1. Tell students that they will be studying the evidence for the evolutionary transition from a fish-like ancestor that lived in the water to a salamander-like ancestor that walked on land. Provide students with the Skeletal Structures of Four Vertebrates PDF Document to begin looking at the basic anatomy of the forelimb. Discuss the following questions:

1. What similarities and differences do you see in these bone structures?
2. What might this say about the common ancestors of four-limbed vertebrates?

2. To interpret the fossil record or understand whether a fossil is a transitional form, we need to know the parts of the structures important in the transition. Tell students that they will begin with a human because we are most familiar with this structure. Ask them to start by identifying the following structures on the human forelimb diagram: humerus, ulna, radius, carpals, metacarpals, and phalanges. Then ask students to identify the structures listed above on the forelimbs of the amphibian and the bird. Discuss the following questions:

1. What do you notice about the placement and structure of the humerus, radius, and ulna in each of the skeletons?
2. Why might these three animals show similarities and differences in the structure of their forelimbs?

Note: It is important that students understand that, given our common ancestry with all four-limbed vertebrates, we should be able to compare the forelimb of a human with the forelimb of other terrestrial vertebrates. We expect there to be differences because we inhabit very different environments and are required to do very different tasks with our forelimbs, but we can also find many similarities.

Finally, ask students to take a look at the structure of the fin of a ray-finned fish. The anatomy of a fish fin is quite different from that of the previous animals. Label the axial and radial structures with the students. Ask students to summarize similarities and differences between the pectoral fin of the fish and the pectoral limb (forelimb) of the other animals.

Note: Students should notice that the pectoral fin of the fish (even though it did share a common ancestor with the human, amphibian, and bird at some point in time) is very different. The pectoral fin of a ray-finned fish is made up of many small rays that consist of linear arrangements of bones. The specific bones can be difficult to discern, but are generally comprised of radial (closest to the body) and axial (furthest from the body) bones.

3. Explain to students that we can use the similarities among species to construct a cladogram, a diagram used to illustrate evolutionary relationships among species by analyzing certain kinds of characters, or physical features, in the organisms. In the course of evolution, a novel, heritable trait will emerge in some organism. This trait—which corresponds to a particular form of one character (for example, the presence or absence of vertebrae)—will be passed on to its descendants. Two organisms that share such a new, or derived, trait or group of traits are therefore more closely related to each other than to organisms that lack those traits. By treating recently evolved characteristics differently from ancestral characteristics, this technique emphasizes evolutionary relationships over structural similarities. Hand out the Cladistics: Definition of Terms PDF Document and review the cladogram definition and illustration with students.

To make a cladogram, we compare the traits of one group of species with those of an "outgroup," a group that is not thought to be closely related to the group being studied. For example, when looking at the ancestry of certain mammals, we can compare their traits with those of non-mammalian vertebrates, such as birds. In doing this, we find that all mammals and birds have vertebrae and four limbs, so they must have descended from a common ancestor. Thus, we can conclude that these traits were present in some ancestor of these particular mammals. Have students fill in the Student Version of the Character Matrix for Constructing a Cladogram PDF Document, indicating a "0" if the trait is absent in the group and a "1" if the trait is present. Students should do this in pairs, then explain and compare their answers in small groups. Finally, have students check their answers as you review the exercise with the whole class. Note: Answer key for teachers is included in the PDF Document.

Next, have student pairs use the data in the character matrix to build a cladogram to explore the common ancestry of different organisms. Students should follow the protocol below:

1. Identify characters that all individuals share—shared inherited characters that are typical of the whole group.
2. Find out which character(s) is/are shared by all but one species. That species will be the first to branch off the cladogram.
3. Continue to analyze the remainder of the species by finding which ones share the most and least characters, and then placing them one-by-one on the cladogram.
4. Be sure to do the following:
   • Place the names of organisms (or taxa) on the outer tips of branches.
   • At each internode, list the shared derived characters that all organisms at the ends of the branches that follow it share (unless modified later).
   • Record shared derived characters only once, unless you want to indicate that the character evolved several times independently (not probable).

