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Chapter 21

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BIOL 3010
Scott Schau

Chapter 21 The History of Life on Earth Text BookLecture Slide Combined 211How Do Scientists Date Ancient EventsFossils the preserved remains of ancient organismsPaleontologists scientists who study fossils Morphology the body form Age of rocks cannot be determined by just looking at them but the age of rocks can only be determined when comparing one another Strata oldest layers in rocks are known to lie at the bottom and successively higher strata are progressively younger William Smith concluded that o Fossils of similar organisms were found in widely separated places on Earth o Certain organisms were always found in younger rocks than certain other organismso Organisms found in higher more recent strata were more similar to modern organisms than were those found in lower more ancient strataRadioisotopesRadioactivity a method of dating rocks discovered at the beginning of the twentieth century HalfLife a successive time interval in which half of the remaining radioactive material of the radioisotope decays either changing into another element or becoming the stable isotope of the same element14 For C production in the upper atmosphere is about equal to its natural decay 1412 In an organism the ratio of C to C stays constant during its lifetime 14 When an organism dies it is no longer incorporating C from the environment 141412 The C that was present in the body decays with no replacement and the ratio of C to C decreasesAs soon as an organism dies it ceases to exchange carbon compounds with its environment Isotopes in a sedimentary rock do not contain reliable information about the date of its formationo Since they are transported overlong distancesand are deposed in another location But igneous rocks eg lava or volcanic ash that have intruded into layers of sedimentary rock can be datedOther radioisotopes are used to date older rocksDecay of potassium40 to argon40 is used for the most ancient rocksRadioisotope dating is combined with fossil analysis212 How have Earths Continents and Climates Changed over TimeLithosphere is Earths crust which consists of a number of solid plates each about 40 kilometers thicko Floats on a fluid layer of molten rock or magma the magma circulates because heat produced by radioactive decay deep in Earths core sets up convection currents in the fluid The plates move because magma rises and exerts tremendous pressure Where 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 when they occur under water such valleys are known as trenchesWhere plates are pushed apart ocean basins may form between them Continental Drift movement of the lithospheric plates and the continents they containThroughout Earths history the plates that carry the continents have drifted apart and moved back together numerous timesPlate movement has affected climate sea level and the distribution of organisms Increase of Oxygen in AtmosphereIncrease in atmospheric oxygen have been largely unidirectional Oxygen first in atmosphere 38 bya billion years agoOxygen concentrations began to increase significantly about 24 billion years ago when some prokaryotes evolved the ability to split water as a source of hydrogen ions for photosynthesis The waste product is O 2 One lineage of these oxygengenerating bacteria evolved into the cyanobacteria These organisms formed rocklike structures called stromatolitesThe cyanobacteria liberated enough O to allow the evolution of oxidation reactions as the energy source for the synthesis of 2ATPWhen oxygen first appeared in the atmosphere it was poisonous to the anaerobic prokaryotes that inhabited Earth at the time
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