Professor Meg Kassabaum climbed a ladder down into the earth, where bits of pottery and charcoal and stripes of soil and sand sketch the story of the ancient Native Americans who once lived in the Mississippi Valley.

Her research team has spent the past four weeks at a site on Smith Creek not far from the state line investigating a transformative period in history. The site contains artifacts from two distinct cultures, which saw a transition from hunting and gathering to agriculture, the University of Pennsylvania anthropologist said.

The two societies — the Coles Creek mound builders and later Plaquemine culture — also have their own social structures and distinctive artwork, which Kassabaum showed in pottery found in the recent dig, which was beginning to wind down Thursday afternoon.

Her team has explored three sites west of Woodville, Mississippi. Two of the digs have focused on a pair of mounds, while the third is in an area between them, an area the researchers call a central plaza that is now a family’s front yard.

They’ve unearthed arrowheads, animal bones from food, and pieces of a broken pipe. They’ve also found pottery, decorated, though not with human or animal figures. The Coles Creek people often slapped the still-wet clay with paddles bound in rope before their pots and bowls were fired, transferring the texture of the cords. The Plaquemine carved intricate geometric designs on their pieces, such as one find that shows an interlocking scroll pattern.

The mounds were likely constructed by the Coles Creek some time between 800 A.D. and 1000 A.D., though the site was probably inhabited by the Plaquemine for 300 years afterward, Kassabaum explained.

One of the mounds is believed to have been the foundation for a structure like a temple or communal building. Another nearby mound not under excavation was probably used for burial, while the third, partially eroded from the nearby creek, is a mystery, though Kassabaum said it could have been the foundation for a stage or open gathering space.

Field Supervisor David Cranford said the mound builders probably used the sites for special occasions like feasts and ceremonies.

The mounds range in height from 12 feet to 30 feet, the tallest being the one believed to have been used as the foundation for a temple or communal building. Within that mound, Kassabaum pointed to stripes of brown, yellow and orange that represent different stages of construction as generations built on top of the work of their forebears. The professor compared the appearance to a layer cake.

She noted a striking layer of white sand hauled from the nearby creek. It would have taken more effort to use the sand than nearby dirt or clay, and Kassabaum considered its significance. Perhaps the white color had some symbolic meaning. Maybe the builders wanted to connect the site to the creek, upon which the builders relied for food, water and travel.

The dig at the central plaza revealed the biggest surprises, Cranford said. The team found evidence of post holes, believed to be for houses that would indicate later groups lived at the site full-time.

“It’s really cool to see the posts when they line up. … (The inhabitants are) changing the way the site is being used,” Cranford said.

The exact number of residents hasn’t been determined and will require further study, Kassabaum said. The site may have supported 15 to 20 people or as many as 200. Teams on future trips may be able to look for other signs of structures that could provide a more complete picture.

While excavating the central plaza, the team also found a human skull embedded in the ground, but did not dig further. Kassabaum’s team contacted Choctaw and Chickasaw tribal leaders, who asked that the researchers halt their excavation, cover the remains when they leave and not allow photography of the skull out of respect for their religious beliefs. So, the team does not know how old the remains are.

It’s also unclear what relationship the Plaquemine may have had to the Coles Creek mounds. Cranford said they may have regarded the mounds similarly to how modern Americans view battlefields from the Civil or Revolutionary wars.

“The meaning keeps on changing based on the people who are using it,” he said.

“It’s like a big jigsaw puzzle. A lot of the pieces are missing and we don’t always have a clear idea of how they were used.”

One of the biggest changes between the two societies was that, unlike the Coles Creek culture, the settled Plaquemine practiced agriculture. Teams have found preserved bits of burned corn. Additional food crops included plants now considered weeds, like chenopodium and amaranth. Others are more recognizable to modern palates, like squash.

The people at the Smith Creek site also enjoyed hickory nuts, pecans and sunflower seeds, as well as various fish, turtle and deer, as evidenced by bones found in the digs.

How, when and where agriculture developed in the Mississippi Valley is still an area of exploration for anthropologists, said University of North Carolina professor Vin Steponaitis, who worked with Kassabaum on the Mississippi Mound Trail project.

Corn agriculture arrived in the valley later than other sites in the eastern United States, he said. Researchers believe that may be because the region is rich in food resources that can be hunted or gathered, but Steponaitis hopes transitional sites like Smith Creek can provide new evidence.

Leadership and political power in the Mississippi Valley cultures is also a burning question for anthropologists. By the time the French encountered the Natchez, the descendants of the Plaquemine, the tribe already had a hereditary nobility, Steponaitis said. Earlier inhabitants of the valley are believed to have had much more egalitarian communities, Kassabaum said.

Steponaitis wants to know when those ruling families began to come into power. Centralized government often arrives after a society has moved to a settled, agricultural lifestyle, he said. But the Mississippi Valley peoples may have been different.

“Building mounds is not easy,” he said. “You have to move a lot of people in the same direction.”

Monumental architecture tends to arise when a group has a leader at the top who can mobilize labor, the professor continued. So maybe the mounds are evidence of some kind of chiefdom that predated agriculture.

“It would sort of turn that (typical) sequence … on its head,” Steponaitis said.

Kassabaum’s team hasn’t found any chiefs’ burial sites at Smith Creek, and the professor said they may never. The chiefs would be identifiable by their lavish tombs set apart from other remains, she said.

But the team still has more to explore, and Kassabaum expects to return next year and see what artifacts are still buried.

Follow Steve Hardy on Twitter, @SteveRHardy.