Bones Of Contention: Biology, Not Behavior, Shapes Us Says UA Researcher

FAYETTEVILLE, Ark. — Award-winning research from a University of Arkansas doctoral student warns anthropologists against inferring too much about ancient activity patterns and lifestyle habits from the evidence found in bones.

The physiological features that many scientists use to assess the physical demands of a people’s lifestyle may be the result of biological factors rather than behavioral ones, says Elizabeth Weiss, a physical anthropologist and recent graduate of the Environmental Dynamics Ph.D. program in Fulbright College.

"In a way, my research is a cautionary tale. It shows that a lot of the differences you see in bones are due to biological determinants like age, sex and size of the individual." Even the geographic location of a population can affect the morphology of their bones, Weiss said. "You have to account for those factors before you jump to conclusions about activity patterns."

Her findings prompted the American Association of Physical Anthropologists to award her the Mildred Trotter Prize for best student research in the field of bones and teeth, presented at the 2002 meeting in Buffalo, NY.

"The Trotter Prize is among the most competitive and prestigious student awards in the organization because it represents research on bones and teeth — what many physical anthropologists study," said Peter Ungar, associate professor of anthropology and Weiss’ advisor at the U of A. Ungar, himself, won the award in 1992 for his doctoral research on reconstructing diets from microscopic tooth wear in monkeys and apes.

Weiss’ doctoral project attempted to determine if the morphology of human arm bones correlated to specific, strenuous patterns of activity. She examined the humeri, radii and ulnae of more than 400 human specimens, a sample group that represented six distinct populations — the Georgia Coast, Illinois and Pecos Pueblo American Indians, the Aleuts of Alaska, the Tsimshin of British Columbia and numerous Euro-Americans.

Weiss categorized these populations according to lifestyle, distinguishing between those who routinely rowed on the open ocean (Aleuts and Tsimshin), those who rowed on inland waters (Georgia Coast), and those who did not row (Pecos Pueblo, Illinois and Euro-Americans).

She knew that human bones respond to physical stress in predictable ways. Strenuous physical activity stimulates bone to build upon itself, resulting in a thicker cross-section. In addition, physical exertion causes muscles to strengthen and grow, and bigger muscles etch larger patterns onto bones at the points where they attach. Therefore, Weiss hypothesized that the people with the most strenuous activity pattern — rowing on the ocean — would show thicker arm bones with larger muscle markers.

Indeed, the ocean rowers matched this pattern. But something made Weiss look deeper: "The females from the Aleut and Tsimshin groups also had more robust cross-sections than males from the other populations — despite the fact that females didn’t row," she said.

So Weiss began to wonder if other factors — factors unrelated to activity — could determine bone development. Using X-rays and CT scans to measure the exact cross-sectional thickness of each bone and using information known about each specimen — such as age, sex and size — Weiss began analyzing the data to see if she could identify these other factors.

Because muscle markers are visible on the surface of bones, it’s tempting to interpret their size as a gauge of how much strength an individual acquired in life. But Weiss’s study proved that anthropologists have to look beyond the surface to rule out pre-existing determinants of muscle marker size.

For example, Weiss found a strong correlation between muscle markers and cross-sectional thickness. This implies that individuals with thicker bones also show larger muscle markers, regardless of whether they developed thick bones genetically or through rigorous activity.

In addition, Weiss’ research showed that muscle markers correlate to the overall size of the person, with larger individuals exhibiting larger muscle markers. Such correlations are particularly relevant when examining individuals from northern regions. Colder climates tend to select for individuals with larger, stockier frames because such body types are more thermodynamically efficient. Thus, the thicker bones and larger muscle markers of the Aleuts and Tsimshin could be just as easily attributed to their physical build and geographic location as to their patterns of behavior.

"Basically, the research shows that the process of bone development is very complex, and you can’t draw conclusions about activity before you’ve accounted for everything else," Weiss said.

In fact, Weiss’ research was made even more complex by her decision to focus on arm bones, according to Ungar. Most studies of bone morphology and its relation to behavior examine the femur because the upper thigh participates in a very limited range of activities — mainly walking. The fact that people use their arms for such a variety of activities and uses makes it hard to isolate the effects of specific behaviors. Weiss circumvented this difficulty by focusing on one of the most rigorous upper body activities — rowing.

"Her thesis isn’t the final word on this debate, but it’s an important work nonetheless," Ungar said. "Any subsequent research will have to cite her project because it’s a benchmark study for cross-sectional geometry of the upper limb."

For more information about Weiss’ research, visit http://www.anthrosciences.com/ or www.geocities.com/cutop/.