Teaching Evolution, Activity 6

Teaching about Evolution and the Nature of Science

Chapter 6

ACTIVITY 6

Understanding Earth's Changes Over Time

Comparing the magnitude of geologic time with spans of time in a person's lifetime is difficult for many students. In this activity, students use a long paper strip and a reasonable scale to represent visually all of geologic time, including significant events in the development of life on earth as well as recent human events. The investigation requires two class periods and is appropriate for grades 5 through 12. The activity is adapted with permission from the Earth Science Curriculum Project.¹

Standards-Based Outcomes

This activity provides all students with an opportunity to develop understandings of the earth systems as described in the National Science Education Standards. Specifically, it introduces them to the following concepts:

A mathematical scale representing the length of geologic time.
The relationship of time between human events, events in earth's history, and the total age of the earth.
The formation of the sun, the earth, and the rest of the solar system from a nebular cloud of dust and gas 4.6 billion years ago.
The estimation of geologic time by observing rock sequences and using fossils to correlate the sequences at various locations. A current method of dating earth materials uses the known decay rates of radioactive isotopes present in rocks to measure the time since the rock was formed.
The ongoing evolution of the earth system resulting from interactions among the solid earth, the oceans, the atmosphere, and organisms.
The evidence for one-celled forms of life—the bacteria—extending back more than 3.5 billion years. The evolution of life caused dramatic changes in the composition of the earth's atmosphere, which did not originally contain oxygen.

Science Background for Teachers

Geologic time is largely subdivided on the basis of the evolution of life and on the amount and type of crustal activity that occurred in the past. Geologic time is ordered both relatively and absolutely. For relative dating the sequence in which rock strata formed is important; to explain the complete time scale for all of geologic history required correlating rock formations throughout the world. Fossils are important guides in this correlation as scientists assigned relative dates to the world's rocks according to a proposed sequence of life (fossil evidence).

Radiometric dating provides absolute ages for events in the earth's history. Radiometric dating techniques apply the decay rates of selected naturally radioactive isotopes to stable daughter isotopes to determine how long the unstable parent isotopes have been decaying. Fairly accurate dates have been determined for the events beginning in the Cambrian era; this comprises about 12 percent of the earth's history.

Materials and Equipment

The following materials will be needed by each group of two students:

A paper strip, such as adding machine tape or shelf paper,
A meter stick,
Masking or cellophane tape.

If students use the scale suggested—1 millimeter to 1 million years and 1 meter to 1 billion years—a paper strip 5 meters in length efficiently accommodates the 4.5-billion-year time scale. Make copies of Student Investigation Sheet A on page 91 ("Approximate Ages of Events in Years Before the Present"). Students will use this page to conduct their investigation.

Instructional Strategy

Engage — Ask students how long a million years is. How would students count or measure a million of anything? Use this discussion to help students arrive at the question: How does a million years, or even the time since the last ice age, compare with the age of the earth? Suppose you want to make a visual model showing a time line of the earth's history, how would you proceed?

Explore — Provide students with Student Investigation Sheet A. Have them decide how to represent these events in a time-ordered sequence. Provide a roll of paper tape on which to plot the model.

Students might need help in understanding how to set up a scale that can be displayed in the classroom or adjacent hallway. A reasonable scale is 1 millimeter to 1 million years, 1 centimeter to 10 million years, and 1 meter to 1 billion years. Depending on available space, larger unit distances will be easier to work with. Regardless of the scale the students choose, the last million years will be difficult to plot. Allow students to work out a scale on their own. However, to avoid undue confusion and frustration for some, review student progress after the first few minutes and be ready to ask leading questions or make suggestions.

Allow students time to agree on a reasonable scale, mark the locations of each event on their time scale, and resolve the problem of trying to fit the events from the last 1 million years in the allotted space. When appropriate, encourage students to construct a separate, and larger, scale for marking the most recent events.

Explain — Discuss the long period of time in the earth's history before evidence of simple life forms, such as algae, appear in the fossil record. Note that time spans between significant "firsts" become shorter and shorter as you move closer and closer to "today." Compare and discuss expanded scales used to show more detail in the recent past. Discuss the role of scale in helping visualize and better understand the extremely long time span of the geologic time scale and the connections to biological evolution.

Elaborate — Challenge students to develop an extended time scale to mark special events in their own lifetime and that of their parents, grandparents, or another adult. Have them calculate the percentage of the earth's history for which there is evidence of life, the percentage of the earth's history for which there is fossil evidence of the first humanlike animals, or the percentage of the earth's history during which dinosaurs lived.

Evaluate — Ask students to calculate the length of a paper strip necessary to represent all of geologic time when using the extended scale they used to show the most recent events. Have students write a short news article explaining their scale of geologic time and the evolutionary changes in the earth's lithosphere, atmosphere, and biosphere.

Student Investigation Sheet A

Approximate Ages of Events in Years Before the Present

2. First known plants (algae), 3.2 billion years ago.

3. First known animal (jellyfish), 1.2 billion years ago.

4. Beginning of the Cambrian and first abundant fossils, 550 million years ago.

5. Beginning of the Ordovician and first backboned animals, 500 million years ago.

6. Beginning of the Silurian and the first land plants, 440 million years ago.

7. Beginning of the Devonian and the first amphibians, 400 million years ago.

8. Beginning of the Mississippian, 350 million years ago.

9. Beginning of the Pennsylvanian and the first reptiles, 305 million years ago.

10. Beginning of the Permian, 285 million years ago.

11. Beginning of the Triassic and first dinosaurs, 245 million years ago.

12. Beginning of the Jurassic and first mammals, 205 million years ago.

13. First birds, 150 million years ago.

14. Beginning of the Cretaceous, 140 million years ago.

15. Beginning of the Paleocene and first primates, 65 million years ago.

16. Beginning of the Eocene, 60 million years ago.

17. Beginning of the Oligocene and first elephants, 35 million years ago.

18. Beginning of the Miocene, 25 million years ago.

19. Beginning of the Pliocene, 5 million years ago.

20. First humanlike animals, 2 million years ago.

21. Beginning of the Pleistocene and ice ages, 1 million years ago.

22. Last ice age, 10,000 years ago.

Convert the following to years before the present:

23. Mount Vesuvius eruption destroys Pompeii, A.D. 79.

24. First U.S. satellite orbited, 1958.

25. First man on the moon, 1969.

26. Last New Year's Day.

27. Today.

Notes

Investigating the Earth.

PDF Worksheet

Student Investigation Sheet A