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Social Science: Anthropology
: Environment
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Publication Date: Fall 2005
To fully understand the prehistoric cultures of the Southwest, ASU scientists study how the landscape influenced societal and economic changes. They want to know how these changes, in turn, transformed the landscape.
In 2001, Hoski Schaafsma proposed a study of several acres of land in a riverine area north of Phoenix. The Arizona State University graduate student’s goal was to investigate the effects of ancient Hohokam farmers on the soils. He wanted to determine whether their activities influenced the kinds of plants that grew there today.
His advisor, John Briggs, was skeptical. Briggs pointed out that the site had been abandoned by the Hohokam nearly 1,000 years ago. Besides, he reasoned, a century of cattle grazing and more than a decade of soil-churning ATV traffic probably had altered the place beyond recognition. Intrigued by Schaafsma’s idea, he gave him a green light to proceed.
To their mutual surprise, Schaafsma discovered that the Hohokam’s silt-trapping dams had altered the soils in their riverside fields. To this day, only a monoculture of creosote bushes has managed to take root. By comparison, the surrounding landscape that was largely untouched by the indigenous farmers hosted an average of 28 plant species.
“It’s mind-boggling to me that a prehistoric human culture planting corn with a stick still has an impact on modern-day vegetation,” says Briggs, a grassland ecologist with ASU’s department of ecology, evolution and environmental science.
Briggs wasn’t the only professor at ASU intrigued by the idea that prehistoric land uses could influence modern-day ecosystems. Scientists refer to such phenomena as “legacy effects.” For more than a year, ASU anthropologists Keith Kintigh and Katherine Spielmann attended informal meetings with their colleagues Their discussion focused on the interplay between landscapes and the ancient cultures that inhabited them. They had ideas to test. Spielmann suggested a research project at the little-known pueblos of Agua Fria National Monument in central Arizona.
The monument area provided the perfect test site. Located 40 miles north of downtown Phoenix, it is within easy driving distance for the researchers and their students.
However, unlike other major prehistoric sites in the Southwest, the archaeological features at Agua Fria have barely been documented. Some sketchy information does exist. That information raises a number of tantalizing questions for researchers.
Sometime between the late 1200s to about 1400, a group of prehistoric people occupied pueblos on a cluster of mesas at the monument. No one knows who they were or where they came from. No one knows why they chose to set up their homes and farm fields in these arid, high-elevation grasslands with their drying winds and thin, rock-studded soils. And no one knows, why, after less than 200 years of occupation, they disappeared from the site.
“It’s our Easter Island,” Kintigh says. The site’s greatest appeal was the availability of land—1,000 acres, to be exact. The Agua Fria National Monument was formally dedicated in 2000. Former U.S. Secretary of the Interior Bruce Babbitt drew the boundaries to include 10 major pueblos, agricultural fields, and hundreds of tiny field houses.
This greater inclusiveness marks a new era in the preservation of archeological sites, Kintigh says. Until recently, the preservation of prehistoric ruins often entailed drawing tight boundaries around existing structures. Much of the research was limited to these postage-stamp sites. But archaeologists knew that the land needed to support its occupants may have ranged tens of square miles beyond the boundaries of their excavation pits.
Archaeologists can’t understand ancient Egypt simply by studying the pyramids. Using that logic, Kintigh says that researchers can’t hope to fully understand the prehistoric cultures of the Southwest without studying how the landscape influenced societal and economic changes. They need to know how these changes, in turn, transformed the landscape.
The ASU researchers are taking a more landscape-focused approach to studying the indigenous people at Agua Fria. The archaeologists need the expertise of scientists like Briggs—not just as consultants, but as collaborators. Kintigh says that such integrated research is almost unprecedented in both anthropology and ecology. For example, in the past, archaeologists routinely called upon other specialists for help. Mammalogists can identify animal remains found in prehistoric garbage dumps known as middens. Palynologists can pinpoint the kinds of crops grown in ancient agricultural fields by analyzing residual pollen grains found in the soil. But on the Agua Fria project, the scientists shared the same research question and charted research plans in tandem.
Working together in 2004, the ASU scientists began to unravel some of the mysteries posed by the pueblo people of Agua Fria. Their initial focus was at Pueblo La Plata. The site has an estimated 160 rooms. It is the biggest and the most accessible ruin in the monument.
To understand how the occupants may have impacted the landscape, the researchers’ plan called for setting up two transects. Each transect is a line of intensive study. One transect was located about 550 yards from the pueblo ruins in an area heavily used by humans. A second transect was known as the control transect. It was set up more than half a mile away. Foot traffic probably would have been far lighter in this area.
Ecology and archaeology students worked at regular intervals along each transect. They documented such things as rock cover—everything from gravel to boulders—as well as herbs, cacti, shrubs, and trees. They also took note of human debris. They charted broken bits of pottery and chipped stones left over from the tool-making process.
The site revealed some curious patterns. Surrounding the pueblo, the researchers discovered what they called a donut hole—an area of parched soils largely devoid of rocks. They think that pueblo builders chose material closest to the site for building their structures. As a result, rock cover was depleted in the immediate area surrounding the ruins.
Not surprisingly, the number of rocks increased as students moved away from the ruins. But as the number of rocks increased, so did the plant cover. To this day, the donut hole immediately surrounding the ruins supports far fewer plants than the rocky terrain on the pueblo’s periphery or in outlying areas.
“The pueblo people moved rocks for a variety of purposes,” Briggs explains. “As a result, they continue to have an impact on today’s landscape.”
In early 2005, the ASU researchers started mapping the ancient agricultural fields near the pueblos. They want to know if the agricultural infrastructure built by early Agua Fria farmers also left discernable impacts on these cultivated areas.
Kintigh says that such legacies have been dramatic at other prehistoric landscapes. The fields surrounding the Zuni pueblos of New Mexico are just one example. Ancient farming practices there left soils so compacted and depleted of nutrients that they are incapable of supporting plants to this day.
The ASU scientists know that key questions can’t be answered without shared expertise. The archaeologists pointed out linear alignments, known as check dams, on the hillsides around Pueblo La Plata. Only then did Briggs see the work of human hands in what looked like a natural landscape.
Humans have impacted the environment in many ways during the Industrial Age. That is not new. “We’ve been impacting the environment ever since we became a society in human evolutionary times,” Briggs says. “It’s really changed my perspective of what ‘natural’ really means.”
Unlike ecologists, Spielmann says that archaeologists take for granted that there is “human modification of the landscape.”
“There’s not much out there that’s untouched,” she adds. “But we had no idea about the complexities of soil, or what you could read from plants. This project has made us more sophisticated thinkers about one another’s disciplines.”—Adelheid Fischer