A GLIMPSE OF EARLY AGRICULTURE

BY JULIAN SMITH

An artist’s rendition of ancient farmers tending their crops at the Las Capas site. Those farmers built an extensive network of irrigation canals.

Several archaeological projects in southern Arizona have revealed that this area has been farmed for thousands of years by people who devised sophisticated irrigation systems.

Last summer and fall, drivers on Interstate 10 in Tucson, Arizona, could see an archaeological dig in progress near the Ruthraff Road exit. Desert Archaeology, a local cultural resource management firm, was excavating part of a prehistoric site called Los Pozos in advance of a highway improvement project. The project is the latest investigation at the site, which stretches for over a mile between the highway and the Santa Cruz River. Los Pozos dates to the Early Agricultural Period, roughly between 2000 B.C. and A.D. 1, a period during which irrigated maize agriculture was introduced to northwest Mexico and the U.S. Southwest from Mesoamerica. Over the past few decades, researchers in the Tucson area have found that farming occurred much earlier than was once thought and that it affected the lives of residents in profound ways.

During the two millennia of the Early Agricultural period, local cultures gradually shifted from mobile hunting and foraging to a more sedentary lifestyle centered around growing maize, a transition that had profound effects. The Santa Cruz River floodplains and terraces offered the perfect setting for irrigation agriculture: a dependable and plentiful water source and regular floods of rich sediment, said Ian Milliken, Pima County Cultural Resources Project Manager, who is involved in the Los Pozos investigation. (Tucson is located in Pima County, and the county government is a consulting party in this project.) Residents learned how to build dams, terraces, and canal systems that watered individual garden plots. “Their primary crop was maize, which was originally thought to have arrived by 1000 or 500 B.C.,” according to Jim Watson of the Arizona State Museum, who has worked at numerous Early Agricultural period sites.

Since the 1990s, however, a series of projects in the Santa Cruz River basin near Tucson have pushed these dates back significantly. “They really redefined our understanding of the origins and adoption of agriculture in the Sonoran Desert,” Watson said. Most, if not all, of this research has been done by cultural resource management firms hired by Pima County, the City of Tucson, and the State of Arizona.

The Las Capas site, near where the Rillito and Cañada del Oro rivers feed into the Santa Cruz, is one of the largest and best preserved Early Agricultural period sites in the Southwest. Las Capas was probably home to no more than 100 people at a time, said Jim Vint of Desert Archaeology, who led an excavation of the site in 2008 and 2009 in advance of the expansion of a water reclamation plant. Over the centuries, however, these people left behind dozens of round and oval pithouses, thousands of storage and roasting pits, and acres of irrigated fields. Radiocarbon dating showed maize was being grown here by 2100 B.C., and possibly as early as 3700 B.C.

Small clusters of dirt-capped houses stood on low rises just outside the floodplain. Fields in the floodplain were crisscrossed by dozens of irrigation canals of varying width that brought water to hundreds of individual agricultural plots, each one roughly 250-square feet and bordered by soil berms. The extent of the fields and the sophistication of the canal system impressed Vint, and he thinks they showed how invested in agriculture the people were.

In 2015, a backhoe operator who was working at a construction site in Tucson near Interstate 10 uncovered something that thrilled archaeologists: a set of perfectly

Archaeologists use backhoes at Las Capas to expose broad areas by removing several centimeters of earth at a time. As features such as pithouses and storage or roasting pits are exposed, their edges are marked with paint.

preserved footprints about three feet below the surface. Forensic experts analyzed the prints and concluded they had been made by seven to nine individuals, including a toddler and a dog. Radiocarbon dating of samples and flood deposits showed they were made around 1000 B.C. and were one of the oldest sets of footprints discovered in the Southwest. Further excavations showed the footprints encircled agricultural plots that were about thirty-by-forty-five feet in size. Bordered by raised berms, these plots were planted with maize and irrigated by three canals that connected to the Santa Cruz River. Over 4,000 people came to see the prints when the site, named Rillito Fan, was briefly opened to the public that winter. “Footprints are something everyone can recognize, no matter what age,” Milliken said.

An aerial view of Las Capas’ network of canals and agricultural plots, which are outlined with white paint. The area that was farmed is darker due to clay deposited by repeated irrigation and the accumulation of decayed plant material.

The footprints around the two excavated fields showed in detail how the ancient farmers operated their irrigation system. First, they dug out part of one corner of the berm and plugged the lateral canal with mud to divert water into the various plots. To drain excess water, they walked to the far corner and dug another opening in the berm. “Traditional ecological knowledge is very evident in Rillito Fan,” Milliken said. The fields were created with just the right amount of slope to let water distribute evenly through each plot, but not flood the fields too deeply. Canals were made just wide and deep enough to carry water to the furthest point in the system. “Archaeologists usually try to infer behavior from the material assemblage,” he said. But by being able to see exactly where the ancient farmers walked, “we have confirmation that this is exactly the technology they were using to flood their field system.”

These methods showed how the inhabitants were further refining the irrigation techniques seen at Las Capas. “It confirmed they weren’t just experimenting, but mastering this art of agriculture earlier than we expected,” said Milliken. “They were brilliant strategists in understanding hydrology and the landscape.” Trench profiles revealed other fields and canals above and below the ones with the footprints—signs of generations of people returning again and again for the abundant water and fertile soils, building and repairing irrigation systems in the process.

