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Ch22Lecture_posted - 22 22 The History of Life on Earth The...

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1 22 The History of Life on Earth 22 2 The History of Life on Earth • Defining Biological Evolution • Determining Earth’s Age • The Changing Face of Earth • The Fossil Record • Major Patterns in the History of Life on Earth • Rates of Evolutionary Change within Lineages • The Future of Evolution 22 3 Defining Biological Evolution • Understanding evolution is important because the features of all organisms are best understood in the light of evolution. • Science offers predictive power . • It is also important because humans are becoming powerful agents of evolutionary change.
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2 22 4 Defining Biological Evolution Biological evolution is a change over time in the genetic composition of a population of organisms. • Some changes can occur rapidly enough to be manipulated experimentally; others take place over very long time frames. • An understanding of the long-term patterns of evolutionary change requires thinking in time frames spanning many millions of years and imagining conditions on Earth that are very different from those we observe today. 22 5 Evolutionary time • The earth is about 4.6 billion yrs. old • Life originated about 3.8 billion yrs ago … • How can we keep/know time at this scale? 22 6 ht p:/ www.uky.edu/KGS/education/images/time460.gif
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3 22 7 Evolutionary Time • Before about 1850, most people believed that the Earth was a few thousand year’s old. • Then, geologists began to demonstrate that some rocks were very ancient. How? 22 8 Determining Earth’s Age * * * * * * * * * youngest oldest Comparison of fossils from different places Provides relative ages of various rock layers 22 9 Determining Earth’s Age • To provide absolute ages, need other methods • Dating with radioactive isotopes § Radioactive isotopes decay in a predicatable pattern over long time periods. • ½ life of Tritium = 12.3 years § Start with 120 units of tritium § At 12.3 years, have 60 units q ½ original § At 24.6 years, 30 units q 2 half lives, ½ x ½ = ¼ original § At 36.9 years, have 15 units q 3 half lives, ½ x ½ x ½ = 1/8 original
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4 22 10 Determining Earth’s Age • Carbon-12 has a half life of 5700 years. § 4.8 billion years > 87,000 half lives • 2 more useful isotopes § Potassium 40 ( 40 K) has a ½ life of 1.3 billion years § 238 U has a ½ life of 4.5 billion years • We can measure the current amount of these isotopes, but how do we know what quantity rocks started with? 22 11 Determining Earth’s Age Sedimentary rock is composed of rocks of different ages, weathered for different lengths of time, and moved over great distances Volcanic rock begins as magma, with a known composition (and cools quickly). The lava can be dated by the decay of potassium-40 to argon-40. • Sedimentary layers and rocks are dated by
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This note was uploaded on 10/18/2008 for the course BIO 187 taught by Professor Douglasgreen during the Fall '07 term at ASU.

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Ch22Lecture_posted - 22 22 The History of Life on Earth The...

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