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
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 guides 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
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 Berkeleys 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 theyre interesting and instructive, and should be helpful
in presenting the unit. These suggestions will be reflected in revisions
of this guideand in future guides on other key aspects of evolution.
Wed like to do more! And wed 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
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
units 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 spansbefore they are introduced to the
whys and wherefores of the current scientific classification of the
All well and good, but our colleagues commented on how and why we selected
the boundaries of our five time spans. Its 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 guides 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 thisin whatever way the time spans are
depictedis 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 theyve 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 lifes 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.
Its 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 organismsboth
plants and animalsthat 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 modelsas
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.
Youve 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.
Its 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 teachers guides also evolvethrough many trial
versions and with an active revision file for every guide. There is
one kind of organism however, thatwhile it had its own evolution
in the early stages of lifes developmenthas since kept on
surviving and thriving through all evolutionary time divisionseven
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
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 theres only one direction for
changetoward more complexity. But very few creatures move in
that direction. Occasionally a couple of species dribble off in the
direction of complexity, but that doesnt define a trend or a
thrust. The most outstanding feature of lifes 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 Ill have done a good deed.
By the same tokenand with all due respect for bacteria, pond
scum, and all other living organisms, past and presentwe 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!