The transition from nomadic hunting and foraging to subsistence farming spurred significant social changes, Watson said. As settled communities and populations grew, people had to organize themselves to face new challenges such as how to allocate farmland and water. (The latter is still a divisive issue in the Southwest today.) As successful

harvests became essential to survival, people had to learn to work together on a larger scale to build and maintain large canal systems. “All those things require leadership and collaboration and cooperation to make the whole thing work,” said Bruce Huckell of the University of New Mexico, who has worked at Early Agricultural-period sites.

The social and cultural shift didn’t happen overnight, Vint said, but instead was a slow and stable process. “There was a certain amount of agriculture, but still a lot of foraging and gathering of wild plants.” (Some researchers call this phase “farmaging.”) At Las Capas, for instance, maize made up only a portion of the local diet for the first centuries of the Early Agricultural period, possibly less than a third of residents’ average annual calories. The rest came from hunting animals like rabbits and deer, and collecting plants like mesquite and sedges. Maize farming was adopted quickly, but its effects spread over a thousand or more years before significant changes in social organization appeared, according to Sarah Herr of Desert Archaeology. “It’s not a game-changer in the way you might think,” she said.

One question that scientists have debated is whether irrigation developed independently in Mesoamerica and the Sonoran region of northwest Mexico and the U.S. Southwest, or if the technology came north through migration or word of mouth. Bill Doelle of Archaeology Southwest said he wouldn’t be surprised if canal irrigation was a local development that was adapted to maize. “It’s a simple concept that could be done with reasonable levels of human labor,” he said. “They’re not moving massive amounts of earth.”

As an experiment, Vint had his crew members build a plot of their own using the same kind of mesquite sticks ancient farmers would have used. They found that two

A number of figurines, such as this one which dates from 710 – 530 B.C., have been recovered from early agriculture sites. These figurines could have symbolized fertility, rain, and ancestor worship.

people could dig a plot complete with berms in just thirty-five minutes. “It’s not as long as you might think; twenty to twenty-five people could construct a pretty big system in a couple of weeks,” Doelle said. “Then all you have to do is maintain it.”

In any case, Watson said, “the big lesson is how [the Tucson projects] show a long trajectory of social and technological buildup. At Las Capas you get to see it progress over 2,000 years,” he said. Over time, families lived near fields for longer periods and returned more often. Settlements grew and residents relied more on maize. “All that led to more social complexity,” he said.

A number of sites in the Santa Cruz basin dating to the Late Cienega phase (400 B.C.-A.D. 50) show this increasing complexity. In the mid 1990s, excavations at a different section of Los Pozos uncovered an agricultural settlement. This part of the site included hundreds of round pit structures built for storage and shelter, with artifacts ranging from projectile points and shell beads to the remains of a dog and a turtle.

One pit structure yielded an unusual set of artifacts including a fossilized horse tooth and the vertebra of a mastodon or mammoth. A gastropod fossil, an oblong piece of siltstone, and a fragment of quartz found nearby may have been part of the collection. Any one of these items would have been noteworthy; finding them all together suggests the house’s occupant played a special role in society. (Native American shamans made medicine bundles that held similar items during the Historic period.) “The curation of unique artifacts could point to increasing ritual complexity in these Early Agricultural communities,” Watson said. “As groups grew larger and more socially complex, the ritual sphere would have also needed to grow more complex.” Larger structures dated to the Cienega phase (800 B.C.-A.D. 50) are thought to have been built for communal use, he said. The Tumamoc Hill site, which sits on the peak of the like- named hill, 700 feet above the Santa Cruz River in downtown Tucson, has a round structure over twelve feet in diameter dated to the Late Cienega phase. (The terraced site is also encircled by over a mile of low stone walls, one of the earliest examples of communal labor in the Southwest.) The Santa Cruz Bend and Clearwater sites both span the entire Cienega phase and have structures larger than typical family pithouses. An important area of future research is the degree and scale of contact between southern Arizona and northern Mexico, according to Vint. “We don’t have a good understanding of early agricultural development in northern Sonora,” he said. “That would help connect the dots in terms of the route and timing of maize coming up from the south. But the international border creates a lot of problems for archaeologists, especially lately.” The cooperation and support of local government and governmental organizations has been crucial to archaeology in the Santa Cruz basin, according to Milliken. Public works projects cover wide areas that, when excavated, can give a fuller picture of a site than a typical narrow excavation trench. “In profile, what appear to be canals could be natural or man-made,” he said. “But if you strip a large area, take sediment off by increments, you can see features in plan [top-down] view.” Some backhoe operators are skilled enough to scrape soil away centimeters at a time.

Development projects often require digging deep as well as wide, which uncovers large amounts of data. “Economic progress and development in the area have led to these discoveries. Watson said. “Who knows how long they would have stayed buried otherwise?”

JULIAN SMITH is the author of Aloha Rodeo: Three Hawaiian Cowboys, the World’s Greatest Rodeo, and a Hidden History of the American West.