• Informally assess individual and group knowledge by observation during the Life on Earth game or by collecting students’ papers.
• During the discussion of the Cambrian period, check for understanding by asking students:

o   “Why was the Cambrian period important in Earth’s history?”

o   “Can you name several organisms that existed during this period?”

o   “What are some causes of the extinction of species?

• Collect and check the Cambrian Fossils worksheet.

Part 1: Timeline of Life on Earth

To prepare, write three headings from left to right across the board:

• 3.5 to 4 Billion Years Ago
• 570 to 600 Million Years Ago
• 10 Million Years Ago to Present

List the events below on large index cards with masking tape on the back; make four sets of cards. Also, write the events on the board in random order:

• Unicellular organisms appear
• Multicellular organisms appear
• Simplest organisms appear
• Plants and animals appear
• Most complex organisms appear
• Humans appear

Instructions:

1.      Ask students to work alone to write the events on paper in the correct order.

2.      Divide the class into four teams, and allow time for the teams to discuss their individual answers with one another. The team will need to agree on an order to present to the class. Observe to make sure that students are given a chance to defend their answers in their groups. While they are discussing, erase the events from the board, but not the dates in the headings. Give each team a set of event cards.

3.      Select one student from each team and give them time to tape the events under the headings in the correct order.

4.      Tell each group how many events they have correct, but not which ones. Give them some more time to rearrange the cards. Continue the game until a group has the correct order on the board.

Answer KEY:

Show students the clock and explain, “If the history of all life on Earth happened in 24 hours, modern-day humans appeared only 30 seconds ago. For most of Earth’s history, there were only unicellular organisms; they appeared around 4:00 a.m. Multicellular organisms are fairly recent on Earth; they arrived at 9:05 p.m. This time period is called the ‘Cambrian explosion’ because so many different kinds of organisms appeared over a relatively short time. Dinosaurs appeared around 11:00 p.m.” (see www.pbs.org/wgbh/nova/transcripts/3111_origins.html) Ask students, “What do you think is the reason for the appearance of multicellular organisms on Earth?” Collect student responses and address questions as needed.

Timeline: Life on Earth

Use a document camera to display the Timeline of Life on Earth (S-6-3-1_Timeline of Life on Earth.doc). Point out the oldest and most recent ends of the timeline. Point out and define the “Cambrian period” as “the geologic period at the beginning of the Paleozoic era, with the appearance most major groups of multicellular organisms except vertebrates.” Have students write the definition in their notes. Ask students, “Why is it sometimes called the ‘Cambrian explosion?’” (because the diversity of life appeared over a relatively short time period.)

Tell students that the Cambrian period marks the beginning of the Paleozoic era (point out the Paleozoic era). The Cambrian period gets its name from a place in Wales where the first fossils from this time period were found. The Cambrian period lasted for about 33 million years, from about 543 million years ago until 510 million years ago. Ask students why the Cambrian period is so important to Earth’s history. (More complex organisms appeared at that time.)

Attach a string from one end of the classroom to the other and explain to students that it represents the history of Earth. Show them which end is the present day. Explain that they will be making a timeline along the string. Use a document camera to display the Timeline of Life list (S-6-3-1_Timeline of Life.docx). Have students work in pairs. Assign an event to each pair of students (depending on class size) and give each pair an index card, marker, and paper clip or clothespin. Have students write the events on the index cards, and then have the class attach their index cards to the string in the correct order. Assist them with spacing the events (approximately). Have students return to their seats.

Point out the fact that the Cambrian explosion (appearance of multicellular organisms) did not happen until relatively recently. Ask students, “Why do you think so many kinds of organisms appeared so quickly during the Cambrian period?” (Student answers may include climate change or more oxygen in the atmosphere.)

Define extinction. Show students that major extinctions have happened repeatedly throughout Earth’s history. Tell students that extinctions occur when the environment changes and the characteristics of organisms are not suited for survival.

Ask students, “What kinds of environmental changes do you think caused the extinctions?” (Student answers may include climate change, asteroid impacts, and ice ages.) Have students list possible causes of extinctions in their notes.

Optional: Show a video about the origins and evolution of life, such as the Geologic Timeline (click on video #10, titled Geologic Time), found at http://www.learner.org/vod/vod_window.html?pid=321.

