INQUIRY &
I N V E S T I G AT I O N
Chromosome Connections:
Compelling Clues to Common
Ancestry
R E C O M M E N D AT I O N
L A R RY F L A M M E R
ABSTRACT
The Chromosome Connection
J Students compare banding patterns on hominid chromosomes and see striking
evidence of their common ancestry. To test this, human chromosome no. 2 is
matched with two shorter chimpanzee chromosomes, leading to the hypothesis
that human chromosome 2 resulted from the fusion of the two shorter chromosomes. Students test that hypothesis by looking for (and finding) DNA evidence
of telomere segments at the fusion site, thus reinforcing the likelihood of our
common ancestry with chimps and showing that we all carry the molecular fossils of telomere fusion! Students see how multiple lines of evidence make a compelling case for common ancestry.
You may ask, what does all this have to do with chromosomes?
When the chromosomes from a certain stage of the cell cycle are
stained in a special way, they will appear under a microscope to have
a series of dark and light bands (Figure 2). When many photos of
each chromosome are carefully analyzed, diagrams of each can be
developed for easier comparison of details.
Each pair of chromosomes in an organism’s full set of chromosomes
has its own unique pattern of bands, characteristic for each species.
When the chromosomes from two different species are compared and
are found to have very similar banding patterns, what do you supKey Words: Hominid chromosomes; common ancestry; DNA fusion;
hypotheses tested.
pose this suggests? These matched patterns, as we find with bullet
marks, indicate a common source – a shared biological origin of those
chromosomes. The chances that two sets of identical complex banding
BANG! What was that? If you actually heard this sound – with the
patterns had independent unrelated origins are vanishingly small. We
unmistakable qualities of a gunshot – you certainly would notice it
conclude that those two species must have had a
and wonder what was happening. Depending
common origin – a common ancestry.
on where you are, this sound could signal a real
In Figure 3 (adapted from Yunis & Prakash,
crime, with real bullets. And sooner or later,
Each pair of chromosomes
1982), we compare, side by side, four of the
crime-scene investigators may gather, looking
in an organism’s full
chromosomes (nos. 3, 4, 5, and 6) from two
for clues to what happened.
different species we’ll call C and G. Do you see
Crime-scene investigations are among
set of chromosomes has
the identical or nearly identical patterns on the
the closest applications of the process of
corresponding chromosomes? These line patscience to real-world questions – especially
its own unique pattern of
terns are too complex to exist in two species by
to explain events of the past that were not
bands, characteristic for
chance. What does that suggest? If you said that
witnessed by anyone living today. Analyzing
they must have a shared source – or a common
clues produced by an unwitnessed earlier
each species.
ancestor – you would be right. Furthermore,
event is fascinating science in its own right
when you learn that nearly all the chromoand characterizes the “historical” sciences
somes from one species appear to be identical in whole or in part to
(e.g., astronomy and paleontology). As you may know, a bullet
those of a second species, this strengthens the likelihood that those
used in a crime can be linked to the gun that fired it by analyzing
two species have a common ancestor. Ask your students to count the
the scratch marks on the bullet (Figure 1).
total number of clear differences between the C and G chromosomes
When a second bullet (safely fired from the suspect gun) has a
shown here and record that number privately as “count 1.”
pattern of marks identical to those on a bullet found at the crime
Announce that the two species being compared here are chimscene, we can be very confident that those two bullets came from the
panzees (C) and gorillas (G), and that their chromosome similarisame gun. In other words, they had a common origin: the gun that
ties clearly point to them having a common (shared) ancestor at
fired them (Wallace, 1966).
The American Biology Teacher, Vol. 75, No. 2, pages 108–113. ISSN 0002-7685, electronic ISSN 1938-4211. ©2013 by National Association of Biology Teachers. All rights reserved.
Request permission to photocopy or reproduce article content at the University of California Press’s Rights and Permissions Web site at www.ucpressjournals.com/reprintinfo.asp.
DOI: 10.1525/abt.2013.75.2.7
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VOLUME 75, NO. 2, FEBRUARY 2013
Figure 1. Matched bullets from a common source.
ask them to raise their hands for the opposite
result: their “count 1” was less than their “count
2.” Because counting differences involves some
judgment, they will vary, but one count will usually be higher. Also, tell your students that they
need to realize that these are only four chromosomes out of the 23 or 24 pairs of chromosomes
that these three species have, but even those
other chromosomes are remarkably similar. At
this point, project a copy of Figure 5 on your
screen. This shows the diagrams of each chromosome found in four hominid species, with
closely matched chromosomes side by side. This
figure is available directly from http://www.indiana.edu/~ensiweb/
lessons/chr.4.all.pdf (courtesy of Jorge Yunis, 1982). From all of this,
you can probably agree that both species P and species G are certainly very similar to C (and therefore to each other), which means
that they all probably shared a common ancestor at some time in the
distant past.
