What’s in This Unit?

Fossils are millions—even billions—of years old. New fossil discoveries can provide cutting-edge evidence about the history of life on Earth. In fact, in addition to fossils of other early species, paleontologists discover about 14 full dinosaur specimens every year.

In the Evolutionary History unit, students will take on the role of student paleontologists investigating a Mystery Fossil, which serves as the anchor phenomenon for the unit. This fossil is based on a real cetacean (whale) fossil excavated in Pakistan in 2000. The students’ task is to determine the Mystery Fossil’s evolutionary history so that they can accurately place the specimen in a museum exhibit. To gain an understanding of how paleontologists determine relationships between species, students use the Evolutionary History Simulation to analyze real fossil evidence and explore relationships on an interactive evolutionary tree. With a fossil collection at their fingertips, students identify similarities and differences among the skeletal structures of both extinct and living species. Students also use the Natural Selection Simulation to revisit principles of natural selection, applying this concept to understanding how one species becomes two. They read several articles about evolution, speciation, and natural selection, and they create models to show their thinking. By the end of the unit, students can use their analysis of skeletal structures to determine where they should place the Mystery Fossil in the museum, according to what type of organism the evidence shows it to be most closely related to—whales or wolves.

Why?

Investigating the relationships between a newly discovered fossil and other species via the anchor phenomenon motivates students to analyze evidence just as paleontologists do. Throughout the Evolutionary History unit, students are asked to make comparisons between skeletal structures of the Mystery Fossil, whales, and wolves. The unit builds on students’ questions and initial observations about the fossils in order to engage in advanced, careful analysis of fossil evidence. Their role as student paleontologists motivates them to make more complex and sophisticated comparisons between the Mystery Fossil and other species. Through their analyses of the Mystery Fossil and numerous other species in the Evolutionary History Simulation, students learn that species share similar structures because they descended from a common ancestor. They also learn that differences in structure arise due to natural selection and speciation over vast amounts of time. Immersing students in the anchor phenomenon by placing them in the role of preparing a museum exhibit that accurately introduces the Mystery Fossil to the world motivates them to present evidence for evolution to an outside audience. This encourages students to think deeply about how structural similarities and differences among fossils provide evidence for evolution.

How?

Evolution is a complex and dynamic topic. Single-celled life formed in the ocean over 3.5 billion years ago. Over time, life diversified (forming algae, sea sponges, jellyfish, and sharks) and moved to land (forming amphibians, reptiles, birds, fungi, and plants). Some land animals eventually evolved characteristics that brought them back into the ocean (e.g., whales). In this unit, students explore the unity and diversity of life through their investigations of the body structures of living and extinct species.

In Chapter 1, students are introduced to the Mystery Fossil and begin to address the Chapter 1 Question: Where in the museum does this new fossil belong? Students learn that paleontologists need to use careful and precise observations when they examine evidence from bone structures and are asked to follow this paleontological practice themselves. Using Species Cards, students examine body structure similarities between living and extinct organisms. Establishing that all living organisms have at least some similar body structures provides a starting point for students to understand that all living things are related and that species inherit their body structures from their ancestor populations. Students compare body structures using Species Cards and the Evolutionary History Simulation. They read the article “How You Are Like a Blue Whale,” which describes how paleontologists use body structures to explain that whales and humans are related even though they appear to be very different. At the end of the chapter, students consider similarities between the body structures of whales, wolves, and the Mystery Fossil.

In Chapter 2, students begin to consider why similar body structures (e.g., the hand bones of different species) can be very different, and address the Chapter 2 Question: How did wolves, whales, and the Mystery Fossil become so different from their common ancestor population? Students review concepts of natural selection and then consider how the mechanism of natural selection would play out over time, leading to speciation. Each student reads one of three articles describing a unique instance of speciation from the article set Where Do Species Come From? Students then use the Natural Selection Simulation to model speciation. Next, they explore how small changes that can result in speciation can accumulate over evolutionary time, resulting in very large differences between species. They end the chapter by considering differences in the same bone structures of whales, wolves, and the Mystery Fossil.

In Chapter 3, students set out to answer the Chapter 3 Question: How can we tell if the Mystery Fossil is more closely related to wolves or to whales? This chapter is focused on placing species on an evolutionary tree. Students consider similarities and differences in the shared structures of common ancestors and descendants as they build physical models of hypothetical species on a model evolutionary tree. Using the Evolutionary History Simulation, students investigate shared structures, and differences in shared structures, to determine how to place a variety of ancient whales on the Cetacea branch of the evolutionary tree. Students learn that certain diagnostic structures (structures that are shared by two species but not by a third) can be used to determine relative relatedness. By the end of the chapter, students are prepared to argue for which species they think the Mystery Fossil is more closely related to: wolves or whales.

Chapter 4 introduces students to a new anchor phenomenon to explore: Is the Tometti fossil more closely related to ostriches or to crocodiles? The Tometti fossil is based on the Sinosauropteryx fossil found in China. Students are asked to analyze evidence about ostriches, crocodiles, and the Tometti fossil and then discuss their understanding of the possible relationship between these three species during the Science Seminar. Students are encouraged to argue for either possibility, and it is acceptable for them to reach either conclusion as long as they support their argument with evidence. Students then write a complete argument for homework.