Common Ancestry

Sorting Seeds

Students sort images of seeds using a classification scheme of their own design. This exercise is intended to demonstrate how living things can be organized by similarities and differences in traits, preparing students to consider the similarities in living things in subsequent exercises.

• Distribute sets of Seed Cards to individual or pairs of students, instructing them to make observations about the seeds then group them by a scheme of their choice.
• Have students report about the scheme they used, discussing similarities and differences in schemes.

Example groupings: Size, color, shells, two distinct halves, type (nuts, beans, seeds), etc.

• Humans are natural organizers.
• Living things can be organized by their similarities and differences.

Students create their own organization and classification system based on observed characteristics of seeds.

20 minutes

Tree Diagrams

The video introduces tree diagrams. It prepares students for subsequent activities in which they will use tree diagrams in hypothesizing about common ancestry based on several lines of evidence.

Project to the whole class, pausing as needed to discuss.

• Similarities among living things might indicate relatedness.
• Organisms with the most similarities tend to be more closely related.
• Tree diagrams are ways to organize hypotheses about the relationships among living things.

Tree diagrams are introduced as a system for organizing information and hypotheses about the relationships among organisms.

10 minutes

Fish or Mammals?

This series of activities explores the ancestry of cetaceans (whales, dolphins, and porpoises). The Case Study document presents data from comparative anatomy, fossils, embryological development, amino acid sequences, and DNA transposons.

The Evidence Organizer helps students follow the path that scientists took to understand the ancestry of cetaceans. Each new piece of evidence led to a more-detailed understanding: first that cetaceans are more similar to mammals than to fish; then that they descended from a four-legged mammal that lived on land; later still that they descended from an even-toed ungulate; and finally that their closest living relative is the hippopotamus.

Tip: If students are still having trouble interpreting tree diagrams, go through the practice with trees extension.

Once students have finished filling in their Evidence Organizers (and they understand the evidence), give them a copy of the Argumentation document. Here they will match the appropriate pieces of evidence with the provided claims and reasoning.

• Fossils, anatomy, embryos, and DNA sequences provide corroborative lines of evidence about common ancestry, with more closely related organisms having more characteristics in common.
• (Argumentation) In a scientific argument, evidence is data or observations that support a claim.
• (Argumentation practice) Choose evidence that supports a given claim and is consistent with a given line of reasoning.

Students interpret fossil, anatomical, embryological, and DNA data to determine the ancestry of cetaceans.

Students find patterns in fossil, anatomical, embryological, and DNA data to determine relatedness.

Students practice choosing the appropriate evidence that supports a claim and is in line with given reasoning.

90 minutes

Extension Activity: A Tale of Two Pandas

This second case study provides more practice with the evidence for common ancestry. In addition, it explores the scientific process, highlighting the fact that sometimes lines of evidence appear to contradict one another. The reading level and the evidence presented here are more challenging than in the Fish or Mammals case study.

The Tale of Two Pandas video introduces the question, "Is the giant panda a bear or a raccoon?"

The Case Study document presents evidence from behavior, anatomy, fossils, and DNA. Note that different lines of evidence support different conclusions.

The Teacher Guide describes how you can assign the lines of evidence to different groups of students to evaluate. Using the Evidence Organizer, students organize their evidence to support one of three claims.

To wrap up, present the Conclusion (pdf document) to summarize how more recent DNA evidence finally led to a scientific consensus: that the giant panda shares a more recent common ancestor with bears.

• Fossils, anatomy, embryos, and DNA sequences provide corroborative lines of evidence about common ancestry, with more closely related organisms having more characteristics in common.
• (Argumentation) In a scientific argument, evidence is data or observations that support a claim.
• (Argumentation practice) Choose evidence that supports a given claim.

Students find patterns in anatomical, fossil, and DNA and protein data to determine relatedness.

Students interpret anatomical, fossil, and DNA and protein data to determine the evolutionary relationships of the giant panda.

60 minutes

Evidence for Common Ancestry

With the Fish or Mammals? series, students have seen how multiple lines of evidence can help answer puzzling questions about common ancestry—and how multiple lines of evidence generally point to the same conclusion. This activity is intended to reinforce the idea that lines of evidence tend to corroborate one another, while also making the point that many questions about common ancestry are not so puzzling. Students explore a collection of inferences about the relationships among species, which are based on various lines of evidence.

Have students explore individually or in pairs. An optional guiding worksheet is provided.

• Fossils, anatomy, embryos, and DNA sequences provide corroborative lines of evidence about common ancestry, with more closely related organisms having more characteristics in common.

30-40 minutes

Common Ancestry: It's in our DNA

This slide presentation circles back around to the DNA > Protein > Trait connection explored in the previous module (Shared Biochemistry) and puts it in context as the phenomenon that underlies all of the other lines of evidence for common ancestry.

Project to the whole class. You may wish to review the main science ideas.

• DNA codes for proteins. Collectively, proteins are responsible for an organism's traits.
• DNA underlies the similarities and differences in fossils, anatomy, and embryos.
• More closely related organisms have more genes in common.

The DNA > Protein > Trait (cause) connection is put in context as the phenomenon that underlies all of the other lines of evidence (effect) for common ancestry.

5-10 minutes

Common Ancestry: It's in our DNA (online slideshow)

Online Phylogenetic Tree

Choose any two species on the phylogenetic tree to see the relative sizes of their genomes, the number of shared and unique genes, and the amount of time that has passed since they diverged from a common ancestor.

After two organisms are selected, overlapping circles representing their genomes will appear.

Have students explore individually or in pairs as they fill in the worksheet.

• All organisms have some genes in common.
• More closely related organisms have more genes in common.
• Tree diagrams are visual representations of evolutionary history that depict patterns of common ancestry and speciation over time.

Students use an interactive tree diagram to find general patterns in genetic data and the relationships among organisms.

10-15 minutes

Computers with internet access (online activity has no sound)

Formative Assessment

This quick formative assessment checks to see how well students understand tree diagrams and evidence for common ancestry.

10 minutes

Extension Activity: Geologic Time in 92 Days

A daily exercise equating geologic time to 92 school days. Paper cut-outs representing 50 million years are introduced each day and build upon one another to create a timeline. A key explaining the events and biologic forms in each 50-million year square is included. This activity integrates with the Geologic Time page from Learn.Genetics.

Before moving on, make sure your students understand the following:

• The similarities among living things suggest relatedness.
• Fossils, anatomy, embryos, and DNA sequences provide corroborative lines of evidence about common ancestry, with more closely related organisms having more characteristics in common.
• DNA underlies the similarities and differences in fossils, anatomy, and embryos.