Talk about a sweet deal—prehistoric peoples of Mesoamerica may have traded chocolate for gems from the U.S. Southwest, a new study suggests.

Traces of a chemical found in cacao—the main ingredient in chocolate—were found in several drinking vessels from various sites in Pueblo Bonito, a complex of sandstone "great houses" in Chaco Canyon, New Mexico.

Ancestral Puebloan peoples built the complex, the epicenter of the ancient Chaco culture, in stages between A.D. 850 and 1150.

But cacao, a tropical fruit that grows in Central and South America, was cultivated in prehistoric times only in Mesoamerica, a region that stretches from Mexico to Costa Rica (see map).

The findings suggest the New Mexico complex also served as a trading hub for Mesoamericans and Puebloans between the 11th and 14th centuries—and that the two groups had a "much tighter connection" than previously thought, said study leader Dorothy Washburn.

Visiting Mesoamericans may have bartered cacao beans for gems unique to the Southwest, such as turquoise, which is known to have been mined by Puebloans in what's now New Mexico. (Take a chocolate quiz.)

"We've erected this wall between the Southwest and Mesoamerica for the whole of prehistory, [when] it was just one area—the Americas," said Washburn, of the University of Pennsylvania Museum of Archaeology and Anthropology in Philadelphia.

"This is just another way of seeing how these people interacted."

Ordinary People Also Drank Chocolate?

Archaeologists already knew that some trade existed between Mesoamericans and Southwest peoples. For instance, the remains of parrots and copper bells that originated in Mesoamerica have shown up in places such as Pueblo Bonito.

Scientists have also found remnants of turquoise in the ruins of the ancient Maya city of Chichén Itzá, located in present day Mexico, which has no turquoise deposits.

In 2009 archaeologists reported the discovery of the cacao chemical theobromine in vessel fragments near Pueblo Bonito—suggesting cacao had reached the Southwest.

This finding prompted Washburn and colleagues to test 75 vessels that had been used by both Chaco elites and farmers who lived near the complex.

Traditional methods of testing chemical residues require crushing or boiling vessel fragments. But study co-authors William Washburn and Petia Shipkova—both of Bristol-Myers Squibb in Princeton, New Jersey—were able to use a noninvasive technique that washed the insides of the vessels with water.

Shipkova tested this water with a highly sensitive instrument that can detect nanogram concentrations of theobromine.

The results showed that 50 of the 75 vessels had theobromine—including the commoners' vessels.

Finding the chemical in the farmers' vessels was "most unexpected," Dorothy Washburn said. "We were bowled over. ... That meant that a lot of people had access to [chocolate]."

John Henderson, a Mesoamerican expert at Cornell University who was not on the study team, was similarly struck by the idea of ordinary folks drinking chocolate, normally a beverage of the elite.

He suggested that the Chaco great houses may have put on huge feasts that would have allowed commoners to intermingle with elites.

Frothy Chocolate a Favorite Prehistoric Drink

Chocolate was almost always on the menu in Mesoamerica, where "any important social occasions included cacao," Henderson said.

When Spanish conquererors first encountered the 16th-century Aztec emperor Montezuma, they described a lavish banquet featuring more than 50 big jars of foam-topped liquid cacao, said Dorothy Washburn, whose study appeared in March in the Journal of Archaeological Science.

The Puebloans would have probably made this crude form of chocolate much like the Mesoamericans—by taking the dried beans and mixing them with water and other substances, she added.

To make the bitter mixture palatable, people would have added in chili peppers, honey, or vanilla beans. For the finishing touch—a layer of foam—a preparer may have used a stirrer similar to an eggbeater to whip up a froth or might have poured the liquid from jar to jar.

There's a physical foundation for this ancient cacao obsession: Theobromine stimulates the heart and relaxes the airways, helping people breathe easier, co-author William Washburn noted.

This "invigorating" effect, he said, is part of the reason Mesoamericans would give cacao to soldiers to pump them up prior to battle.

Chocolate Chemical Not From Native Plants

After their initial experiments, the study team consulted native plant databases and concluded the theobromine found in the Chaco vessels did not come from another type of local plant, such as Ilex vomitora, a species of holly.

Cornell's Henderson said the team went to "extraordinary efforts" to rule out another explanation for the theobromine's presence.

The scientists overall were "very careful" with their scientific techniques, "so there isn't a chance that their surprising result is the case of contamination," he added.

Likewise, the theory that Southwest peoples traded turquoise for cacao also "seems plausible," although there's no direct evidence of such an exchange, Henderson said.

Indeed, without more chemical evidence that offers hard science, "the story is not told," Dorothy Washburn said. "We do not have the penultimate understanding of the [culture at] Chaco.