The History of Life on Earth Chapter Overview How do scientists date ancient events? How have Earth?s continents and climates changed over time? What are the major events in Life?s history? How do Scientists Date Ancient Events? Evolutionary changes occur at different rates short term long term (millions of years) Fossils Remains of ancient organisms preserved in rock. Information on body form or morphology, and where and how the organisms lived. How do Scientists Date Ancient Events? How do Scientists Date Ancient Events? Earth?s history is recorded in rocks rock layers are called strata Undisturbed layers of sedimentary rock Nicolaus Steno How do Scientists Date Ancient Events? Combining the fossil and rock information led to the following conclusions: Fossils of similar organisms are found many places on earth Certain fossils are found in younger layers and others in older layers Organisms found in higher strata are more similar to modern organisms How do Scientists Date Ancient Events? Radioactive dating Radioactive isotopes An isotope of an element with an unstable nucleus Decay in a regular pattern Half-life The time it takes for ½ of an isotope to decay Dating a Sample Must estimate the amount of isotope that was present originally 14C ? dating is most commonly used once an organism dies, 14C begins to decay Look at ratio of 14C to 12C The Changing Face of Earth Earth has undergone major physical changes during its history. Continents have moved Alfred Wegener was the first to suggest that the continents have moved Continental drift Earth?s crust consists of plates floating on a fluid layer of molten rock (magma) The movement of plates. Continent position influences sea levels, ocean currents and climate patterns. Plate Tectonics & Continental Drift Changes in Sea Levels Earth?s Atmosphere has Changed Early atmosphere probably contained little to no free oxygen Bacteria began using photosynthesis Cyanobacteria Stromatolites Evolution of oxidation reactions to synthesize ATP. Earth?s Atmosphere has Changed These changes in the atmosphere affected the evolution of life O2 was initially poisonous Evolution of aerobic metabolism More efficient Aerobic organisms began to replace anaerobes O2 levels continued to rise Evolution of eukaryotic cells Multicellular organisms. Earth?s Atmosphere has Changed Earth?s Climate has Changed Earth?s climate has shifted between hot/humid and cold/dry conditions Volcanoes and life Most volcanic eruptions have local effects Mt. St. Helens Late Permian period (275 MYA) Continents collided causing massive volcanic eruptions: pangea Large amounts of ash in the atmosphere triggered glaciers Extraterrestrial Events? Meteorite collisions occur yearly Large meteorites have caused several extinctions Evidence for meteorite collisions Disfigured rocks Craters Iridium Major Events in the History of Life on Earth Earth?s history is divided into eras and periods Precambrian Lasted more than 3 billion years Microscopic prokaryotes, eukaryotes evolved Evolution of soft bodied animals All life existed in the oceans The Cambrian Period (542-488 MYA) Oxygen levels approached current levels ?Cambrian explosion? large #?s of new species evolved Most species lived in the sea Several large continents formed Many species today can trace their lineage back to the Cambrian Explosion Paleozoic Era (542?251 MYA) Divided into 6 periods 3 mass extinctions Radiation of marine groups, plants and animals move to land, forests appear on land Formation of Pangaea (~290 mya) Large single continent Caused volcanic eruptions, glaciers formed resulted in extinction of 96% of all species The Mesozoic Era (251-65 MYA) Pangaea begins to separate distinct floras and faunas Climate warmed O2 levels rose Increased diversification of reptiles Jurassic period (age of dinosaurs) Evolution of mammals. Meteorite collision at end of era causes a mass extinction The Cenozoic Era (65 mya ? the present) Continents moved to current positions Diversification of mammals 4 major ice ages Evolution of hominids began ~6 mya Homosapiens appeared 200,000 years ago
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