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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 sampling “tour” of the major kingdoms of life, those organisms that biology is all about.

PREPARATION: 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 - prefereably a quiet one, and mice or rats), fungi, protozoans, algae, and bacteria. Try to include unusual organisms, e.g., carnivourous plants, sensitive plant, "Miracle Leaf" (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 in 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 fossil reconstructions (see Fossils below). If there are pages in their text showing examples of these organisms, have their 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.

When you get to microorganisms, project a few examples on your screen, then give your students a quick introduction to using your 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 you like, 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, (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 typical of (and unique to) different periods of geological time. To do this, it helps to have a 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 died out in the Permian Extinction event (~ 250 mya).
2. Placoderm fish (with armor plates covering head and thorax) first appeared in the Silurian (~440 mya), then died out in the late Devonian (~360 mya).
3. Dimetrodon, one of the sail-backed pre-mammals (synapsids) that lived around 260 mya in the early Permian, then went extinct during the End-Permian Extinction event (~250 mya) when more than 90% of marine animals disappeared.
4. Ammonites fossils (similar to today’s chambered nautilus), first appeared in the Devonian (~400 mya) but they died out about 65 mya, when the non-avian dinosaurs also went extinct.
5. Ambulocetus, a whale ancestor that lived in the Eocene (~ 48 mya).
6. Modern Horse: Earliest fossils date back to only about 3.5 mya. There are no modern horse fossils from an earlier time, only earlier horse-like animals, the ancestors of modern horses.
7. Homo erectus: If possible, have a full scale resin replica of this (and/or other) fossil hominin skull. Students should see that it's clearly similar to a modern human skull, but there are several important differences. Homo erectus emerged about 2 million years ago, and went extinct about 70,000 yrs ago. Hopefully, you can give your students the experience of studying this and several other species of early humans in the Skulls Lab.

Again, the point of all of this is not to know precisely when each group existed, but that they represent major changes in the kinds of life on Earth over hundreds of millions of years, most of which are not living anywhere today. In addition, most modern life forms are NOT found in the ancient fossil record; they have emerged only in the past several million years (and this includes humans). From studies of the known fossil record, compared with the known living species, it has been estimated that more than 99% of all kinds of life that ever existed are extinct today. We (and all the other living forms of life today) are among the remaining 1%. Who knows how long we'll survive?

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.