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This lecture introduces you to the history of life on earth. In this chapter, students will be able to understand: 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? Why do evolutionary rates differ among groups of organisms? | The History of Life on Earth 21 The History of Life on Earth 21.1 How Do Scientists Date Ancient Events? 21.2 How Have Earth’s Continents and Climates Changed over Time? 21.3 What Are the Major Events in Life’s History? 21.4 Why Do Evolutionary Rates Differ among Groups of Organisms? 21.1 How Do Scientists Date Ancient Events? Many evolutionary changes take place over long periods of time. To study long-term evolutionary change, we must think in time frames spanning millions of years; and imagine conditions very different from today’s. 21.1 How Do Scientists Date Ancient Events? Fossils are preserved remains of ancient organisms, they tell us about body form or morphology, and where and how they lived. Earth’s history is recorded in rocks. Layers of rocks are called strata. 21.1 How Do Scientists Date Ancient Events? Relative ages of rocks can be determined by looking at strata of undisturbed sedimentary rock. The oldest layers are at the bottom, youngest at the top. First observed in the 17th century by Nicolaus Steno. Chapter Opener 2 Younger Rocks Lie on Top of Older Rocks 21.1 How Do Scientists Date Ancient Events? In the eighteenth century, geologists realized that fossils could also be used to age rocks. Certain fossils were always found in younger rocks, others were found in older rocks. Fossils in more recent strata were more similar to modern organisms. 21.1 How Do Scientists Date Ancient Events? Radioisotopes can be used to determine the actual age of rocks. Radioisotopes decay in a predictable pattern. Half-life is the time interval over which one half of the remaining radioisotope decays, changing into another element. Figure 21.1 Radioactive Isotopes Allow Us to Date Ancient Rocks Table 21.1 Each radioisotope has a characteristic half-life. 21.1 How Do Scientists Date Ancient Events? To date an event, we must know (or be able to estimate) the concentration of the radioisotope at the start of the event. For 14C, production in the upper atmosphere is . | The History of Life on Earth 21 The History of Life on Earth 21.1 How Do Scientists Date Ancient Events? 21.2 How Have Earth’s Continents and Climates Changed over Time? 21.3 What Are the Major Events in Life’s History? 21.4 Why Do Evolutionary Rates Differ among Groups of Organisms? 21.1 How Do Scientists Date Ancient Events? Many evolutionary changes take place over long periods of time. To study long-term evolutionary change, we must think in time frames spanning millions of years; and imagine conditions very different from today’s. 21.1 How Do Scientists Date Ancient Events? Fossils are preserved remains of ancient organisms, they tell us about body form or morphology, and where and how they lived. Earth’s history is recorded in rocks. Layers of rocks are called strata. 21.1 How Do Scientists Date Ancient Events? Relative ages of rocks can be determined by looking at strata of undisturbed sedimentary rock. The oldest layers are at the bottom, youngest at the top. First observed in
