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uman evolution is an important and intriguing area of biology (Alles & Stevenson, 2003). The significance of evolution as a component of biology curricula, at all levels, can not be overstated; the need to make the most of opportunities to effectively educate students in evolution as a central and unifying realm of biology is paramount. Developing engaging laboratory or classroom activities that investigate human evo- lution (e.g., DeSilva, 2004) can therefore be of significant value to students and educators. This report describes an exercise involving comparative anatomy of hominid skulls, centering on the use of digital imaging to generate measurement data for comparison and analysis. Here, hominid refers to the family Hominidea; that is, all modern and extinct Great Apes—including humans, chim- panzees, gorillas, and orangutans. Rather than as a step-by- step fixed protocol, this laboratory is presented with options that emphasize adaptability to various pedagogical approaches and instructional levels. Versions of this laboratory have been used in college evolution and introductory biology courses; adaptations are likely to be of value for other courses, includ- ing non-majors’ biology, comparative anatomy, and secondary school biology. Features of skulls (commercially obtained skull casts) are identified and compared by students in order to develop an evolutionary analysis centered on functional anatomy. The focus of this exercise is on structural features that relate to three characteristics central in human evolutionary history: brain size, posture, and mastication. As with other published activities designed to explore hominid evolution using skull comparisons (Nickels, 1999; Nickels, 1987), various quantita- tive and qualitative measures are introduced and employed. However, this exercise also extends this approach in a novel way by employing quantitative craniometric relationships made using digital imaging.
This activity can accommodate a variety of approaches to investigate specific differences between species, relate struc- ture and measures to function, and apply observations and data to evolutionary paradigms. For example, based on initial exposure to skulls and discussion or background reading, students can develop and test hypotheses about how particu- lar measures indicative of braincase size will compare among species. Interpretations of data collected can be made in light of current understanding of the time course and ramifications of changes in form and function associated with human evolu- tion. Aspects of evolution are introduced in a hands-on manner in this exercise, which also supports the study of human anato- my and emphasizes functional and comparative anatomy.
Methods
As introduction to the exercise, some basic relevant func- tional anatomy is first reviewed using skull models, illustra- tions, and select reference materials (e.g., Lewin, 2005; Lewin & Foley, 2004; Campbell, 1998). Instructors have, of course, a number of options as to how this material is presented and integrated, ranging from lecture to assigned reading to more hands-on, active learning approaches. Characteristics focused on in this introduction reflect major differences among homi- nids involving: 1) the size of the brain case (and hence the size of the brain), 2) muscular and skeletal elements of mastica- tion, and 3) body posture and support of the head. Emphasis is placed on encouraging students to consider interrelations between differences in skull structure and characteristics such as intelligence, behavior, nutrition, and way of living. Students first then develop familiarity with anatomical features (Table 1) relating to their comparative analysis. Skull casts of hominids used in this exercise are avail- able from a number of specialty suppliers (e.g., Bone Clones®, Canoga Park, CA; www.boneclones.com) and general vendors (e.g., Ward’s Natural Science, Rochester, NY; www.wardsci. com). The number of students per lab group can be adapted
H
J O H N C. R O B E R T S O N
INVESTIGATING HUMAN EVOLUTION
Using Digital Imaging & Craniometry
JOHN C. R OBERTSON is Associate Professor, Department of Biology, Westminster College, New Wilmington, PA 16172; e-mail: [email protected].
O N L I N E I N Q U I R Y & I N V E S T I G AT I O N
DIGITAL IMAGING & CRANIOMETRY 37
based on resources and the specific design of the activity. As an example protocol, each group works with a total of four specimens (skulls), choosing any combination from the four groups listed in Table 2. Species listed in this exercise are broadly representative of hominids; other species can be added or substituted.
To facilitate comparison of the different species, quantitative and qualitative mea- sures are made of features on different skulls. Examples of quantitative data that students are specifically asked to incorporate into their comparative analysis include the follow- ing measures:
- braincase size (in cm 3 )
- condylar position index (ratio of CD: CE x 100, see Figures 1 and 2)
- supraorbital height index (ratio of FB:AB x 100, see Figures 1 and 2)
- maxillary prognathism index (ratio of HE:DE x 100, see Figures 1 and 2)
- angle of forehead (see Figures 1 and 2)
- post-orbital constriction index (as % of skull width; see Figure 3) Students are asked to develop their own model for estimating braincase volumes (rulers and measuring tapes are provided). To generate the other quantitative measures listed, students take digital images (typically JPEG files) of skull casts using standard digital cameras of at least 1 megapixel resolution. Quality images are effectively obtained with the casts placed on a table-top under ambi- ent room lighting. Appropriate software (e.g., PowerPoint) is then used to overlay lines and points in specific anatomical locations on the skull images. Details of digital imaging, image manipulation, and instructions for placement of lines and points for craniometry are provided to students (Figures 1 and 2). By measur- ing (directly on the computer screen or using a ruler on printouts) length of specific line segments and calculating ratios, students generate craniometric data. Note that because the ratio data have no units, relative magnification of the skulls in individual images need not be standardized. Forehead slope is mea- sured using a protractor.
