Evolutionary Changes in Primates



Evolutionary Changes in Great Apes

INTRODUCTION

Great apes include humans, chimpanzees, gorillas and orangutans. Together they form the taxonomic family Hominidae and their ancestors. When paleontologists discover fossils they must determine type of organism that was fossilized. They must also determine whether the fossils are from recent or ancient organisms. If a skull were discovered and determined to be a large primate skull, the next step would be to determine whether it is of an ape or a human. Because evolutionary change has occurred in both groups, the skull could be of early or modern ape or early or modern human. Since humans and apes evolved along separate lines, certain physical characteristics can be used in an attempt to classify the skull as belonging to either ape (early or modern), early human or modern human. Techniques similar to the ones used in this investigation are used by anthropologists, paleontologists, and archeologists.

OBJECTIVES

1. Examine skull diagrams of 3 Hominid skulls.

2. Measure or observe and record specific skull structures and features.

3. Evaluate evolutionary changes that have occurred in these organisms.

PROCEDURE

PART A: SKULL CHARACTERISTICS and RELATIONSHIPS

1. Ratio of Brain Area to Face Area

The rectangles over the skulls in Figure 1 represent the area of the brain (upper rectangle) and face area (lower rectangle) of each skull.

A. Determine the area of each rectangle by measuring the length and width in centimeters and multiplying the two measurements together.

B. Record in the data table the brain and face areas of all 3 specimens.

C. Determine the ratio of brain area to face area for each skull by dividing the brain area by the face area. Carry the division to two decimal places. Always use face area as 1 in the ratio. For example, a skull with a brain area of 50 cm2 and a face area of 120 cm2 has a face area to brain area ratio of 0.42 to1.

D. Record the ratios in the proper column in your data table.

2. Cranial Capacity

A. Measure the diameter in centimeters of the circle in each skull. Multiply the cranial diameter by 200. This gives the cranial capacity (brain volume) in cubic centimeters. Note: The volume of sand that will fill the cranial cavity is the method used when an intact skull is available. The technique of multiplying by 200 is suitable only for these diagrams.

B. Record the cranial capacity for each skull in your data table.

Data: Comparison of Hominid Skulls

| | | | |

| |Specimen 1 |Specimen 2 |Specimen 3 |

| | | | |

|Brain area cm2 | | | |

| | | | |

|Face area cm2 | | | |

|Ratio of | | | |

|Brain to Face | | | |

|area | | | |

| | | | |

|Cranial Capacity | | | |

|cm3 | | | |

| | | | |

|Jaw Angle | | | |

|Distance | | | |

|across Front | | | |

|of jaw cm2 | | | |

|Distance | | | |

|across Back | | | |

|of jaw cm2 | | | |

|Ratio of | | | |

|Front to Back | | | |

|distances | | | |

|Number of | | | |

|Teeth | | | |

| |M = P = |M = P = |M = P = |

|Number of | | | |

|each type of tooth |C = I = |C = I = |C = I = |

| | | | |

|Sagittal Crest | | | |

| | | | |

|Brow Ridge | | | |

|Identity: | | | |

|Human, Gorilla or Australopithecus | | | |

3. Jaw Angle (Prognathism)

In front of each skull are two lines, one running parallel through the nose. These two lines are to be used for measuring how far the jaw protrudes forward.

A. With a protractor, measure the external angle (the angle toward the right) formed by the two lines in each skull .

Figure 2

For example, the jaw angle in Figure 2 is 45o.

B. Record the angles in the proper column of your data table.

4. Lower Jaw Shape

The ratio between the distance across the jaw back compared to the distance across the jaw front can be used to compare jaw shapes of the three organisms in Figure 3.

A. Measure in centimeters the distance across each jaw from one dot to the other on the back teeth.

B. Measure the distance across each jaw using dots on the front teeth.

C. Record the distances for each jaw in your data table.

D. Determine the ratio between the distance across the back of the jaw to the distance across the front of the jaw for each skull. Use the distance across the back of the jaw as the 1 in the ratios. (Divide front distance by back distance.)