After students have constructed their cladograms, ask them to discuss their hypotheses about the common ancestry of these organisms.

4. (Optional) If you want to show students how scientists did similar work when studying Devonian fish fossils, you can display this Sample Data Matrix HTML Document from the Nature Web site. Scroll to the bottom of the document to see the matrix. The characters (#1-114), which are described in the first five pages, can be read in vertical columns in the matrix.

5. (Optional) Ask students to to use the Tree of Life Web Project to develop a new cladogram showing the evolutionary relationships of the following organisms: an actinopterygian fish (such as a salmon), a sarcopterygian fish (such as a lungfish or coelacanth), an amphibian (such as a salamander), a bullfrog, a lizard (such as an iguana), a snake (such as a rattlesnake), a bird (such as a chicken), a crocodile, a turtle, a cow, a human, and a chimpanzee. This can be done in a lab as a whole class, or for homework. Begin by orienting the class to the Web site. Make sure students understand that they are looking at a single cladogram that is separated into pages or levels on the Web site. Then have students work in pairs to draw their new cladogram. More advanced students can provide names for each of the nodes as well as the tip species. The cladogram that students construct should look like the Sample Cladogram PDF Document.

6. Have students watch the Fish with Fingers QuickTime Video. Then, provide students with the Sample Cladogram PDF Document. Ask students to study the cladogram. Make sure that they understand that Eusthenopteron, Panderichthys, Tiktaalik, Acanthostega, Ichthyostega, and Tulerpeton represent species from the fossil record.

Note: If you need more information on these species to share with your students, you can find them on the Tree of Life Web Project or the Devonian Times Web site. You can also find images of the fossils to share with students.

7. (Optional) For more information about Devonian fish fossils listed in the sample cladogram, show students the Summary of Late Devonian Fish and Early Tetrapods PDF Document. Advanced students can try to construct a timeline that shows when they think each of these fossil species existed. They can then place the fossils in the timeline based on the age of the rock in which the fossils were found. They can then use information from the Tree of Life Web Project and the Devonian Times Web site to add information about the characters of these fossil species to their cladogram.

Explain to students that we are most interested in the transition from a fish-like ancestor to a tetrapod. Given what students have learned about the placement of these animals (the lobe-finned fish-like Eusthenopteron and Panderichthys, and the early tetrapod-like Acanthostega and Ichthyostega) in the cladogram, ask them to consider where they would look (habitat, environment, time period) and what would they look for (what fossil characters would qualify as intermediary) to further delineate this transition.

8. Watch the Transitional Tetrapod Fossil QuickTime Video to see Neil Shubin describe the discovery of Tiktaalik in rocks on Canada's Ellsmere Island.

Discuss the following questions with students:

1. What characters link Tiktaalik to fish-like ancestors?
2. What characters link Tiktaalik to more tetrapod-like ancestors?

9. As noted in the previous video, the structure of the pectoral fin of Tiktaalik provides evidence of how organisms may have made the transition from living in an aquatic environment to moving about on land. Have students color the different structures of the skeletons on the Pectoral Limb PDF Document, being sure to use the same color for the same structures in different species. Students should then cut these diagrams out and place them on the cladogram with their corresponding organisms. Ask students: How does the structure of its forelimbs help explain Tiktaalik's place on the cladogram?

10. (Optional) To further understand the significance of Tiktaalik, it is important to also examine some of the other characters that define Tiktaalik as a transitional form (in addition to the pectoral fin/forelimb). Have students read one of the following summary articles. Ask them to list and describe the structures that indicate Tiktaalik's more primitive ancestry as well as the structures that relate to the origin of tetrapods.

• Tiktaalik roseae
• Understanding Evolution: What has the head of a crocodile and the gills of a fish?