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1MO3_Chapter 9.docx

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Jon Stone

From Shrew to Ape Textbook CaveatsPrimitiverecall that there is no such thing as higher or lower organisms primitive or advancedPopulation recall that population comprise individuals in the same species that live in the same areaThe use in current and maybe better described as a community OverviewEvolutionTo understand human evolution completely one must considereach step in the lengthy process by which a population comprising small solitary shrew like insectivores scurrying through leaf litter in a dark Cretaceous forest descended a lineage that led ultimately to organisms like us During the Permian and early Triassic periods much of the worlds fauna was dominated by therapsids a diverse group of reptiles that possessed traits such as being warmblooded and covered with hair that linked them to the mammals that evolved later The early mammals were probably mousesized nocturnal creatures that fed mainly on seeds and insects They had internal fertilization but still laid eggs By the end of the Mesozoic era 65 million years ago placental and marsupial mammals that bore live young had evolved According to Darwins theory complex adaptations are assembled gradually in many small stepseach step favoured by natural selection Modern humans have many complex adaptations like grasping hands bipedal locomotion tool making abilitieslanguage and largescale cooperationGeological Time ScaleThe origin of the human lineage is a late event in Earth historyFirst True MammalsAt the end of the Triassic period therapsids which were reptiles with mammalian traits mostly disappeared o One lineage of therapsids evolved and diversified becoming mammals The Cenozoic era saw spectacular radiation of all the mammals following extinction of the dinosaursThe Origin and Evolution of Mammals Early Triassic periodTherapsids o Warrmblooded furcovered reptiles Late Triassic early Jurassic periodFirst mammals dinosaurs rule End of CretaceousDinosaurs extinct mammal radiationOverview Conditions To understand how modern humans evolved one must be familiar with the geological climatic and biological conditions under which the descent with modification occurredReconstructing Ancient EnvironmentsEarth has changed over time o Continental driftClimate change o Gradual cooling and dying trend in last 20 million yearsChanging environments o Lead to adaptations Geology and ClimateThe earths continents have moved significantly in the past 200 million years from a large mass called Pangaea to the seven continents we recognize today Continental drift affects evolution by putting up barriers that isolate species and by effecting climate change o Ratites in Southern Hemisphere Chapter 26 Climatic ChangeThe climate has changed substantially during the last 65 million years first becoming warmer and less variable and then cooling and becoming more variableEvolution produces adaptation but what is adaptive in one environment may not be adaptive in anothero If the environment remained the same over the course of human evolution then the kinds of evolutionary changes observed in the hominin fossil record such as increases in brain size bipedalism and prolonged juvenile dependence would have to be seen as steady improvements in the perfection of human through time then evolution would have to track a moving target If the world had become warmer rather than colder during this period then our human ancestors would probably have remained in the safety of the trees and would not have become terrestrial or bipedal The positions of the continents have changed relative to each other and to the poles One of the factors that have contributed to this change is the movement of the continents or continental driftContinental drift is important to the history of the human lineage for two reasonso Oceans serve as barriers that isolate certain species from others so the position of the continents plays an important role in the evolution of species
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