Episode Two of the Evolution series introduced students to the Cambrian Explosion, sometimes called the Biological Big Bang. In a relatively brief period of time many new animal forms appeared in the fossil record. But just how big of a bang was it? Was it a firecracker pop, a nuclear blast, or something in between? And what does it mean for contemporary evolutionary theory? That's what your students will investigate in this activity.
This activity is a class research project in which students will gather information about animal phyla and their first appearance in the fossil record. When they have gathered and compiled the information, they will be asked to compare their information about when the phyla first appeared and the predictions of contemporary Darwinian theory.
Introduce the activity by telling students they will be investigating the Cambrian Explosion, mentioned in Episode Two of the Evolution series. Hand out the instruction sheet, report form, filled-in sample report form, and list of references on the following pages.
Go over the instruction sheet with students, introducing the concept of the phylum and going over the directions for the activity and for filling out the report. Tell them that they should be prepared to orally present the results of their research to the rest of the class. If students want, they can include pictures with their presentation--which might help other students understand what representative members of the different phyla look like. Emphasize, however, that the presentations will be very short, and limited to the information contained in their report forms. Set a due date for the reports that seems realistic for your students and your schedule.
At the beginning of the class period when students will be presenting their reports, draw a table (chart) on the black board. Along the left side of the table, list the phyla in alphabetical order. Along the top of the table, from left to right, list the following geological periods: Precambrian, Cambrian, Ordovician, Silurian, Devonian, Carboniferous, Permian, Triassic, Jurassic, Cretaceous, Tertiary, Quaternary. The Precambrian is the earliest time period, while the Quaternary is the latest. (The Precambrian is actually an geological era. For the purposes of this activity, however, the distinction is unimportant.)
We have provided an example of such a table on the page with the title: "Tracking Biology's Big Bang: First Appearances (Sample)." Note that this is for illustrative purposes only and does not contain all the phyla or geological periods.
As students give their presentations, have them plot the first appearance of the phyla they report on. Let them know that the "lower" part of a period is the earliest, and the "upper" part is the most recent.
Episode Two of the Evolution series introduced you to the Cambrian Explosion, sometimes called the Biological Big Bang. In a relatively brief period of time many new animal forms appeared in the fossil record. But just how big of a bang was it? Was it a firecracker pop, a nuclear blast, or something in between? And what does it mean for Darwin's theory of evolution? That's what your class will investigate in this activity.
Each student in your class will be assigned an animal phylum to report on. A phylum (phyla for plural) is the broadest classification of animals. As opposed to a single species, like a chimpanzee, a horseshoe crab, or a horsefly, a phylum takes in a wide variety of animals. The phylum that contains humans also contains elephants, squirrels, canaries, lizards, guppies, and lampreys. Indeed, it contains every animal with a backbone--and then some.
If the differences within a phylum are great, the differences between phyla are vast. As much as a chimpanzee may differ from a fish, it differs even more radically from a sea urchin or a worm. In fact, you could say it's built on an entirely different architectural theme, or body plan.
Because phyla are so different from each other, the appearance of new phyla in the fossil record tells us something about how fast change is happening--depending, of course, on how good the fossil record is. The appearance of a new phylum every few million years or so might indicate gradual, steady change. The appearance of several new phyla at once, however, could indicate that something very different was going on.
The information you'll need to get will be specified on a form that the teacher will hand out. The form will ask for a description of the phylum, the names of some animals that belong in the phylum, and the period that the phylum first appeared in the fossil record. (Note, though, that some phyla alive today do not appear in the fossil record. As part of your investigation, you'll be asked to indicate whether this is the case.) You may also be asked to include pictures of some of the animals that belong in the phylum.
Your teacher will give you a list of suggested sources where you can find the information you need. Once you and your classmates are done gathering information, you will plot the appearance of each phylum on a chart to see the pattern that emerges from the fossil evidence.
You can find a great deal of information about animal phyla in paleontology and introductory biology textbooks. The best place to find these texts is in your local college or university library, but you can also find them in your local public library or school library. The more recent the texts, the better.
You can also find useful information on the Web--though the quality of information can vary. The best information is usually found on Web pages that document their facts with references to articles in professional journals. To search for information on the Web, go to Google (www.google.com) or any other large search engine. Use the name of the phylum you've been assigned as a search term. You can use it alone or with other search terms, such as "paleontology," "fossil," and so on.
Keep in mind that the classifications of some animals have changed over the years. For example, some classes of animals thought to belong to one phylum have been recognized as phyla in their own right.
Contemporary Darwinism holds that all living organisms descended from a single "universal" ancestor. All the plants, animals, and other organisms that exist today are products of random changes and natural selection.
According to contemporary Darwinism, nature acts like a breeder, carefully scrutinizing every organism. As useful new traits occur, they are preserved and passed on to the next generation, while harmful traits are eliminated.
Though each change is small, these changes eventually accumulate to produce new tissues, organs, limbs or other parts. Given enough time, organisms may change so radically that they bear almost no resemblance to their original ancestor--or to their distant cousins alive today. Thus, humans, squids and dragonflies differ dramatically from their alleged single-celled ancestor. And they differ as dramatically from each other.
If we were to chart the appearance of new animal phyla, the chart would look something like the following diagram. As animals slowly diversify by accumulating changes, more and more new phyla begin to emerge, but it's a long, gradual process.
How do you think this compares with what your class found about the first appearance of the animal phyla? As you look at the chart you filled out in class, do you see increasing numbers of phyla appearing over time? Or do you see another pattern? If so, what kind of pattern do you see? Does it conflict with contemporary Darwinism? Why or why not?