Study Guide for Chapter 21

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University of Toronto Scarborough
Biological Sciences
Mary Olaveson

Biology Chapter 21 History of Life on Earth Danish physician Nicolaus Steno was the first to realize that the oldest layers of sedimentary rock, strata, lie at the bottom, and successively higher strata are progressively younger - Major Features of an explanatory theory for evolutionary change based on two major propositions: o Geologist William Smith used this finding to determine certain clues about fossils found in sedimentary rock. He concluded: Fossils of similar organisms were found in widely separated places on Earth Certain organisms are constantly found in younger rocks than others Organisms found in higher, younger strata were more similar to modern organisms than those found in deeper strata th Despite this, a method of dating rocks was not available until the 20 century, after radioactivity was discovered Radio Isotopes Half Life - The time it takes for a radioactive isotope to decay by 50% -Palaeontologists use the ratio of carbon-14 to carbon-12 to date fossils that are less than 50,000 years old. Earlier than 50,000 years, so little carbon-14 is left that it is undetectable -Dating rocks more than 50,000 years old requires estimating isotope concentrations in igneous rocks (rocks formed when molten material cools). Usually the decay of potassium-40 to argon-40 has been used for dating ancient events. -Radioisotope dating of rocks, combined with fossil analysis, is the most powerful method of determining geological age -However, in places there are no igneous rocks palaeontologists use palaeomagnetic dating, which relates the ages of the rocks to patterns in Earths magnetic field at the time they were formed -Using these methods, palaeontologists have divided history into eras, which are in turn subdivided into periods. The boundaries between these divisions are based on striking differences noted in the study of the fossils in the rocks Eras (youngest to oldest)
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