Part Five -The Patterns and Processes of Evolution 463

21. The History of Life on Earth 464

21.1 How Do Scientists Date Ancient Events? 465 Radioisotopes provide a way to date rocks 466 Radioisotope dating methods have been expanded and refined 467 21.2 How Have Earth’s Continents and Climates Changed over Time? 468 Oxygen has steadily increased in Earth’s atmosphere 468 Earth’s climate has shifted between hot/ humid and cold/dry conditions 470 Volcanoes have occasionally changed the history of life 471 Extraterrestrial events have triggered changes on Earth 471 21.3 What Are the Major Events in Life’s History? 471 Several processes contribute to the paucity of fossils 472 Precambrian life was small and aquatic 472 Life expanded rapidly during the Cambrian period 472 Many groups of organisms diversified 473 Geographic differentiation increased during the Mesozoic era 476 The modern biota evolved during the Cenozoic era 477 Three major faunas have dominated life on Earth 479 21.4 Why Do Evolutionary Rates Differ among Groups of Organisms? 479 “Living fossils” exist today 479 Evolutionary changes have been gradual in most groups 479 Rates of evolutionary change are sometimes rapid 480 Rates of extinction have also varied greatly 480