Name:
Block:
Investigation: Common Descent
Evolutionists believe in the theory of “universal common decent” which states that a group of
organisms is said to have common descent if they have a common ancestor. One way we can
demonstrate this is by examining the DNA of organisms. The more similar two organism’s genes are the
more closely related these two organisms may be on the evolutionary tree.
We will create a paperclip model to give us an idea of how geneticists look at genes and deduce how
closely related two organisms are.
Below is an evolutionary tree (phylogenetic tree) that is based on similarities and differences in genetic
sequences. Think of the lines as branches of a tree, the closer the branches are the more closely related
two organisms. The crotches of the tree illustrate a common ancestor and the further down the tree the
earlier the life form.
Kangaroo
Cow
Goat
Sheep
Llama
Donkey
Horse
Monkey
Gibbon
Gorilla
A
Chimpanzee
Human
1. From point A, work with your partner and develop a hypothesis to explain how the Gorilla,
Chimpanzee and Human are related to one another and to species A.
My hypothesis:
2. Working in your groups synthesize strands of DNA according to the following specifications: Each
color paper clip represents one of four nitrogen bases: Yellow = Cytosine, Green =Guanine, Blue=
Thymine and Red = Adenine
A. Synthesize a strand of DNA that has the following sequence:
G-G-C-A-T-A-A-A-C-C-A-A-C-C-G-A-T-T-A
Label this strand “human DNA”. This strand represents the gene for hemoglobin, a protein found on red
blood cells.
B. Synthesize a strand of DNA that has the following sequence:
G-G-C-C-C-C-T-T-C-C-A-A-C-C-G-A-T-T-A
Label this strand “Chimpanzee DNA”. This strand represents the gene for hemoglobin, a protein found
on red blood cells.
C. Synthesize a strand of DNA that has the following sequence:
G-G-C-C-C-C-T-T-C-C-A-A-C-C-A-G-G-C-C
Label this strand “Gorilla DNA”. This strand represents the gene for hemoglobin, a protein found on red
blood cells.
D. Synthesize a strand of DNA that has the following sequence:
G-G-C-C-G-G-C-T-C-C-A-A-C-C-A-G-G-C-C
Label this strand “Common Ancestor DNA”. This strand represents the gene for hemoglobin, a protein
found on red blood cells of the common ancestor for the gorilla, chimpanzee and human.
3. Compare the human DNA to the chimpanzee and gorilla DNA by matching the strands base by base.
Fill in the table below:
Human DNA compared to:
Chimpanzee DNA
Number of matching bases
Number of unmatched bases
Gorilla DNA
a. After analyzing the data, what can you conclude about the relationship between humans, gorillas and
chimpanzees?
b. Does the data support your hypothesis in step 1? Explain.
4. Compare the common ancestor DNA to the human, chimpanzee and gorilla DNA by matching the
strands base by base. Fill in the table below:
Common ancestor DNA
compared to:
Human DNA
Number of matches bases
Number of unmatched bases
Chimpanzee DNA
Gorilla DNA
a. According to the data which DNA is most similar to the common ancestor DNA?
b. What can you infer about the evolutionary relationship between these species?