Of course, your students will be asking “What is species P?”
Delay answering! The suspense is most engaging. Ask them to guess.
Write down their guesses on the board, without revealing whether
any guess is right or wrong. Eventually, you must give in and tell
them that P is us – people – Homo sapiens. Furthermore, in careful
studies of the differences in these chromosomes, it turns out that
chimps are more like us than they are like gorillas (Figure 6)!
Figure 2. Stained chromosome: photo and diagram.
some time in the past that was neither a chimp nor a gorilla. That
ancestor produced two offspring in the distant past that were slightly
different (genetically) and probably became part of two different
populations that became reproductively separated, and their offspring
inherited those chromosome differences. Over many more generations, additional changes accumulated in each population, eventually resulting in two new species: C (chimps, Pan troglodytes) and
G (gorillas, Gorilla gorilla). We find many similar but different species
today that show various stages in their growing genetic dissimilarity.
In Figure 4 (adapted from Yunis & Prakash, 1982), we compare
this same selection of chimpanzee chromosomes (C) with another
species (P). With so much of their banding patterns being identical,
you would have to conclude that these two species must also have
had a common origin – a shared ancestor. Ask your students to
study these chromosomes, count the obvious differences between C
and P for all four chromosomes combined, and privately record the
result as “count 2.”
When all have finished their counts, ask your students to raise
their hands if their “count 1” was greater than their “count 2.” Then
THE AMERICAN BIOLOGY TEACHER
Figure 3. Compare 4 chromosomes (3, 4, 5, 6) between
2 species (C and G).
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Figure 4. Compare 4 chromosomes (3, 4, 5, 6) between 2
species (C and P).
In fact, those studies tell us that the chromosomes of both chimpanzees and gorillas have changed more than our chromosomes
have since the split, which suggests that we may be more like our
common ancestor than chimps and gorillas are (Yunis & Prakash,
1982). Again, remind them that these are only 4 of the 23 pairs of
chromosomes in us – and of the 24 pairs of chromosomes in chimps,
gorillas, and orangutans.
A Molecular Fossil: Chromosome 2
J If your students pick up on this difference (23 vs. 24), perhaps, with
a few hints, you can then shift to our chromosome 2, and its comparison with a chimp’s two shorter chromosomes, 2a and 2b.
In Figure 7 (adapted from Yunis & Prakash, 1982), these two
chimp (C) chromosomes (2a and 2b) are positioned next to our no.
2 (P), and they match perfectly! How could this happen? You might
consider two possible explanations (hypotheses):
A. The common ancestor had one long chromosome that split in
half sometime in the chimp population, to make the two short
ones that we see in chimps today, while we kept our one long
chromosome 2 to the present, or…
B. The common ancestor had two short chromosomes that fused
end-to-end early in the unique human branch, to make the
long chromosome 2 we see in us today, while the chimp branch
kept its two short chromosomes.
For reasons discussed in the Chromosome Fusion lesson (below), we
choose explanation B to test. We should then predict what we will
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find only if the hypothesis is true. So, if fusion of the two chromosomes is what happened and you closely examine our chromosome 2,
what might you expect to find in the supposed fusion area that could
be considered evidence of this fusion? Is there, perhaps, something
special about the DNA in that region?
Something that is not obvious here, and was unknown to Yunis
and Prakash (1982), is that we now know the DNA sequence in the
ends of chromosomes (called “telomeres”). They typically consist of
a long series of tandem repeats, like this (showing only one DNA
strand here, assuming the usual complement on the other strand):
ttagggttagggttaggg…etc., with ttaggg repeated many hundreds of
times. So, if two short chromosomes (like those found in chimps
today) came together end-to-end to make our chromosome 2, we can
predict that we should find the remains of those telomeres near the
middle of our chromosome 2. On the other hand, finding telomere
sequences in the middle of a chromosome that was not the result of
fusion would be very unlikely. By 1990, we knew the DNA sequence
near the middle of our chromosome 2, so that hypothesis (choice B
above) could be tested by looking for those telomere remnants in the
supposed fusion zone of our chromosome 2 (Ijdo et al., 1991).
