In this lesson, students learn about the fossil record, the primary type of evidence scientists use to piece together the history of life and to support and refine the theory of evolution. The lesson begins with an overview of the fossil formation process, then covers the evolution of land-dwelling animals from fish, and finally looks at how some mammals (whales) ended up back in the water.
- Discover how scientists use fossil evidence to trace the evolution of various species
- Understand methods used to date fossils
- Learn about some of the most important evolutionary transformations in the history of life
- Three class periods
- Record of Time PDF Document (for the teacher)
- Becoming a Fossil Video
- Laetoli Footprints Video
- Radiometric Dating Video
- Fish with Fingers Video
- Tetrapod Limbs JPEG Image
- Evolving Ideas: How Do We Know Evolution Happens? Video
- Whales in the Making PDF Document
- Whale Evolution Data Table Worksheet PDF Document
Before the Lesson
- Read the background essay that accompanies each resource to gain information that will help you facilitate class discussion.
- Read the Record of Time document for detailed information on fossil formation and the various methods used for dating fossils.
- Make copies of the Whales in the Making handout and the Whale Evolution Data Table Worksheet (PDF) worksheet -- one for each student.
- Prepare a vertical timeline of the Cenozoic era on paper. Make the timeline 65 inches tall, and label the top the present and the bottom 65 million years ago (Mya). Label every million years, with one inch equal to one million years (My). Highlight the Eocene epoch (55-34 Mya). Display the timeline as a reference tool for the whale evolution part of this activity.
Part I: Fossil Formation
1. Have students watch the Becoming a Fossil video. Then discuss the following:
- Why do most living things not leave fossils behind?
- How are fossils formed?
- How are fossils uncovered?
- How do scientists determine the age of fossils?
2. Have students watch the Laetoli Footprints video. Then discuss the following:
- What was the unusual series of circumstances that caused the Laetoli footprints to be preserved? Does this combination of events say anything about why such footprints are a rare find?
- What do the footprints at Laetoli tell scientists about the way the creatures that made them moved?
3. Have students watch the Radiometric Dating video. Then discuss the process of radiometric dating, as well as other methods of dating fossil finds.
Part II: Evidence of the Evolutionary Process
4. Have students watch the Fish with Fingers video. Then discuss the following:
- What did old theories say about the evolution of land-dwelling animals, and why was paleontologist Jenny Clack dissatisfied with these explanations?
- What evidence did Clack find to disprove old theories?
- What explanation of the evolution of land animals can Clack give based on current fossil evidence?
5. Have students examine the Tetrapod Limbs image. Then discuss the image with students, focusing on these specific questions:
- What are the similarities and differences among the seven limbs shown?
- How would scientists explain why these very different species all have limbs with five digits?
- What is the difference between a homologous structure and an analogous structure? Name some examples of each.
6. Show the Evolving Ideas: How Do We Know Evolution Happens? video and discuss the following:
- What can we learn from fossil evidence?
- What specific fossil evidence points to the whale's evolution from land to water?
Part III: Whale Evolution
7. Pass out copies of the Whales in the Making handout and the Whale Evolution Data Table Worksheet (PDF) worksheet. Have students work in teams of two. Ask them to cut out the six fossil boxes from the handout and gather information about each fossil from resources in the Evolution Library (http://www.pbs.org/wgbh/evolution/library) or in books from the school library.
8. Ask each team of two to prepare an Eocene epoch timeline on paper, using the same scale as the classroom model (one inch equals one million years). Their timelines should be twenty-one inches long. Each million years should be labeled, with 34 Mya at the top of the timeline and 55 Mya at the bottom.
9. Have teams mount fossil boxes 1 and 2 from the handout at the proper locations on their timelines. Point out the large gap between these two fossils. Then have students add the remaining fossils in order by the age of the fossil (from youngest to oldest).
10. Discuss the following:
- What typical whalelike traits were apparently the earliest to appear? What apparently evolved much later?
- As each "missing link" was found, how many new gaps were formed? What is the relationship between gaps and fossils?
- To find fossil evidence to fill the largest remaining gap in whale evolution, what age sediments would you search?
- What distinguishing traits would you expect to find in whale fossils of that age?
Check for Understanding
To help students synthesize what they've learned about evolution from these activities, ask them to discuss the following:
- Why is it difficult to find an evolutionary trail of fossil species leading from a common ancestor?
- What have the Laetoli footprints and the bone structure of the Lucy fossil taught paleontologists about the way Lucy moved?
- What can a creature's way of moving say about its evolution?
- How did fossil evidence change scientists' ideas about the transition from life in water to life on land?
- How did fossil evidence change scientists' ideas about the transition of mammals from land back to water?
- Explain why the absence of transitional fossils does not mean that evolution didn't take place.