Who is Your Closest Relative

Problem/Question

Which primate (orangutan, gorilla, chimpanzee, gibbon) is most closely related to humans?

Background Information

When organisms reproduce they must pass their genes (DNA) to their offspring. Most of the time, the genes (DNA) get copied correctly. Sometimes the genes (DNA) get copied incorrectly. This is called a mutation. Sometimes mutations are deadly and the offspring don’t survive. Sometimes the mutations happen in a part of the DNA sequence that doesn’t code for anything, and there is no change in the traits. Sometimes the mutations create a variation for a particular trait. This variation could allow the offspring to survive longer and reproduce more often. The mutation thus gets passed down from generation to generation. Mutations therefore create variation. This variation may lead to new species.

To study the evolutionary relationships between organisms alive today, various methods can be employed to estimate when those organisms may have diverged from a common ancestor. Having this information for a group of living organisms would allow one to construct a cladogram. This cladogram depicts in graphic form these divergences over time and accounts for the current various species in question. Such a tree for five current species might look like the chart below:

In the past, much of this work was done by making observations of anatomy and physiology and with comparisons in fossil records. More recently, techniques have been developed in molecular biology for performing such comparisons. One of the newest and most quantitative methods is to compare the nucleotide sequence in a particular segment of DNA common to two or more living organisms. Often referred to as a molecular clock, this method utilizes a predictable rate at which mutations occur in DNA sequences. Those organisms that show the greatest number of nucleotide sequence differences are considered to have diverged from a common ancestor (following separate evolutionary paths) the greatest number of years ago (e.g., Nos. 1 and 5 in the diagram at the above). Organism 1 and 2 have the fewest nucleotide differences and thus shared a common ancestor more recently than organism 1 with the rest.

Procedure

You are going to compare the sequence of one gene from 5 different species. Here is how you will do it.

1.  Obtain the handout of DNA strips from human, chimpanzee, gorilla, gibbon, and orangutan. Cut out the strips according to the handout.

2.  Using the technique demonstrated by your teacher count the total number of bases (A, T, G, C) for human, chimpanzee, gorilla, gibbon, and orangutan. Enter the number into the data table.

3.  Circle the bases that don’t match the human DNA for the chimpanzee, gorilla, gibbon, and orangutan. Count them and enter the numbers into the data table.

4.  Calculate the number of bases that match the human DNA for the chimpanzee, gorilla, gibbon, and orangutan and fill in the data table.

5.  Calculate the percent DNA match to human DNA for the chimpanzee, gorilla, gibbon, and orangutan.

Data

chimpanzee / gorilla / gibbon / orangutan
Total number of bases
# of bases that don’t match human DNA
# of bases that match human DNA
% match to human DNA

Analysis/Conclusion Questions—Answer in complete sentences.

Fill in the following cladogram. Write the name of the species (orangutan, chimpanzee, gorilla, gibbon) into the blank that is provided. The most closely related species goes closest to human. Write the percent DNA match into the box underneath. Note human is already written and all humans have a 100% match with each other.

1.  According to the data, explain which primate is most closely related to humans. What is your evidence?

2.  Which primate did humans evolve from a common ancestor most recently?

3.  Explain what a mutation is.

4.  Explain why the chimpanzee DNA had some bases that don’t match (in sequence) the human DNA.

5.  What can be 3 results of mutations?

6. Explain what variations can lead to.