When we go into the online database of DNA for our chromosome 2 and search for the telomere sequences in the exact region
where fusion must have happened, we do, indeed, find what is
clearly the tandem repeats of …ttagggttaggttaggg, then suddenly
aatcccaatcccaatccc…, the complementary sequence in the telomere
DNA of the other short chromosome. Actually, the sequence isn’t
exactly like this, because of mutations since the fusion time several
million years ago, but it’s very close! In a very real sense, these damaged telomere sequences in our chromosome 2 are actually molecular
fossils – the remains of the telomeres of two separate chromosomes in
our ancestor – and every one of us has those molecular fossils in our
cells! Our prediction is confirmed, reinforcing our confidence in the
way we got our chromosome 2 (Ijdo et al., 1991; Fan et al., 2002).
Your students can actually do this search, following the directions in
the Chromosome Fusion lesson on the ENSI website (see list below).
Or, if you lack the time or computer access to do this, you can use
a copy of the DNA strands in the fusion zone, taken from the data
bank and provided in that lesson for your students to search directly
for their molecular fossils.
At this point, perhaps you can see another prediction about our
chromosome fusion hypothesis that we could test. Look at the diagram of our chromosome 2 next to the two matching chimp chromosomes (Figure 7). What do you see that we could look for from
the fusion of those two short chromosomes? If you need a clue, look
at the number of centromeres (constricted parts) in each chromosome. Do you see it? Obviously, the centromere in one of the short
chromosomes continues to function as the centromere of our chromosome 2 (chromosomes can have only one centromere). But what
happened to the centromere in the other short chromosome? What
would you expect to find in the region of our chromosome 2 that
coincides with that centromere? Perhaps some kind of DNA evidence
for the remains of that centromere? In 1992, Avarello and colleagues
reported their discovery of the centromere evidence from that other
fused chromosome. Hypothesis strengthened: fusion confirmed
again.
Once scientists figured out the chimp genome (Chimpanzee
Sequencing and Analysis Consortium, 2005), we could compare human DNA with chimp DNA. When we compare all the
VOLUME 75, NO. 2, FEBRUARY 2013
Figure 5. All chromosomes from four hominid species: human, chimp, gorilla, and orangutan. With kind permission of Jorge
Yunis (Yunis & Prakash, 1982).
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Multiple Lines of Evidence & Fair Test
J Figure 6. Chromosome differences (Yunis & Prakash, 1982).
When several different lines of evidence, and their confirmed predictions, all consistently point to the same event, we call this consilience,
one of the strongest kinds of evidence for what really happened. This
is not a consensus, with scientists agreeing about an interpretation
of data (or evidence). Rather, these are different kinds of independent
data that all fit a particular hypothesis and are not all consistent with
any other hypothesis. The chance that there is another explanation
has become less and less. Unfortunately, many students in school don’t
get to experience consilience, so they miss seeing one of the most compelling features of science, especially for the science that points consistently to evolution – the origin of species from previous species.
These lessons using hominid chromosomes also serve as an
excellent example of the Fair Test gauntlet that further strengthens
scientific conclusions (Nelson, 2000). Fair Tests are challenges to
two or more alternative possible solutions (hypotheses) to a problem.
When the test of these hypotheses does not have the same basis
used in forming those solutions, and could potentially support any
of those alternatives, we have a Fair Test. When the Fair Test results
are analyzed according to several criteria, we should find that one
solution is better than the others – meaning that it explains all the
observations of the problem better than the alternatives. We call this
the Best Explanation. This is what science is usually looking for.
Why Hominid Chromosomes?
J Figure 7. Chromosome 2 from species P and chromosomes
2a and 2b from species C (Yunis & Prakash, 1982).
corresponding genes among apes and humans, we generally find
identical or nearly identical sequences. As anyone who knows how
DNA transcription and translation work, nucleotide replacement
mutations in DNA can still produce functionally equal proteins –
even if you get a few different amino acids here and there in noncritical parts of the protein. In addition, most of the differences occur
in noncoding (non-gene) segments and reflect the randomness of a
regular rate of change that can be used to approximate how much
time has passed since they branched from their respective shared
ancestor. Examination also reveals that the chimpanzee branch had
more DNA changes than the human branch since they separated
from their common ancestor (Caswell et al., 2008). So now we have
another DNA confirmation of the common ancestry of humans and
chimps, plus an indication of how long ago this branching happened
(about 3–6 million years ago).
