essential enzyme for them Chapter 20.5 Time Line for Life’s Origin and Evolution Time Line Page 328
