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_______________________________________________________ 20 LIFE'S ORIGIN AND EARLY EVOLUTION Impacts, Issues: Looking for Life in All the Odd Places A. Some organisms can live in extreme environments. 1. Thermus aquaticus can live in hot water at 176F. a. A heat-resistant enzyme taken from this prokaryote is used in PCR. b. This has resulted in a new line of inquiry—bioprospecting. 2. Some organisms can survive near hydrothermal vents on the seafloor (230F), others in acidic springs (pH near 0), and still others cling to life on mountain glaciers. B. Nanobes, one-tenth size of bacteria, are growing 3 miles below Earth in 170 degree Celsius rocks. 20.1 In the Beginning A. The big bang describes the instantaneous distribution of all matter and energy throughout the universe. 1. Gaseous particles condensed into stars, where nuclear reactions began giving off light. 2. Dying stars left behind clouds of dust and gas, which began to cool and form our solar system. B. Conditions on the Early Earth 1. About 4.6 billion years ago remnants of exploding stars began to condense into planets around the sun. a. The earth was initially very hot, but cooled to form an outer mantle and partially-molten core. b. First atmosphere probably consisted of gaseous hydrogen, nitrogen, carbon monoxide and carbon dioxide. ****Oparin/Haldane’s Hypothesis c. Free oxygen levels were low until about 2.2 billion years ago. 2. When the earth’s crust cooled, water condensed, rains began, and pools of chemicals began to form. 1 Chapter Twenty C. Abiotic Synthesis of Organic Compounds 1. Within 200 million years life had originated on its surface, but how? a. Evidence from neighboring bodies in our solar system indicates that precursors for building biological molecules must have been present on the primitive Earth. b. Energy in the form of sunlight, lightning, and heat from the Earth's crust was also present. 2. Could large organic molecules have formed spontaneously and then evolved into the molecular systems of life? a. Stanley Miller used a lab apparatus to demonstrate synthesis of amino acids from a mixture of hydrogen, methane, ammonia, and water under abiotic conditions. b. Other possibilities include the arrival of organic compounds from outer space and synthesis of biological molecules near hydrothermal vents in deep seas. 3. The assembly of proteins, DNA, and other complex organic compounds could have been facilitated by clay templates that brought them together in the same place and time. a. Even if molecules were formed spontaneously, they would have quickly hydrolyzed unless clay templates served to hold the molecules together for condensation reactions. b. Experiments by Sidney Fox showed that when amino acids are placed in water and heated, they spontaneously order themselves into small protein-like molecules which Fox called "proteinoids." 20.2 How Did Cells Emerge? A. Origin of Agents of Metabolism 1. The structure of the first proteins dictated their behavior, including some which acted as enzymes, leading to metabolic pathways. 2. Possibly the porphyrin ring (a component of both chlorophyll and cytochromes) was the electron transporter of the first metabolic pathways. Life's Origin and Early Evolution 2 B. Origin of the First Plasma Membranes 1. The metabolism in living cells cannot occur without a barrier against the chemical actions on the outside. 2. Proto-cells were probably membrane-bound sacs containing nucleic acids that served as templates for proteins. 3. Membrane-bound sacs can form spontaneously, incorporating proteins and fatty acids. C. Origin of Self-Replicating Systems 1. From accumulated organic compounds emerged replicating systems consisting of DNA, RNA, and proteins. 2. An RNA world may have preceded DNA's dominance as the main informational molecule. 3. How DNA entered the picture is not yet clear, but we do know that some reactions were more probable than others—not random. 4. Sidney Fox heated amino acids to form protein chains, which when allowed to cool self-assembled into proteinoid spheres that were selectively permeable. 5. David Deamer combined fatty acids and glycerol to form long-tail lipid molecules that self-assembled into small, water-filled sacs resembling cell membranes. 20.3 The First Cells A. The Golden Age of Prokaryotes 1. The original prokaryote line split into archaebacteria, eubacteria, and a line leading to eukaryotes. 