The DIVERSITY of LIFE: An Overview
In our predominantly urban and suburban world, and with eyes increasingly focused on screens, young people today seldom encounter diverse forms of life. If you ask your students, you may find that lots of them have not learned much about the great diversity of life in the world. This includes past and present life. Therefore, partly to provide some experiences for the deprived ones, and partly as a review for the experienced ones, be sure to provide an early unit that takes your students on a quick sampling “tour” of the major kingdoms of life, those organisms that biology is all about.
PREPARATION: Make your classroom come ALIVE! Bring together in your classroom as many real objects as possible that represent the diversity of life: actual living plants, animals (e.g., guppies or goldfish, salamanders and/or frogs, lizard or turtle, bird - preferably a quiet one, and mice or rats), fungi, tarantula, protozoans, algae, and bacteria. Horsehair worms (nematomorphs, e.g., from a watering trough) are always fascinating. Try to include unusual organisms, e.g., carnivorous plants, sensitive plant, "Mother of a thousand plants" (Kalanchoe pinnata), with leaves that sprout "baby" plants along their edges, etc. Include some of the non-living products of some of these examples, e.g., an animal horn and/or antler, clam or snail shell, bark and wood from a tree, coral skeleton, bath sponge, etc..
Include some fossils of these, as well, along with a big wall chart around your room of the geological time scale. In addition, prepare a slide series with vivid photographs of typical and unusual representatives of each group of organisms, especially those for which you don’t have living examples, and for those groups that consist mainly of microscopic organisms. Include some slides of reconstructions of extinct organisms (see Fossils below). If there are pages in their text showing examples of these organisms, have those page numbers handy, and have them turn to those pages as you go over your survey of the living world.
Also, have your classroom microscopes (and/or microscope projector) handy and ready to use. And, about a week or so before doing this unit, prepare a hay infusion: boil some alfalfa in pond water, then, when cooled, add some samples of pond water, river water, a little soil, and some fresh alfalfa (from local feed store).
PRESENTATION: You might start your presentation by asking your students to name a number of living things. As they do this, list them on your board or overhead. Try to cluster examples of each group (animals, plants, etc.), without naming the group. With prompting, try to get every student to name an organism. Eventually, ask why they think you have grouped them as you have. Hopefully, someone will see that you’ve grouped the animals together, the plants together, etc. Ask if they can think of any organisms that are not in those groups. Hopefully, some will begin to mention things like “bacteria,” “mushrooms,” etc. Then ask “What about this dinosaur bone?” And this opens the door for fossilized extinct organisms they can suggest. From there, you can launch into your prepared (short) slide series, and the real organisms in your room, that represent examples of each kingdom.
Microorganisms: project a few examples on your screen, then give your students a quick introduction to using their microscopes effectively and safely, enough so that, in a day or so, they can begin looking at fresh preps on slides made from the hay infusion water, with added coverslips. If you time this right, your infusion should be rich with various ciliates, bacteria, and algae. If your budget allows, you can supplement with pure cultures (from bio supply house) of particular species they may read about, e.g., Amoeba, Paramecium, Euglena, Volvox, Oscillatoria (a filamentous blue-green bacteria, formerly “blue-green algae," that slowly waves its strands side-to-side! If you get this, ask your students how they think they do that). To provide some structure and focus, have them identify (using a sheet with named pictures of a variety of typical pond-water creatures) and draw examples of several different organisms they find.
If your scopes have measuring scales, show them how to determine the approximate dimensions of those organisms, and ask them to add those measurements to their drawings. If they have access to pictures with parts labeled, have them look for those features, include them in their drawings, and label them. Keep in mind that the major focus here is not the details of structure, size, or identification, but just some familiarity with some common microorganisms in their environment, and some careful observation with that super tool: a microscope.
Cells: With the microscopes out, you might also want to use the opportunity to have them take a look at some cells, the basic unit of all life (e.g., onion, cheek, Elodea, etc.) hopefully refreshing material that they should have studied in middle school. Doesn’t need to be much detail (like names and functions of all the organelles, or details about structure and function). Mainly a chance to see some plant cells, animal cells, noting a few key differences, and compare with the one-celled organisms found in the hay infusion.
Fossils: Be sure to include several key fossils (and pictures of fossils and fossil reconstructions). It’s helpful to have fossils that are found locally, and also typical of (and unique to) different periods of geological time. To do this, it helps to have a large scale geological time line conveniently posted that you can point to for the ages of your fossils. [See the instructions for room-size time scale in our Time Machine lesson, item #1 under Materials]. Emphasize that the organisms that became these fossils are not alive today, they’re extinct. Good examples could include (show pictures):
1. Trilobites: look somewhat like today’s horseshoe crabs, these creatures first appeared in the early Cambrian (~ 540 mya) and were abundant until they died out in the Permian Extinction event (~ 250 mya).
A good ENSI lesson to help students get a real sense of deep geological time, AND the stepwise emergence of each of the different classes of vertebrates over many tens of millions of years, is Patterns in Time. Be sure your students understand how each successive class of vertebrates retained the basic features of its predecessors, PLUS a new feature or two (modified from previous structures) added on. Study the chart in that lesson showing that progression. This strongly suggests that each successive class evolved from its predecessor class.
Following your “exploration” of the living world, you might ask “Why do you suppose we tend to call certain organisms “plants” or “animals,” etc.? And why do we refer to different kinds of animals as being in certain animal sub groups, like “mammals” and “fishes?” [They share certain features in common for each group.] And what do we call this, putting organisms into certain groups? [Classification]. Right! So let’s take a closer look at the thing called “Classification,” and see what that tells us.