E. Record the ratios in the proper column of your data table.

5. Numbers and Types of Teeth

A. Count the number of teeth for each lower jaw in Figure 3.

B. Count the number of each tooth type for each lower jaw.

C. Record in your data table the total number of teeth for each lower jaw and the number of each type. Use “M” for molar, “P” for pre-molar, “C” for canine, and “I” for incisor.

6. Sagittal Crest

A bony crest running across the top of a skull for muscle attachment is called a sagittal crest. (It is really pronounced on the skull of the Gorilla.)

Indicate in your data table whether a sagittal crest is absent or present in a skull

7. Brow Ridge (Supraorbital Ridge)

Directly above the eye sockets is a thick bony ridge. This ridge may be absent or present in a skull.

Indicate in your data table whether or not a brow ridge is present.

PART B – INTERPRETATION OF DATA

The following information will help you evaluate your recorded data and answer the questions in the Analysis section.

Note: Gorillas and humans are modern and Australopithecus is not. Consequently it is wrong to argue that gorillas evolved into Australopithecus then into humans.

1. Ratio of Face Area to Brain Area

An increase in brain area compared to face area is a trait of modern humans.

2. Cranial Capacity

An increase in brain size as measured by cranial capacity is characteristic of more of more modern hominids. Modern humans have the largest cranial capacity of all closely related primates.

3. Jaw Angle

Jaw angle increasing toward 90o is a trait of modern humans. Less of a protruding jaw is characteristic of a smaller jaw relative to cranial capacity.

4. Lower Jaw Shape

Gorillas have a jaw in which both sides are parallel to one another. They have a ratio of 1:1 for front to back distance. Modern humans have a more V-shaped jaw with a ratio of front to back distance less than 1:1.

5. Sagittal Crest

This bony ridge is associated with stronger jaw muscles. Reduction in the size of the lower jaw in modern humans has resulted in a corresponding reduction in the size of this ridge.

6. Brow Ridge

Loss of this ridge is a trait of modern humans.

Figure 3

Specimen 1 Specimen 2 Specimen 3

(1/2 natural size) (1/2 natural size) (1/2 natural size)

ANALYSIS : Gorilla, Australopithecus, Human Comparison

1. Use information in part B to identify the 3 skulls as Human, Gorilla and Australopithecus.

2. How do the Australopithecus and gorilla skull compare in terms of the following traits?

a. Cranial capacity

b. Lower jaw shape

c. Jaw angle

3. Which of the traits listed above does Australopithecus share with modern humans?

4. Using brain to face area ratios, does Australopithecus more closely resemble gorilla or human?

5. Copy the table for all 12 traits. For each trait decide which 2 species are most similar and check mark those 2 boxes.

|Trait |Gorilla |Australopithecus |Human |

|1. Brain area | | | |

|2. Face area | | | |

|3. Brain to face ratio | | | |

|4. | | | |

|continue with traits 5-12 | | | |

6. Where does Australopithecus fit in this evolutionary tree?

( Australopithecus lived before the common ancestor and is an ancestor of both humans and gorillas, or….

( Australopithecus lived after the common ancestor and is an ancestor only of humans, or….

( Australopithecus lived after the common ancestor and is an ancestor only of gorillas.

Choose one scenario and explain why it is correct using your data.

Data, numbers, logic!

7. The size and strength of teeth and jaws suggest the type and toughness of food an animal eats.

a. Explain possible selective pressures that would result in stronger or weaker jaws in great apes. b. Do you think it is possible that the jaws of the common ancestor were weaker than the jaws of modern gorillas? Why?

8. Suppose you find a distorted jawbone and note that there are 16 teeth in it. Explain why this information may or may not be helpful in determining whether the fossil is from a modern or ancient human or gorilla.

9. Suppose you find a distorted jawbone with most of the teeth missing. The canine teeth, however, are present and appear quite large. Which animal might this be from?

-----------------------

Figure 1

Brow Ridge

Figure 1

Specimen 3

1/2 natural size

Sagittal Crest

Specimen 1

1/2 natural size

Specimen 2

1/2 natural size

Australopithecus

1/2 natural size

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