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EEB204H1 - Study Notes 2.docx

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Ecology & Evolutionary Biology
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Corey Goldman

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EEB204H1 STUDY NOTESCHAPTER SEVENTEEN THE HISTORY OF LIFEHOW DID LIFE BEGINMuch of our knowledge of the evolutionary history of life comes from the work of paleontologists the scientists who study fossilsthUntil the 19 century most people thought that new members of species sprang up all the time through spontaneous generation from both nonliving matter and other unrelated forms of lifeModern scientific ideas about the origin of life began to emerge in the 1920s via Alexander Oparin and John BS HaldaneAn oxygenrich environment tends to keep molecules simpleThe atmosphere of the young earth must have contained very little oxygen and that under such atmospheric conditions complex organic molecules could have arisen through ordinary chemical reactionsSome molecules could persist in the lifeless environment better than others and would become more common over timeis called prebiotic before life evolutionPrebiotic chemical evolution gave rise to progressively more complex molecules and eventually to living organismsStanley Miller and Harold Urey set out in 1953 to simulate prebiotic evolution in the laboratoryThey simulated the oxygenfree atmosphere of early earth by mixing these components in a flask an electrical spark mimicked the intense energy of early earths lightning storms simple organic molecules appearedOthers have produced amino acids peptides nucleotides adenosine triphosphate ATP and other molecules characteristics of living thingsElectricity is not the only suitable energy source other energy sources include heat or ultraviolet light have also been shown to drive the formation of organic molecules in experimental simulations of prebiotic conditions Additional organic molecules probably arrived from space when meteorites and comets crashed into earths surface When small molecules known to be present in space were placed under spacelike conditions of very low temperature and pressure and bombarded with UV light larger organic molecules were producedToday most organic molecules have a short life because they are either digested by living organisms or they react with atmospheric oxygenEarly earth though lacked both life and oxygen so organic molecules would not have been exposed to these threats The prebiotic molecules must have been threatened by the suns high energy UV radiation because early Earth lacked an ozone layerBecause there was little or no oxygen gas in the atmosphere and therefore no ozone formationUV radiation can provide energy for the formation of organic molecules but it can also break them apartBeneath rock ledges or at the bottom of even fairly shallow seas would have been protected from UV radiation organic molecules may have accumulatedSmall molecules accumulated on the surfaces of clay particles which may have a small electrical charge that attracts dissolved molecules with the opposite charge would have been sufficiently close together to allow chemical reactions between themSmall organic molecules catalyzed the formation of larger more complex molecules including RNABecome the building blocks of the first living organisms The current DNAbased system of information storage evolved from an earlier systemA prime candidate for the first selfreplicating informational molecule is RNA the catalytic RNA molecule is called ribozymeFound dozens of naturally occurring ribozymes that catalyze a variety of different reactions including cutting other RNA molecules and splicing RNA fragments together also been found in ribosomes where they catalyze the attachment of amino acid molecules to growing proteins replication of small RNA moleculesLife arose in an RNA worldRNA served as both the informationcarrying genetic and the catalyst for its own replicationAfter reaching a sufficiently high concentration perhaps on clay particles the nucleotides probably bonded together to form short RNA chainsMolecular evolution in the RNA world proceeded until by some still unknown chain of events RNA gradually receded into its present role as an intermediary between DNA and protein enzymes Contained within some kind of enclosing membraneIf a vesicle happened to surround the right ribozymes it would form something resembling a living cellWe could call it a protocell structurally similar to a cell but not aliveNucleotides and other small molecules might have diffused across the membrane and been used to synthesize new ribozymes and other complex molecules the vesicle may have divided with a few copies of the ribozymes becoming incorporated into each daughter vesicle Theory is by no means certain Most biologists accept that the origin of life was probably an inevitable consequence of natural lawsThis proposition cannot be tested definitively WHAT WERE THE EARLIEST ORGANISM LIKEEarth at first was quite hotMeteorites smashed into the planet and the kinetic energy of these extraterrestrial rocks was converted into heat on impactMore heat was released by the decay of radioactive atomsThe rock composing earth melted and heavier elements such as iron and nickel sank to the center of the planet where they remain molten even todayOnce liquid water was available the prebiotic evolution that ultimately led to the first living organism could beginThe period in which life began is known as the Precambrian eraThe first cells to arise in earths oceans were prokaryotes probably obtained nutrients and energy by absorbing organic molecules from their environment metabolized anaerobically Eventually photosynthesis appeared the very earliest photosynthetic bacteria probably used hydrogen sulfide gas dissolved in water The shortage of hydrogen sulfide set the stage for the evolution of photosynthetic bacteria that were able to use the planets waterWaterbased photosynthesis converts water and carbon dioxide to energetic molecules of sugar releasing oxygen as a byproduct Introduced significant amounts of free oxygen to the atmosphere for the first timeCombined with iron atoms to form huge deposits of iron oxide rust is abundant in rocks formed during this periodThe accumulation of oxygen in the atmosphere of early earth probably exterminated many organisms and fostered the evolution of cellular mechanisms for detoxifying oxygenThe ability to use oxygen in metabolism through aerobic respiration to generate useful energy for the cell Once potential prey population appeared predation would have evolved quickly were probably prokaryotes that were larger than typical bacteria The predatory cells were able to envelope smaller bacteria in an infolded puch of membrane and in this fashion engulf whole bacteria as preyBy about 17 billion years ago however one predator probably gave rise to the first eukaryotic cell
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