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Comparing molecules like DNA and proteins (e.g., Molecular
Sequences and Primate Evolution at http://www.indiana.edu/~
ensiweb/lessons/mol.prim.html) reveals clear signs of evolution, but
we can’t directly see those molecular details, so these rather abstract
kinds of evidence are probably not as convincing to students as clues
that are more visual. Fossils can also be used and are certainly visual
(see the Skulls Lab at http://www.indiana.edu/~ensiweb/lessons/hom.
cran.html). However, their features showing gradual change are often
quite subtle and not always obvious to the novice. But when students
can actually see the striking similarities of chromosome banding patterns side-by-side from living species, it becomes very hard to ignore
the obvious relationships that they imply. A good place to insert one
or more of these chromosome lessons would be near the end of your
chromosome genetics unit. In doing this, you also show how evolution fits into other topics of your course (Flammer, 2006).
Furthermore, focusing on human evolution (rather than the
evolution of moths or camels) is particularly engaging to students
and provides many examples of how multiple lines of evidence
(consilience) can be so compelling. ENSI Lead-Teacher Beth Kramer
has aptly pointed out that if students can be convinced that evolution happens in people, then extending that process to all other
organisms is a snap!
Most importantly, the experiences presented here have the
greatest impact when students are actually engaged in making those
observations. Access to Internet databanks of DNA and protein
sequences enables students, with very little guidance, to make these
discoveries themselves. The four interactive lessons below have been
developed and classroom-tested to take students into different aspects
of hominid chromosome comparisons. They are found on ENSIweb,
the website for the Evolution & Nature of Science Institutes. All
these lessons provide freely downloadable handout materials and
suggested strategies for their presentation and assessment. Go to
VOLUME 75, NO. 2, FEBRUARY 2013
the Evolution Lessons Index at http://www.indiana.edu/~ensiweb/
evol.fs.html, click on List of Titles, then, under Human Evolution
Patterns, find these lessons:
UÊ Comparison of Human & Chimpanzee Chromosomes: Intro
to hominid chromosomes.
UÊ Chromosome Connection 2: Comparisons with more details
of differences.
UÊ Chromosome Fusion: Using DNA database to search for
telomere vestiges.
UÊ Mystery of the Matching Marks: Forensic approach, using
interactive PowerPoint.
Acknowledgments
J My thanks to Beth Kramer, ENSI Lead Teacher and colleague
who created the excellent Comparison of Human & Chimpanzee
Chromosomes lesson. She kindly reviewed my draft of this article
and offered several changes that improved it. Also, my thanks to the
person who reviewed my MS, responding with positive comments,
and all the many teachers who used these lessons in their classes and
sent me helpful feedback. Special thanks to the teachers who let me
teach these lessons to their classes.
Caswell, J.L., Mallick, S., Richter, D.J., Neubauer, J., Schirmer, C., Gnerre, S.
& Reich, D. (2008). Analysis of chimpanzee history based on genome
sequence alignments. PLoS Genetics, 4(4), e1000057.
Chimpanzee Sequencing and Analysis Consortium. (2005). Initial sequence
of the chimpanzee genome and comparison with the human genome.
Nature, 437, 69–87.
Fan, Y., Newman, T., Linardopoulou, E. & Trask, B.J. (2002). Gene content
and function of the ancestral chromosome fusion site in human
chromosome 2q13–2q14.1 and paralogous regions. Genome Research,
12, 1663–1672.
Flammer, L. (2006). The evolution solution: teaching evolution without
conflict. American Biology Teacher, 68. [Online article.] Available at
http://www.nabt.org/websites/institution/File/pdfs/publications/abt/
archived-table/2006/068-03-0001.pdf.
Ijdo, J.W., Baldini, A., Ward, D.C., Reeders, S.T. & Wells, R.A. (1991).
Origin of human chromosome 2: an ancestral telomere–telomere
fusion. Proceedings of the National Academy of Sciences USA, 88,
9051–9055.
Nelson, C.E. (2000). Effective strategies for teaching evolution and other
controversial topics. In The Creation Controversy and the Science
Classroom. Washington, D.C.: NSTA.
Wallace, B. (1966). Chromosomes, Giant Molecules, and Evolution, pp. 7 and
81. New York, NY: Norton.
Yunis, J.J. & Prakash, O. (1982). The origin of man: a chromosomal pictorial
legacy. Science, 215, 1525–1529.
References
Avarello, R., Pedicini, A., Caiulo, A., Zuffardi, O. & Fraccaro, M. (1992).
Evidence for an ancestral alphoid domain on the long arm of human
chromosome 2. Human Genetics, 89, 247–249.
THE AMERICAN BIOLOGY TEACHER
LARRY FLAMMER is a retired high school biology teacher and currently
Webmaster for ENSIweb (Evolution and Nature of Science Institutes). He lives
in San Jose, CA. E-mail: flammer4@gmail.com.
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