These quantitative data provide indi- cations of anatomical adaptations relating to shifts in brain size, posture, and diet over hominid evolutionary history (Table 3). Thus — in addition to students’ direct estimate of braincase volume — increased supraorbital height index, lower post- orbital constriction index and decreased angle of the forehead all correlate with greater cranial capacity. Increased condy-
lar position index indicates both a shift in position of the spine associated with evolving bipedalism as well as expan- sion of the posterior skull. Decreased maxillary prognathism indices indicate a reduction in the relative size of the masti- catory components of the skull. In terms of qualitative analysis, there are numerous interspecific comparisons that students can make and relate to functional anatomy. For example, the angle of the foramen magnum can be assessed; this opening tends to be flat in a biped and elevated in a quadru- ped. Access to additional, non-hominid quadruped skulls (e.g., cat or deer) for comparison can help illustrate this point for students. The presence and relative size of a sagittal crest can be evalu- ated; this attachment site can provide an indication of the size and action of the temporalis muscle in chewing. The rela- tive size of the zygomatic arch opening can also indicate size of the temporalis muscle that passes through this open- ing to insert on the mandible. In part, more pronounced brow-ridges help dis- sipate forces associated with chewing; reduction in the size of the supraorbital
38 THE AMERICAN BIOLOGY TEACHER, ONLINE PUBLICATION, MARCH 2007
Table 1. Features useful for students to become familiar with in a functional anatomy- based comparative analysis of hominid skulls.
FEATURE COMMON MEANING
Neurocranium Braincase or cranial cavity
Orbits Eye sockets
Supraorbital torus Brow ridges
Post-orbital constriction Indentations in lateral skull behind orbits
Forehead slope Angle of the forehead (not brow ridges)
Zygomatic arch Cheek bones
Sagittal crest Ridge of bone along medial skull
Prognathism Anterior extension of jaws from skull and face
Foramen magnum Large opening in skull underside for spinal cord
Occipital condyles Bony protuberances lateral to foramen magnum
Temporalis muscle
Chewing muscle; origin on lateral skull, insertion on mandible,
runs through the zygomatic arch
Temporalis line Marking on lateral skull at attachment site of temporalis muscle
Masseter muscle Chewing muscle; origin on zygomatic arch, insertion on mandible
Dental arcade Line formed by teeth in the upper and lower jaws
Table 2. Specimens included in hominid skull evolutionary analysis. Students typically analyze four skulls, selecting one from each of the four groups listed.
Group 1 Gorilla gorilla (Gorilla) Pan troglodytes (Chimp)
Group 2 Australopithecus afarensis (“Lucy”) Australopithecus boisei
Group 3 Homo habilis Homo erectus
Group 4 Homo neanderthalensis Homo sapiens
Figure 1. Schematic provided to students as a
guide for placement of lines and points used to
obtain craniometric data. Horizontal line DE runs
from the posterior most point of the skull to the
anterior extent of the maxilla at the level of the
bottom margin of the orbit. Vertical line AB runs
from the superior-most point of the skull to line
DE. Line F extends horizontally from the top of
the orbit posterior to line AB. Line C runs vertical-
ly up from the center of the occipital condyle to
line DE. Line H originates perpendicular to line DE
at the anterior orbit and extends up beyond the
top of the skull. Angle J is formed by line H and
an intersecting line that follows the forehead
slope (not the brow ridge). See text for descrip-
tion of calculating various craniometric indices.
(e.g., Campbell, 1998). These values might be provided or students may be expected to search for such relevant information as part of the exercise.
Moreover, a variety of media resources (e.g., videos such as Films for the Humanities & Sciences’, “Challenging the Human Evolution Model,” and the Nova production, “In Search of Human Origins, Episode 1: The Story of Lucy”) and popular science literature (e.g., Wong, 2005; Zimmer, 2003; Gould, 1979) can be used in support of or integrat- ed with the described exercise. “The Mismeasure of Man” (Gould, 1996) provides particularly interesting and relevant insights into the history of science and use of dubious craniomet- ric data in advancing a deterministic social agenda.