2. Evolution of the cyclic pathway of photosynthesis in eubacteria tapped a renewable source of energy—sunlight; large accumulations of these cells are seen today as fossils known as stromatolites. 3. By the dawn of the Proterozoic eon (2.7 billion years ago), the noncyclic pathway evolved among cyanobacteria that were producing oxygen. a. An oxygen-rich atmosphere stopped the further chemical origin of living cells. b. Free oxygen permitted aerobic respiration, which now became the dominant energy-releasing pathway. 3 Chapter Twenty B.The Rise of Eukaryotes 1. The oldest complete eukaryotic fossils are about 2.1 billion years old and had organelles. 2. The earliest known organism is filamentous red algae. 20.4 Where Did Organelles Come From? A. Origin of the Nucleus and ER 1. Prokaryotic cells have infoldings of the plasma membrane that may have been the forerunners of the endoplasmic reticulum and nuclear envelope. 2. The appearance of these organelles would have provided a separation of the DNA and enzymes from the surrounding cytoplasm. B. Origin of the Mitochondria and Chloroplast 1. According to the theory of endosymbiosis, one species becomes a resident inside another cell to the benefit of both. 2. Aerobic bacteria could have become the mitochondria; cyanobacteria could have become chloroplasts. C. Evidence of Endosymbiosis 1. A laboratory 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. 2. 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. 3. Chloroplasts are believed to have originated by endosymbiosis too; they resemble cyanobacteria in metabolism and have DNA that is self-replicating. The rest of the notes are for information purposes only - not for quizzing! 20.5 Life in the Paleozoic Era (550 to 240 MYA) A. During the Cambrian period, nearly all of the major phyla evolved; most organisms lived on or near the sea floor (trilobites were a dominant group). Life's Origin and Early Evolution 4 B. In the Ordovician period, the Gondwana continent drifted southward, shallow marine environments were formed, reef organisms flourished, and glaciers formed to trigger the first mass global extinction. C. In the Silurian and Devonian periods, Gondwana drifted northward, reef organisms recovered, predatory fishes flourished, and amphibians and stalked plants were moving onto land. D. In the Carboniferous period, major radiations of plants and animals occurred as land masses were alternately flooded and drained; coal deposits formed. E. In the Permian period, insects, amphibians and reptiles flourished; the formation of a supercontinent called Pangea caused the greatest of all mass extinctions. 20.6 Life In the Mesozoic Era (240 to 65 million years ago) A. Speciation On a Grand Scale 1. The Mesozoic is divided into the Triassic, Jurassic, and Cretaceous periods. 2. Early in the Cretaceous, the supercontinent Pangea began breaking up, favoring divergences and speciation on a grand scale, especially flowering plants, insects, and reptiles. B. Rise of the Ruling Reptiles 1. Early in the Triassic, rather small dinosaurs evolved from reptilian lineage. 2. In time adaptive zones opened up, allowing proliferation of the huge dinosaurs. 3. At the close of the Jurassic, many dinosaurs perished in a mass extinction caused perhaps by plumes of molten material from ruptures in the Earth's crust or impacts of asteroids. 4. Later in the era, superplumes caused global temperatures to increase, which led to a proliferation of photosynthetic organisms. 5. At the close of the era, the last dinosaurs vanished in a mass extinction that may have been due to the consequences of an asteroid impact in Mexico. 20.7 Focus on Science: Horrendous End to Dominance 20.8 Life In the Cenozoic Era 5 Chapter Twenty A. The breakup of Pangea resulted in major changes in land mass configurations, climates, and adaptive zones. B. During the Paleocene epoch, climates were wetter and warmer allowing forests to extend farther north and south than they do today. C. During the Eocene epoch, continued warming saw the emergence of assorted mammals in new habitats. D. From the Oligocene through the Pliocene, an abundance of grazing animals thrived in the woodlands and grasslands. E. Today the distribution of land masses favors species diversity, yet the activities of human civilization that began about 50,000 years ago have accelerated the pace of extinction. 2. Some types of coenzymes (enzyme helpers) have a structure identical to that of RNA nucleotides; furthermore, nucleotide precursors will self-assemble under conditions similar to those on the early Earth. Life's Origin and Early Evolution 6