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Primate Characteristics and Exaptation
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by David Campbell
Ridgeview High School
Orange Park, Florida
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| | The list of characteristics of primates is one of those things that most students memorize in introductory science classes. This little demonstration helps them take their understanding of our common ancestry with primates beyond the rote level and also presents the foundation for understanding an essential concept of evolutionary theory, exaptation. This demonstration can be performed with various levels of students. I have done variations of it with average junior high students all the way up to AP Biology level.
Materials
- one standard roll of school-issue paper towels
- two paper clips, one regular and one jumbo
- one soft rubber object to toss around the room
- one slightly hammy teacher
Opposable Thumbs
Introduce the lesson as you wish. I prefer to start after the students have completed a reading assignment introducing the similarities between primates. Select a volunteer and ask the student to pick up the roll of paper towels with one hand. Present the roll to the student so that the student is most likely to pick it up from your hand by wrapping his hand around the roll. It is critical that you select a student whose hand is large enough to do this. Then lead a discussion on the opposable thumb and its significance for grabbing, grasping, manipulating, etc. It is important when grabbing tree branches that your grip be secure enough so you don't fall off. Falling is bad. The primate can be injured in the fall or become prey for predators lurking below. Gymnasts in the class can be queried on the strength required to maintain the grip at the bottom of a swinging arc when G forces can reach two or three times the force of gravity as occurs during brachiation. Then instruct the student to hold his thumb flat against the side of his hand so that the tip of the thumb is touching the index finger. Present the paper towel exactly as before and ask the student to pick it up without changing the position of the thumb. If the student tries to pick up the roll as before, he will find it impossible. The grip is not as good. Continue to press the student to pick up the paper towel roll without moving the thumb. Eventually, most will grab it on the end by inserting one or more fingers into the hole of the cardboard tube or sliding a hand under the roll to lift it from your hand. This leads to adaptation number two.
The Superior Brain of the Primate
The student, when faced with a problem, used his superior brain to solve it. This is a hallmark of the primates. We are problem solvers. You can use other examples from other primates (chimps standing on boxes to reach things placed out of reach, the manufacture of tools, strategy when attacking other primates, etc.). Ask students about the problem of jumping from one tree branch to another. What must be known by the primates to do this safely? Answers should include distance to the next branch. Misjudging distance can mean falling when you fail to reach the branch or injury when you arrive at the branch with too much speed. See paragraph above concerning the dangers of falling. The primate who misses may become severely embarrassed when the other, we assume more capable primates, ridicule it. Ask how the primates determine this distance. This leads to adaptation number three.
Binocular Vision
As you lead to this topic, quietly pick up the two paper clips and keep them nestled in your hand. This should be done unobtrusively. Students can determine the advantages and disadvantages of primate eye placement and can also determine the degree of binocular vision they possess. Have students hold a pen vertically in one hand and extend that hand to the side. The students should look straight ahead at an object on the far wall and slowly move the extended arm forward until they can just barely see the pen. They must continue looking straight forward while they do this. Notice the position of the pen. This is the limit of their peripheral vision. Have the students cover their eyes on the same side of the body as the hand holding the pen. They should notice that the pen is no longer visible. Now have them continue moving the arm holding the pen forward until the pen is visible to the eye on the opposite side of the head. This is the lateral limit of their binocular vision, the range where the object is visible to BOTH eyes.
Pick a volunteer. I find that one who doesn't wear glasses generally does better with this little exercise. Hold the small paper clip vertically so that at least half of it is visible above your fingers. Explain to the student that you will slowly move the hand holding the paper clip toward the student, and when it is exactly one arm's length away, the student is to reach out with a STRAIGHT ARM to touch the paper clip with the tip of her longest finger. Both of the student's arms must remain at her side until she is ready to reach for the paper clip. In other words, the paper clip and the student's fingertip should touch when the clip is exactly one arm's length away from the student. Assure the student that you will not try to fool her or make it harder than it already is. Most students will be able to accomplish this rather sophisticated test with an accuracy of a few millimeters or less. After the demo, discuss with the students the significance of binocular vision and using the slightly different images from two eyes to judge distance. You can move an eye from arm's length away toward your nose while keeping the object in focus so the students can watch the angle of your eyes change as the object gets closer. You will become quite cross-eyed. At some point during this discussion, you should surreptitiously push the small paper clip into the recesses of your hand and move the jumbo paper clip into position. Explain that you will now show the value of two eyes instead of one. Have the student cover one eye and repeat the test using the jumbo paper clip. Do everything exactly the same as you did for the first test and DO NOT EVEN HINT THAT YOU ARE USING A DIFFERENT PAPER CLIP!! The student will inevitably reach too soon, and her fingertip will be at least 5 to 10 millimeters short of the paper clip. Your students will be impressed.
Discuss the two-eye significance again and ask for questions. If there are none, ask how, if two eyes are required for depth perception, people with one eye are able to drive safely in traffic? The answer is that they assess the relative size of the cars around them to judge distance. They know how big a car is and can do the mental calculations. Relate to the size of a paper clip. Do students know how big a paper clip is? Ask the student who did the demo for you if she knows the size of a paper clip and then ask why she came up short. Then ask what might happen if you used a larger paper clip for the second demo. Guide the class to understanding that the student will reach too soon when the relative size of the paper clip "looks right." Then show them the two paper clips and explain what you did. Relate this to brain capacity and thinking skills. Ask why this single eye accommodation doesn't work when jumping from branch to branch. This paper clip demo has never failed (nearly 100 trials so far), and the student always reaches too soon on the second trial.
I learned this lesson the hard way several years ago in a previous career as a helicopter flight instructor. My student and I were flying just north of a small airfield when I saw a Cessna 150 at our altitude straight ahead and several miles away. I pointed it out to the student, who confirmed that he had the traffic in sight. A few seconds later, the "airplane" passed directly underneath us. What we had actually seen was a radio-controlled model of a Cessna 150 a few hundred yards away, not a full-size Cessna 150 several miles away. Without visual reference, a 4-foot wingspan model airplane a few hundred yards away looked like a full-size airplane several miles away.
At some point you can toss in the rotating shoulder and the flexible fingers. I illustrate the similarity between human hands and ape hands with a photograph of a gorilla hand taken at a local zoo.
Now take all this and ask your students to explain how existing adaptations in our ancestors were used by various hominids to gain evolutionary advantage in "the struggle for survival." The flexible hands and opposable thumb allow us to manipulate things, whether they be tools, writing instruments, or a baseball. Try throwing a football without an opposable thumb, for example. Indoors, toss a small, soft object to students and ask them to throw it back. Ask the class what was involved in making an accurate toss and catch. Answers should include distance to target, mass of object, linear displacement between eyes and catching hand (diving birds of prey usually stretch their legs forward so they are very close to the eyes when making contact with the prey to simplify this problem), curve of flight, motion of the object, etc. The complex brain comes to the rescue. Throwing without a rotating shoulder is very difficult. A flexible hand is needed to grasp the object, and the opposable thumb gives finer control. The same characteristics that allowed our ancestors to thrive in the trees were used by more recent ancestors to produce use tools.
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