1. Explain to students that humans rely on classification systems to bring order to many everyday activities. For example, imagine how difficult it would be to grocery shop if the items were placed randomly throughout the store. Instead, grocery store items are grouped according to their similarities. In other words, they are categorized. Explain that scientists categorize organisms in the same way.

Have students work in pairs, and provide each pair with 10 to 20 of the collected specimens. Ask students to group their specimens according to similarities they observe. Tell them that they can use whatever criteria they want but that they must be able to explain to the class why they grouped the specimens as they did.

During the discussion of each pair's categorization, ask them to consider the following:

• What similarities exist among the organisms within each category?
• What differences exist between different categories of organisms?
• What can be learned about organisms by classifying them?
• Are some organisms in a particular group more similar to each other than to any other organism in that group? Might these organisms form a subgroup?

2. Have students watch the The Common Genetic Code video, which illustrates the genetic similarity among all living things. The video highlights the research of scientist Paul Nurse, who demonstrated that a human gene could function in a yeast cell. Discuss the following:

• What happened when researcher Paul Nurse added human DNA to mutant yeast cells?
• If human genes can repair yeast cells that have lost their ability to replicate, what does that tell us about the two organisms?
• What does it mean to "ronserve" a gene?
• What are "junk genes"?
• Because we share the gene pool with "yeast and the lowly worm," Paul Nurse believes that we should have greater respect for all living things. Do you agree? Why or why not?

3. Show the Common Past, Different Paths video and discuss the following:

• How do the similarities in the developing embryos support the theory of common ancestry?
• What might the developmental similarities seen in the video suggest about the genetic similarities among these animals?

4. Have students work in pairs as they explore the All in the Family Web activity. In this activity they learn how different types of evidence are used to determine evolutionary relationships. Out of a group of three animals, students choose the two they think are most closely related. They can base their choice on appearance, anatomical similarities, developmental morphology, or molecular evidence. As they discover, looks can sometimes be deceiving.

Instruct student pairs to make classification decisions only after discussing reasons for and against the other options. Afterward, have students go back and dig for more evidence to support (or correct) their answers. Remind students that this is the same process scientists go through: forming a hypothesis on the basis of available evidence, then refining or affirming their hypothesis based on the discovery of new evidence.

5. Finish the lesson by discussing the concept of unity among all organisms. Stress the following points:

• All organisms, even those that are distantly related, share many of the same genes -- genes that scientists can now transfer from one organism to another, where they produce the same effect as in the original organism.
• As a result of these shared genes, organisms share many physiological, chemical, and behavioral traits, as well as many developmental processes. These similarities can be observed to varying degrees among all organisms.