We owe our perennial favorite sweet indulgence, chocolate, to Central American Aztec and Mayan people living nearly 4,000 years ago. At least that's what archaeologists have long assumed. But an international team of researchers has uncovered evidence that the plant from which chocolate is made was first cultivated in South America 1,500 years earlier than that.

Past and present are inextricably linked when it comes to the origins of food. "Today we all rely, on one extent or another, on foods that were created by the indigenous peoples of the Americas," says Michael Blake of the University of British Columbia, co-author of a new study published today in Nature Ecology & Evolution. "And one of the world's favorites is chocolate."

The chocolate we know and love comes from cacao beans, which grow inside large pods on cacao trees. Once the beans are harvested, they are left to ferment for several days and then dried. The dried beans are roasted and ground out. The liquid that comes out of this process becomes the cocoa butter used to make your favorite chocolate bar; any residue is turned into cocoa powder.

Archaeologists typically date the first use of cacao to Central America around 3,900 years ago. This assumption was buoyed by various historical accounts, indicating that in pre-Columbian Mesoamerica, natives dried and ground up cacao seeds to make gruels and beverages. Cacao seeds were a form a currency and regularly traded, so large plantations may have sprung up to cultivate them. Cacao also played a central role in the region's mythology, and various Mayan texts and Aztec drawings prominently feature cacao. Even the word "chocolate" derives from the Nahuatl language of the Aztecs (xocolatl is a combination of xocolli, meaning "bitter," and atl, meaning "water").

In terms of genetics, however, the greatest diversity among the cultivated genus (Theobroma cacao) is found in the humid upper Amazonian regions. For Blake and his collaborators, this suggested that perhaps South America, rather than Central America, was the origin of chocolate.

It is known, for instance, that South American people used cacao trees (especially the bark and leaves) as medicine or as a food source, consuming the fresh pulp of the seeds as either a juice or a fermented alcoholic beverage. Archaeologists have found pictorial designs on ancient ceramic vessels from Ecuador and northern Peru that strongly resemble cacao pods.

All this is circumstantial evidence; more direct proof has been lacking until now. Blake and his colleagues examined numerous ceramic artifacts from a Mayo-Chinchipe site in Ecuador known as Santa Ana-La Florida, first discovered by archaeologists in 2002. Radiocarbon dating places the initial occupation of the village between 3,305 to 5,450 years ago. It seems to have functioned as a ceremonial center, with 20 buildings arranged around a central sunken plaza and a series of raised tombs at the east end of the village.

First the team looked for telltale starch grains (complex carbohydrates), found in most plant tissues, including cacao. They found evidence of such grains in the charred cooking residues inside some of the ceramic shards and on the surface of some stone artifacts (bowls, mortars, and pestles)—a clue that cacao was used in the village.

"The use of cacao was something that caught on and very likely spread northwards by farmers growing cacao in what is now Colombia."

Next, they conducted a genetic analysis. There are ten recognized genetic groups of Theobroma cacao, three of which are the oldest domesticated varieties. Blake et al. were able to identify fragments of ancient DNA in the residue, unique to this type of cacao tree.

Finally, there are a number of chemical compounds in cocoa, most notably the flavanols, caffeine, and a bitter alkaloid called theobromine. It's the latter that is relevant to the current study, since it is found in cultivated but not wild varieties of cacao. The team found the telltale chemical fingerprint of theobromine in the residue in several ceramic vessels.

"These three methods combine to definitively identify a plant that is otherwise notoriously difficult to trace in the archaeological record, because seeds and other parts quickly degrade in moist and warm tropical environments," says lead author Sonia Zarrillo of the University of Calgary.

Based on those findings, the team concluded that both cultivated and wild cacao were in common use by the Mayo-Chinchipe villages between 2,100 and 5,300 years ago—at least 1,500 years earlier than previously assumed—both for domestic and ritualistic purposes, such as mortuary offerings. And it implies that the Upper Amazon region is the oldest center for cultivating cacao known to date.

"This suggests that the use of cacao, probably as a drink, was something that caught on and very likely spread northwards by farmers growing cacao in what is now Colombia," says Blake. From there, it likely spread via trade to Panama and other parts of Central America and southern Mexico.

DOI: Nature Ecology & Evolution, 2018. 10.1038/s41559-018-0697-x (About DOIs).