Close the lesson by asking students what they think we can learn about modern life on Earth from studying the Cambrian period. (Accept all reasonable answers.)

Part 2: The Cambrian Explosion

Use an overhead projector or a document camera to display the multicellular organism pictures (S-6-3-1_Multicellular Organism Pictures.docx). Tell students that these are organisms from the Cambrian period and ask, “Do any of these types of organisms still exist today?” (Students should recognize several of the organisms, such as star fish, squid, and sea urchins.)

Ask students why they think most organisms may have lived in the water during the Cambrian period. (Students should respond that most organisms during this period lived in the water because most of the Earth was water, due to high water levels and no formation of glaciers yet.)

Optional: Display a map of Earth during the Cambrian period, available at www.ucmp.berkeley.edu/cambrian/burgess.html.

Explain what Earth’s environment was like during the Cambrian period: “The supercontinents of Laurentia and Gondwana separated into the land masses that became our modern-day continents. At that time, there was only one ocean, known as Panthalassa.” Ask, “What types of natural events do you think occurred during this separation of land masses?” Students can discuss this as a whole class or in small groups. Answers might include earthquakes, new oceans, new land masses, convection currents in the mantle, volcanoes, and plates colliding.

Remind students that the Cambrian period is sometimes called the Cambrian explosion because an incredible diversity of life emerged in a relatively short time, including many major animal groups that are alive today: “Scientists are uncertain what caused the explosion in diversity. It may have been caused by increased oxygen in the atmosphere from photosynthesis in bacteria and algae. Or it could have been caused by a warming climate leading to rising sea levels that flooded the land and created habitats that were suitable for new kinds of organisms.”

Create a concept map on the board like the one shown below. Have students copy the concept map into their notes.

Stress the importance of the Cambrian period and that many animal organisms including humans may exist today through the evolution of these early multicellular organisms.

Conclude by explaining how the Cambrian period may have ended. Explain that cooling temperatures and movement of glaciers most likely caused most of these early multicellular organisms to become extinct. Ask, “How do you think cooling temperatures and glaciers could have caused these animals to become extinct?” Students might give answers such as their bodies could not adapt to cooler temperatures, new predators may have entered the waters, and the waters froze.

List other possible explanations for mass extinction, including changing ocean waters that were cooler with less dissolved oxygen, increased volcanic activity, and meteor impact. Describe each group of multicellular organisms shown in the multicellular organism pictures (S-6-3-1_Multicellular Organism Pictures.docx):

• Brachiopods: Marine invertebrates with bivalve shells that have a pair of arms with tentacles inside; also called “lamp shells.”
• Echinoderms: A group of marine invertebrates that includes starfish, brittle stars, basket stars, sea cucumbers, sand dollars, and sea urchins. Echinoderm means “spiny skin.”
• Mollusks: Invertebrates that have a fleshy, muscular body. The phylum Mollusca includes snails, bivalves, squids, and octopuses.
• Trilobites: An extinct group of arthropods with segments of the body divided into three lobes.

Ask, “Why do most of these creatures have hard protective shells or skin?” Answers should mention protection against bigger prey of the sea and protecting their bodies’ parts so they are not as fragile. You can relate this to the human body and what protects our bones and body parts from being harmed.

Hand out copies of the Cambrian Fossils worksheet (S-6-3-1_Cambrian Fossils Worksheet and KEY.docx). Have pairs of students work together to complete the worksheet.

Extension:

• Students who may be going beyond the standards can find out more about Burgess Shale and report a description to the class (see Related Resources).
• Students who may be going beyond the standards can write a short science-fiction story in which the main character time-travels to the era of unicellular organisms, to the Cambrian period, or other eras. The story should explain what the character observes at each point in the Earth’s history.
• Students who may be going beyond the standards can create a timeline, drawn to scale, where smaller metric units (centimeters and meters) represent a specific number of years in Earth’s history. For example, five hundred million years may represent one meter on a timeline.
• Students who might need an opportunity for additional learning can complete a copy of the Timeline of Life list (S-6-3-1_Timeline of Life.docx). Pair students, including those who may be going beyond the standards, to complete the worksheet together.