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BIOLOGY 1A03 (746)


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Grant B Mc Clelland

Chapter 21 The History of Life on Earth211 How Do Scientists Date Ancient EventsFossils can tell us a great deal about the body form or morphology of organisms that lived there long ago as well as how and where they livedEarths history is largely recorded in its rocks The oldest layers or strata lie at the bottom and successively higher strata are progressively youngerObservations of fossils contained within sedimentary rocksoFossils of similar organisms were found in widely separated places on EarthoCertain organisms were always found in younger rocks than certain other organismsoOrganisms found in higher more recent strata were more similar to modern organisms than were those found in lower more ancient strataRadioisotopes provide a way to date rocksHalf of the remaining radioactive material of the radioisotope decaysRadioisotope dating have been expanded and refined The decay of potassium40 to argon40 has been used to date most of the ancient events in the evolution of lifeFossils in the adjacent sedimentary rock that are similar to those in other rocks of known ages provide additional cluesRadioisotopes dating of rocks combined with fossil analysis are the most powerful method of determining geological age212 How Have Earths Continents and Climates Changed over TimeEarths crust consists of a number of solid plates approximately 40km thick collectively make up the lithosphere It floats on a fluid layer of molten rock magmaMagma circulates bc heat produced by radioactive decay deep in Earths core sets up convection currents in the fluidThe plates move bc magma rises and exerts tremendous pressureWhere plates are pushed together either they move sideways past each other or one plate slides under the other pushing up mountain ranges and carving deep rift valleys underwater known as trenchesWhere plates are pushed apart ocean basins may form between themThe movement of the lithospheric plates and the continents they contain is known as continental driftOxygen has steadily increased in Earths atmosphereIncrease in atmospheric oxygen has been largely unidirectionalThe atmosphere of early Earth probably contained little or no free oxygen gasThe increase in atmospheric oxygen came in two big steps the first step occurred 24 billion years ago when certain bacteria evolved the ability to use water as the source of hydrogen ions for photosynthesisOne group of oxygengenerating bacteria cyanobacteria formed rocklike structures called stromatolites
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