Biology 12

SKILL FOCUS

Performing and recording

Analyzing and interpreting

Communicating results

Investigation 12•A

408 MHR • Unit 4 Evolution

The following information will help you with this

investigation:

The reduced ratio of face area to brain area is a trait

of modern human adults.

An increase in brain size (cranial capacity) is

characteristic of more complex organisms. Modern

humans have the largest cranial capacity of all closely

related primates.

A jaw angle close to 90° is a trait of modern humans.

A bony ridge on the skull, called the sagittal crest, is

associated with strong muscles that move the lower

jaw. The reduced size of the lower jaw in modern

humans has resulted in a corresponding reduction in

the size of this crest.

A less prominent brow ridge is a trait of modern

humans.

Pre-lab Question

How can skulls be used to determine evolutionary

relationships?

Problem

How do the skulls of primates provide evidence for

human evolution?

Prediction

Predict how a modern human skull will differ from the

skull of Australopithecusand the skull of a gorilla.

Materials

ruler protractor

Procedure

1.Your teacher will provide you with a copy of the

skulls (1/4 natural size) of Australopithecus

africanus, Gorilla gorilla, and Homo sapiens.

2.Copy the data table into your notebook.

3.Measure the face area (lower rectangle), brain area

(upper rectangle), and ratio of face area to brain

area for each skull. (For example, a skull with a face

area of 130 cm^2 and a brain area of 60 cm^2 has a

face area to brain area ratio of 1 to 0.46.) Record

the data in your table.

4.Determine the cranial capacity (brain volume) of

each skull by measuring the diameter of the circle in

each skull and multiplying this figure by 200. Record

the data in your table.

5.Measure the jaw angle using the two lines drawn on

the skull. The jaw angle measures how far the jaw

protrudes forward. Record the data in your table.

6.The bony ridge running across the top of a skull is

called the sagittal crest. This crest is used for

muscle attachment. Record the presence or

absence of this crest in each skeleton.

7.Determine the presence or absence of a brow ridge

in each skull.

Gorilla

Modern

Australopithecus human

face area ( )

brain area ( )

ratio of face area to

brain area

cranial capacity ( )

jaw angle

sagittal crest

brow ridge

cm^2

cm^2

cm^3

Skull Differences in Primates

Evolutionary biologists and paleontologists use many techniques, including

comparative anatomy and DNA sequence analysis (comparison of nucleic acid

sequences), to determine evolutionary relationships between species. In this lab,

you will examine primate skulls to determine evolutionary relationships among

primates. One of the skulls you will examine is that of Australopithecus, an early

ancestor of humans. Australopithecusis the oldest known example in which the

opening in the skull that leads to the spine (called the foramen magnum) is found

at the bottom of the skull. Early Australopithecusspecies possessed both ape-like

and human-like characteristics.