Everything Evolves!

Updating Life through Time

Every GEMS guide goes through extensive classroom testing. We pay careful attention to the comments of scores of teachers during a multi-stage pilot and trial testing process. Among many criteria, including aligning major learning goals with national standards and ensuring classroom practicality, testing also provides evidence as to whether the content and its presentation are engaging to students and if major concepts are able to be grasped and applied by students at the pertinent grade levels.

GEMS guides also benefit from scientific review so as to be as accurate as possible. For the new GEMS guide Life Through Time, one of the guide’s authors is an evolutionary biologist and it was reviewed by an internationally recognized scientist. Still, Life Through Time is the most encyclopedic of GEMS guides, with hundreds of illustrations, numerous charts and diagrams, and a whopping 350 pages. Obviously, the subject matter is complex and, in many parts of the country, occasions considerable controversy! We consulted with other scientists on specific issues and made extensive use of the outstanding website of the University of California at Berkeley Museum of Paleontology.

Thanks to post-publication comments by several evolution researchers/professors and science educators at the National Center for Science Education in Oakland and the University of California at Berkeley’s Museum of Paleontology, some corrections, refinements, and clarifications have been suggested. We always welcome criticism and revise guides frequently. Before that happens with this guide, we thought the GEMS website would be an excellent place to provide some information on these comments, because they’re interesting and instructive, and should be helpful in presenting the unit. These suggestions will be reflected in revisions of this guide—and in future guides on other key aspects of evolution. We’d like to do more! And we’d also like to recommend the guide highly, and ask, if you present this unit to your students, for your and their comments. You can order through the GEMS website or by contacting us.

Eras and Time Spans

As explained in the guide, the more than 20 formal time divisions of life delineated by paleontologists and evolutionary biologists are not introduced to students until the end of the unit. Instead GEMS curriculum developers and content experts selected five larger "chunks" of time (called "spans" in the guide). These five spans are represented by the terrarium/aquarium models that run throughout the unit’s activities. Our goal here, similar to other GEMS guides, is to give students their own sense of the vast range and diversity of changes in the story of life on this planet. Based on this experience, students are then motivated to grapple with and critically think about how they might characterize, generalize about, and come up with their own names for the time spans—before they are introduced to the whys and wherefores of the current scientific classification of the time spans.

All well and good, but our colleagues commented on how and why we selected the boundaries of our five time spans. It’s agreed that, for purposes of teaching, grouping time spans together can make sense. But some think it is very important to select the boundaries to emphasize certain major events more directly than the GEMS guide’s time spans do, particularly to focus on the extinction of so many species at the end of the Permian period. On the other hand, it could be educationally powerful for students to discover the significance of these events through their own analysis of the evidence. The common goal here is to help make sure that students come away with an understanding of these key events and an understanding of how and why scientists have drawn these lines.

Another way to accomplish this—in whatever way the time spans are depicted—is to make sure these events are more fully emphasized when the scientific classification of time spans is introduced and when discussing with students the overarching ideas and conclusions gained through the unit.

One of the most original and compelling parts of each major activity/time span in the guide is sparked by scripted role-plays put on by students in which some of the organisms of the period discuss their attributes and interactions and assert their claim to represent the period. This also catalyzes students’ own critical thinking. Student discussion and debate of what they consider the most representative organism, based on evidence they’ve encountered, may well, for any number of reasons, differ from the organism a scientist would select. But the strength of student engagement in this case has a lot to be said for it!

Of Worms, Toadstools, and Models

Another interesting and thought-provoking comment involves instances in the unit of modern animals used as stand-ins or symbolic representations for organisms that preceded them in evolutionary development. This involves a live earthworm (representing an earlier worm) that appears in the terrarium in the first time span, as well as a drawing of the worm and of a toadstool, representing an earlier fungus. In fact, the worms present during the first time span were not the ones we now know under the soil, and the forms of fungus present were far removed from mushrooms! On the one hand, having something alive and/or familiar in the terrarium model is a great plus for student interest; on the other hand one would not want to foster in any way the serious misconception that any modern organism sprang onto life’s stage without evolutionary antecedents! In the current climate, such a misconception could have unfortunate and unscientific anti-evolutionary overtones.