Application
Experience indicates that stu- dents have few problems with techni- cal aspects (imaging and image pro- cessing) of this activity. The expense of skull casts is significant but not prohibitive; the author has found indi- vidual casts available at costs rang- ing from about $100 to about $300, depending on the species, producer, and vendor. Defined study sets of skull casts are also available, usually at some discount over individually purchased casts.
Digital cameras for this exercise are another resource to be considered. Depending on specifics of the assignment, there may not be need for each group to take a large number of images – in which case one or two cameras could suffice for even a large class. Cameras capable of capturing images of only 1 megapixel have been used very successfully for the activity. Presently, higher resolution digital cameras — taking at least 3 megapixel images — can be purchased for under $100. Images are initially captured as data files (e.g., JPEG) to the camera’s internal memory or an optional flash memory card (e.g., SmartMedia or MemoryStick), and can then be downloaded to a computer. For those less familiar with digital imaging, a variety of helpful print (Miotke, 2005) and Internet Resources (e.g., Curtain, 2006) are available. For adding lines and labels to the images, a variety of software programs can be used; basic instructions for image processing in PowerPoint are provided in Table 4.
With many images of hominid skulls readily available in the literature and online (see Internet Resources below), some aspects of this exercise could be done strictly as an image-based activity. However, the hands-on tactile and true three-dimensional advantages make the use of skull casts an excellent learning experience. Additionally, digitizing software programs — a number of which are freely available for down- load, including Image J from the National Institutes of Health
(available online at: http://rsb.info.nih.gov/ij/) — could be inte- grated into this lab to directly generate length, area, and angle measures from skull images. The activity described is highly malleable; depending on the audience and the amount of time available, additional depth of material or activities can be included or the focus and scope can be restricted. Time invested in establishing appro- priate specific outcomes and developing a clear and concise introduction and protocol should contribute to the success of the experience. Based on anecdotal feedback and discussions, students react positively to the exercise. They seem to appreciate the opportunity to work hands-on with skull casts and to enjoy the imaging and collection of quantitative data. Learning about skulls appeals to many students’ interest in human anatomy; many are also quite curious about evidence for and current ideas concerning human evolution. In sum, this comparative activity is an effective and engaging introduction to human evolution that draws together elements of several sub-fields of biology and other disciplines (e.g., anthropology).
Internet Resources
- ArchaeologyInfo.com Site: http://www.archaeologyinfo. com/index.html.
- Becoming Human Site. Institute of Human Origins, Arizona State University. http://www.becominghuman. org/.
Table 4. Basic instructions for moving images into PowerPoint and adding lines and labels.
1. Open a new blank presentation in PowerPoint.
2. Under the Format menu, select Slide Layout. Choose a blank slide layout.
3. Under the Insert menu, select Picture and From File.
4. Browse to the appropriate location of the file. Select (click and highlight) the image you wish to
insert, then click the Insert button.
5. If a resolution box appears, choose the default resolution (768 X 512 pixels).
6. Your image will be placed in the PowerPoint slide.
7. You can resize your image by clicking and dragging on any corner (resizing from any side will
distort the image and may introduce artifacts into the craniometric measures).
8. Under the PowerPoint View menu bar, select Toolbars , then select Picture. A Picture Toolbar
appears that allows you to do some manipulation of the image, including adjusting the bright-
ness and contrast.
9. Under the PowerPoint View menu bar, select Toolbars , then select Drawing. A Drawing Toolbar
appears that allows you to do add lines and labels to your image.
10. When finished labeling the image, under the Edit menu choose Select All. This will highlight the
image and all the labeling that you have added.
11. Place the mouse cursor anywhere on the image and right-click once. A box will appear. Select
Grouping and then choose Group. This will fuse your labels to the image. If you do not do this,
the labels will move if you move or resize the image at a later time.
12. Save the image as:
a. JPEG file interchange format
b. Presentation file
13. To add additional images to the same presentation file, under the Insert menu choose New
Slide and repeat the above steps. In this case, you will end up with a single .ppt presentation file
containing all manipulated images as individual slide pages. Alternatively, you can save images as
separate .jpg image files (1 per image).
40 THE AMERICAN BIOLOGY TEACHER, ONLINE PUBLICATION, MARCH 2007
- Human Origins Program Site. Smithsonian Institution. http://www.mnh.si.edu/anthro/humanorigins/index. htm.
- Online Human Evolution 3-D Gallery. University of California at Santa Barbara. http://www.anth.ucsb.edu/ projects/human/.
- Prehistoric Life Site. BBC. http://www.bbc.co.uk/sn/pre- historic_life/human/.
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