1. Tell students that Earth's history is marked by a series of transformations, mass extinctions, and geologic changes that have taken place over the course of 4.6 billion years. In this part of the lesson they are going to use a Web site to research some of the most important events. Later they'll label the events on a geologic timeline.

2. Divide the class into three groups. Assign one of the following subjects to each group: Transformations, Extinctions, and Geologic Changes.

Tell students to visit the Deep Time Web activity for information about the "events" listed under their group's subject. (These events are listed below.) Have the groups divide up the events so that each student is required to find information about only one or two events during the research period. Recommend that students take detailed notes about the events they research, including when the event happened, a description of the event, and what other events were happening around the same time.

Write the following events on the board, with the exception of the dates and extinction names in parentheses. (This is part of the information students are expected to find.)

Transformations

• First evidence of life (3,850 million years ago [Mya])
• Oldest fossils (3,500 Mya)
• First evidence of soft-bodied animals (900 Mya)
• The Cambrian Explosion (530 Mya)
• First land plants and fish (480 Mya)
• First reptiles (350 Mya)
• First mammals and dinosaurs (220 Mya)
• First birds (150 Mya)
• First hominids (5.2 Mya)
• Modern humans (0.1 Mya)

Extinctions

• Some single-celled animals and soft-bodied animals (Vendian 543 Mya)
• Reef-builders and other shallow-water organisms (Cambrian 520 Mya)
• Twenty-five percent of marine invertebrate families (End Ordovician 443 Mya)
• Fifty to fifty-five percent of marine invertebrate genera (Late Devonian 364 Mya)
• Ninety percent of all species (End Permian 250 Mya)
• About 50 percent of marine invertebrate genera (Late Triassic 206 Mya)
• Dinosaurs and 60 to 80 percent of all species (End Cretaceous 65 Mya)
• Foraminifera, gastropods, and sea urchins (Late Eocene 33 Mya)
• Many woodland, plant-eating herbivores (Miocene 9 Mya)
• Nearly all mammals and birds over 45 lbs. (Late Pleistocene 0.1 Mya)

Geologic Changes

• Formation of the great oceans (4,200 Mya)
• Continents begin shifting (3,100 Mya)
• Rodinia supercontinent breaks up (700 Mya)
• Gondwana forms (500 Mya)
• Great mountain ranges form (425 Mya)
• Formation of Pangaea supercontinent (280 Mya)
• Pangaea supercontinent breaks up (200 Mya)
• Inland seas dry up (20 Mya)
• Global ice ages begin (2 Mya)

3. Following a period of research using the Deep Time Web activity, have students deliver a two- or three-sentence oral report on the information they found about their event(s). Students should include the approximate date of the event, a description of the event, and a brief summary of other events that were happening around the same time. Following each report, give the student the corresponding event tag to place in the appropriate place on the geologic timeline.

4. When all event tags have been placed on the timeline, ask students to identify any patterns they see. Recommend that students look for connections between geological changes and the extinctions and/or transformations that follow. Also, suggest that extinctions often open up opportunities for the survivors and that dramatic transformations sometimes occur in their wake.

5. To help students understand that many of what we consider the most important events in the history of life occurred relatively recently, ask students to imagine compressing the events of 4,600 million years into just one year. What might that look like on a calendar? For example, when during the calendar year would dinosaurs have existed? During which month would life have originated? In which season would modern humans have evolved? After giving students the opportunity to make those analogies, place the month labels in their appropriate places below the geologic timeline. Ask students to continue discussing the events in the history of life -- making clear that comparing geologic time to a calendar year is purely an intellectual exercise designed to help them put such a huge amount of time into perspective.

6. Ask students to choose one of the three subjects -- transformations, extinctions, or geologic changes -- they haven't explored yet. Write the following questions on the board. Instruct students to use information they learned from their fellow classmates, as well as information they find on another visit to the Deep Time Web site, to answer the questions listed below their chosen subject.

Transformations

• What are some of the major evolutionary transformations in the history of life?
• When did these transformations occur, and what types of organisms were involved?
• What fossil examples of such transformations have been discovered?

Extinctions

• Name the five largest mass extinction events in the history of life on Earth.
• When did these mass extinctions occur, and what organisms were affected?
• What kinds of evidence do scientists use to better understand prehistoric extinctions?

Geologic Changes

• How did the continents change from the Precambrian to the Permian to the Jurassic to the Eocene?
• How could the shifting of continents have influenced the distribution and migration of plants and animals?
• What patterns can you see in the movement of landmasses over time?

7. Have students explore the What Killed the Dinosaurs? Web activity and discuss the following:

• What animals were most affected by the extinction that occurred at the end of the Cretaceous period, 65 million years ago?
• What are some of the hypotheses scientists have come up with to explain the extinction of the dinosaurs?
• What types of evidence do scientists use to support those hypotheses?
• Is one of those hypotheses more accepted than the others?
• What group of animals probably benefited most from the extinction of the dinosaurs? Why?