Evolutionary Changes in Primates
Purpose:
• To examine skull diagrams of a gorilla, early human, and modern human.
• Measure, observe, and record specific structures and features of each skull.
• Evaluate evolutionary changes that have occurred in the organisms.
Materials:
Ruler
Protractor
Procedure:
Part I. Skull Characteristics and Relationships
1. The upper rectangle over the skulls in Figure 1 represents the area of the brain. The lower
rectangle represents the face area. Measure each area and multiply the length by the width. Record
your results for each in Table 1.
2. To determine the ratio of the brain area to the face area, divide the brain area by the face area.
Record the ratio in Table 1.
3. To determine the cranial capacity, measure the diameter of the circle in each skull in centimeters,
then multiply the diameter by 200 square centimeters. Record your results in Table 1 as cubic
centimeters.
4. Notice the two heavy lines in the front of each skull. Place the protractor onto each skull as
shown in Figure 2. Read the angle by using the outside scale on the protractor. Record the angles
for each in Table 1.
5. The ratio between the distance across the jaw backs compared to the distance across the jaw
fronts can be used to compare jaw shapes of the three jaws in Figure 3. Measure the distance
across each jaw from one dot to the other on the back teeth. Then measure the distance across
each jaw using the dots on the front teeth. Record the results in Table 1.
6. To determine the ratio of the front to the back jaw, divide the front jaw measurement by the back
jaw measurement and record your results in Table 1.
7. Count the number of teeth in the lower jaw and record the result in Table 1.
8. Count the number of each type of tooth in the lower jaw and record the results in Table 1. Use
"M" for molar, "P" for premolar, "C" for canine, and "I" for incisor.
9. A bony ridge running across the top of a skull for muscle attachment is called a sagittal crest.
Indicate for each skull whether or not a sagittal crest is present in Table 1.
10. Directly above the eye sockets is a thick bony ridge. Indicate whether this bone is present or
absent in each skull in Table 1.
Part II. Interpretation of Data
Use the following information to help you evaluate your recorded data and answer the analysis
questions.
Ratio of Face Area to Brain Area
A reduction in face area compared to brain area is a trait of modem humans.
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Cranial Capacity
An increase in brain size as measured by cranial capacity is characteristic of more
complex organisms. Modern humans have the largest cranial capacity of all closely related
primates.
Jaw Angle
Jaw angle increase toward 90 degrees is a trait of modem humans. Less of a protruding
jaw is characteristic of more complex organisms.
Lower Jaw Shape
Gorillas have a jaw in which both sides are parallel to one another. They have a ratio of 1:1
for back to front distance. Modern humans have a more V-shaped jaw with a ratio of back to front
distance of greater than 1:1.
Numbers and Types of Teeth
Adult modern humans, Australopithecus, and gorillas all have similar patterns in terms of
numbers and types of teeth.
Sagittal Crest
This bony ridge is associated with heavy temporal muscles used to move the lower jaws.
Reduction in size of the lower jaw in modern humans has resulted in a corresponding reduction in
the size if this ridge.
Brow Ridge
Loss of this ridge is a trait of modem humans.