1. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu The History of Life on Earth Table of Contents Section 1 Evidence of the Past Section 2 Eras of the Geologic Time Scale Section 3 Humans and Other Primates Chapter 8

2. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 1 Evidence of the Past Objectives Explain how fossils can be formed and how their age can be estimated. Describe the geologic time scale and the way that scientists use it. Compare two ways that conditions for life on Earth have changed over time. Chapter 8

3. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 1 Evidence of the Past Fossils Life Preserved in Rock Fossils are traces or imprints of living things—such as animals, plants, bacteria, and fungi— that are preserved by geological processes. Chapter 8

4. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 1 Evidence of the Past The Age of Fossils Life Preserved in Rock Estimating the age of rocks and fossils based on its position in sedimentary rock is called relative dating. Absolute dating is a method that measures the age of fossils or rocks in years. The next slide shows how half-lives can be used to date fossils. Chapter 8

5. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 1 Evidence of the Past Chapter 8

6. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 1 Evidence of the Past The Geologic Time Scale * is called the geologic time scale. Divisions in Time Paleontologists have divided the geologic time scale into large blocks of time. Mass Extinctions Some of the important divisions in the geologic time scale mark times when rapid changes happened on Earth. * extinct. Chapter 8

7. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 1 Evidence of the Past Chapter 8

8. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 1 Evidence of the Past The Changing Earth Pangaea German scientist Alfred Wegener proposed that long ago the continents formed one landmass, which he called Pangaea, surrounded by a gigantic ocean. Do the Continents Move? In the mid-1960s, J. Tuzo Wilson of Canada came up with the idea that huge pieces of the Earth’s crust were pushed around by forces within the planet. * plate tectonics. Chapter 8

9. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 1 Evidence of the Past Chapter 8

10. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 1 Evidence of the Past The Changing Earth Adaptations to Slow Change When conditions on the Earth change, organisms may adapt or become extinct. A rapid change, such as a meteorite impact, may cause a mass extinction. But slow changes, such as moving continents, allow time for adaptation. Chapter 8

11. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 2 Eras of the Geologic Time Scale Objectives Outline the major developments that allowed life to exist on Earth. Describe the types of organisms that arose during the four major divisions of the geologic time scale. Chapter 8

12. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 2 Eras of the Geologic Time Scale Precambrian Time Precambrian time is * How Did Life Begin? Scientists think that life developed from simple chemicals in the oceans and in the atmosphere. Energy from radiation and storms could have caused these chemical to react. Chapter 8

13. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 2 Eras of the Geologic Time Scale Precambrian Time, continued Photosynthesis and Oxygen The first cyanobacteria began to release oxygen gas into the oceans and air. Eventually, some of the oxygen formed a new layer of gas in the upper atmosphere. Multicellular Organisms After about 1 billion years, organisms that were larger and more complex than prokaryotes appeared in the fossil record. Chapter 8

14. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Origin of Eukaryotic Cells Chapter 8 Click below to watch the Visual Concept. You may stop the video at any time by pressing the Esc key. Visual Concept Section 2 Eras of the Geologic Time Scale

15. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 2 Eras of the Geologic Time Scale The Paleozoic Era The Paleozoic era began about * Life on Land During the 300 million years of the Paleozoic era, plants, fungi, and air-breathing animals slowly colonized land. Chapter 8

16. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 2 Eras of the Geologic Time Scale The Mesozoic Era The Mesozoic era began about *. Life in the Mesozoic Era Dinosaurs are the most well known reptiles that evolved during the Mesozoic era. The Extinction of Dinosaurs At the end of the Mesozoic era, 65.5 million years ago, dinosaurs and many other animal and plant species became extinct. Chapter 8

17. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 2 Eras of the Geologic Time Scale The Cenozoic Era The Cenozoic era began about* The Age of Mammals The Cenozoic era is sometimes called the Age of Mammals. Mammals have dominated the Cenozoic era the way reptiles dominated the Mesozoic era. The Cenozoic Era Today We are currently living in the Cenozoic era. Modern humans appeared during this era. Chapter 8

18. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 2 Eras of the Geologic Time Scale Chapter 8