Tracking Biology's
Big Bang
Accompanies
the Viewer's Guide, Chapter 2-D, "The Cambrian Explosion"
Overview
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.
Learning Objectives
- Students will gain
an appreciation of the great diversity that characterizes the
animal kingdom.
- Students will be
aware of when various phyla first appeared in the fossil evidence.
- Students will understand
some of the implications of the Cambrian Explosion for contemporary
evolutionary theory.
Directions
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.
When the reports have
been given and the results plotted, hand out the sheet titled, "What
would Darwin predict?" and have the class discuss the questions
it raises.
Tracking Biology's
Big Bang
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.
Directions
Below is a list of
animal phyla. You will be asked to report on one of them.
Acanthocephala
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Annelida
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Arthropoda
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Brachiopoda
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Bryozoa/Ectoprocta
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Chaetognatha
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Chordata
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Cnidaria
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Ctenophora
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Echinodermata
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Echiura
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Entoprocta
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Gastrotricha
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Gnathostomulida
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Hemichordata
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Kinorhyncha
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Loricifera
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Mollusca
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Nematoda
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Nematomorpha
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Nemertea
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Onychophora
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Phoronida
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Placozoa
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Platyhelminthes
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Pogonophora
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Porifera
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Priapulida
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Rotifera
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Sipuncula
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Tardigrada
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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.
Tracking Biology's
Big Bang
Phylum Report Form
Phylum Name:
Description:
Examples of animals
belonging in this phylum: (Give no more than three.)
1.
2.
3.
Do any members of
this phylum appear in the fossil record? (Check one.)
Yes __ No __
According to conventional
dating, when did it first appear in the fossil
record? (Use the
geological period--for example, the lower Permian, upper
Cambrian, etc.)
Where did you get
your information about this phylum?
Tracking Biology's
Big Bang (Sample)
Phylum Report Form
Phylum Name: Arthropoda
Description:
This phylum contains
animals that have segmented bodies, skeletons on the
outside of their
bodies, and jointed appendages (i.e. legs, pincers,
mouthparts, etc.)
Examples of animals
belonging in this phylum: (Give no more than three.)
1. Crabs
2. Lobsters
3. Insects
Do any members of
this phylum appear in the fossil record? (Check one.)
Yes
X No __
According to conventional
dating, when did it first appear in the fossil record? (Use
the geological period--for example, the lower Permian, upper
Cambrian, etc.)
The first appearance
of the phylum was in the lower Cambrian.
Where did you get
your information about this phylum?
- Simon Conway
Morris. "The Cambrian `explosion': Slow-fuse or megatonnage?"
Proceedings of the National Academy of Sciences 95, no. 9
(2000), 4426-4429.
http://www.pnas.org/cgi/reprint/97/9/4426.pdf
Tracking Biology's
Big Bang
Sources
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.
Web
The following Web
sites have information on a large number of animal phyla,
as well as pictures of representative species:
- Phyla of
the Animalia (Lamont-Doherty Earth Observatory, Columbia
Univ.)
http://www.ldeo.columbia.edu/dees/ees/life/slides/oldec/animalia.00.html
- General
Overview of Animal Phyla (Bellarmine University)
http://cas.bellarmine.edu/tietjen/images/general_overview_of_animal_phyla.htm
Print
The following printed
sources may also be helpful:
- Lynn Margulis
and Karlene V. Schwartz, eds. Five Kingdoms: An Illustrated
Guide to the Phyla of Life on Earth. 3rd ed. New York: W.H.
Freeman and Co., 1998.
- Simon Conway
Morris and H. B. Whittington. "The Animals of the Burgess
Shale." Scientific American 241 (1979), 122-133.
- Simon Conway
Morris. The Crucible of Creation. Oxford: Oxford University
Press, 1998.
- Simon Conway
Morris. "The fossil record and early evolution of the
Metazoa." Nature 361 (1993): 219-225.
- Simon Conway
Morris. "The Cambrian `explosion': Slow-fuse or megatonnage?"
Proceedings of the National Academy of Sciences 95, no.
9 (2000): 4426-4429.
http://www.pnas.org/cgi/reprint/97/9/4426.pdf
or
http://www.pnas.org/cgi/content/full/97/9/4426
- Jeffrey
S. Levinton. "The Big Bang of Animal Evolution."
Scientific American 267 (1992): 84-91.
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.
Tracking Biology's
Big Bang: First Appearances (Sample)
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Precambrian
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Cambrian
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Ordovician
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Silurian
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Devonian
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Carboniferous
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Permian
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Triassic
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Annelida
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Arthropoda
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Brachiopoda
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Bryozoa
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Chaetognatha
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Chordata
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Cnidaria
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Ctenophora
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Echinodermata
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Hemichordata
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Mollusca
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Nematoda
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Onychophora
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Phoronida
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Platyhelminthes
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Pogonophora
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Porifera
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Rotifera
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Tracking Biology's
Big Bang
What Would Darwin
Predict?
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?
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