We explain our reasoning on this and other choices in notes and clarifications for the teacher, but we will make further adjustments as needed to respond to this concern. It should be noted, however, that the vast and overwhelming content, theme, and class discourse in the book emphasize how life and its organisms change and evolve over time, how every organism had predecessors. There are numerous examples and illustrations based on fossil and other evidence that depict how paleontologists and other scientists trace how various organisms evolved, how groups of organisms are related to each other, and how, for example, the digestive tracts (or "guts") of organisms evolved over time.

It’s important to understand that the terrarium/aquarium centerpieces for the five time spans are models.

Models have many positive uses; they also have limitations. For example, these can be taken as five snapshots representing the range of organisms—both plants and animals—that existed during the five long time spans. Some of the organisms may have been more prevalent during the early millions of years represented; others during the later stages; still others in the middle. We coalesced this in order to make the backdrops and settings for each time span serve as a memorable and interesting snapshot. We explain this choice and its trade-offs clearly in the guide. It could lead to some misunderstandings about when an organism was most prevalent and/or whether it co-existed with another.

Some teachers may want to have some reflective discussions with their students about this and about the uses and limits of models—as in the GEMS guide River Cutters and others. It was also noted that the depiction of various early life forms in the first time span might fail to convey the billions of years that passed on Earth before any life began and also not make it clear that once life began, there were many more millions of years when pond scum was just about all there was. The text and class discussion do make this and related ideas clear, but the point is well taken.

You’ve Got to Hand It to Bacteria

In sum, we much appreciate the critique we have received. In addition to the larger comments, there are some errors that need correction, including more appropriate placement of several organisms in a time span adjacent to the one in which they now appear and a misidentified reptile. Please see the list for a description of these corrections.

It’s also true that aspects of evolutionary understanding are always evolving! For example, in the past scientists assumed a much closer connection of the arthropods to annelids than is now considered to be the case. Recent work with DNA has also led science to conclude that orchids are part of the asparagus group, and that they appeared much earlier in the evolutionary story than was previously assumed. And many more new understandings are constantly sprouting. We are sure we will continue to benefit from consultations with scientists and from staying alert to the many new developments in evolution and life science.

The title of this article "Everything Evolves" is accurate. As noted, GEMS teacher’s guides also evolve—through many trial versions and with an active revision file for every guide. There is one kind of organism however, that—while it had its own evolution in the early stages of life’s development—has since kept on surviving and thriving through all evolutionary time divisions—even as so many other organisms lived for a time but then became extinct. Those survivors, of course, are none other than bacteria, as the guide dramatically conveys.

The GEMS Life through Time guide is dedicated to the great evolutionary biologist, theoretician, author, and science popularizer, Stephen Jay Gould, who died in 2002. And speaking of bacteria, in a San Francisco radio interview, Gould said:

Gould: When we think about evolution, we have a tendency to focus on the most complex creature at any given time. Once the most complex creature was a bacterium, then it was a jellyfish, then a trilobite, then a fish, and then us. But the history of the most complex creature is not a surrogate for the general thrust of evolution as a whole. At the origin of life you had to have creatures of minimal complexity because, given the nature of chemistry and physics, you can't precipitate a hippopotamus out of the primordial soup. So, you're going to begin with a creature of bacterial grade, the simplest kind of cellular organization. And there’s only one direction for change—toward more complexity. But very few creatures move in that direction. Occasionally a couple of species dribble off in the direction of complexity, but that doesn’t define a trend or a thrust. The most outstanding feature of life’s history is that through 3.5 billion years this has remained, really, a bacterial planet. Most creatures are what they've always been: They're bacteria and they rule the world. And we need to be nice to them.

Question: Nice to bacteria?

Gould: Right. I don't think this leads us to any startling new ideas about how we can keep going, but if I can impart some increased respect for those creatures that we consider simple and inferior, then I’ll have done a good deed.

By the same token—and with all due respect for bacteria, pond scum, and all other living organisms, past and present—we at GEMS think that if we can impart some increased student interest in and critical thinking about evolution, then we too will have done a good deed!