Weekend Reads | Uncovering the Story of Cacao Through DNA Evidence

by Kevin Schofield

This weekend's read is all about how the cacao plant, from which we make chocolate, was domesticated thousands of years ago in Central and South America. A large group of researchers from around the world collaborated on a unique project to gather DNA evidence to understand where cacao plants originated, where they spread, and how they changed over time.

Cacao originates from the Amazon river basin in South America, an area that includes parts of Ecuador, Colombia, and Peru. It's a "hotspot of biodiversity" on our planet, and cacao bears that out: Scientists have identified 11 genetically distinct variants of the cacao plant from different locations in the region. But that's just the beginning of the story, because agricultural practices were well-established in the communities native to the area 5,000 years ago (known as the mid-Holocene period). They knew how to select, cross-breed, and cultivate plants to best serve their needs. Anthropologists also know there was some amount of trade between communities, opening up the possibility that cacao variants from different regions could be shared and further cultivated.

The challenge, however, is in getting samples of the cacao plants that were being grown by farmers five millennia ago. But the researchers found a clever source: residue left on the ceramic bowls, pots, and other food containers that sit in museums' artifact collections from that area and time period. They ultimately collected 378 residue samples from objects covering 19 ancient communities spread across the Pacific coast of South and Central America and what are now Colombia, Ecuador and Peru.

And what the researchers discovered was fascinating. First, there was a huge amount of cross-breeding of the 11 "root" cacao variations, and within that, a large amount of variation in cross-breeds from one community to the next. Second, those 11 original variants see an enormous geographic spread, suggesting trade was far more common and widespread than previously known. Anthropologists have previously inferred some trade routes and trading relationships by analyzing similarities between styles of pots, dishes, and other artwork, particularly along tributaries of the sprawling Amazon River. They had also assumed there was seafaring trade between the communities living along the Pacific coast. But knowing where a particular cacao variant originated and then seeing where it ended up provides strong evidence that there was extensive trade activity — and not necessarily only with neighbors. For example, cacao genetic variants that were native to Peru, on the southern end of the Amazonia range, made it all the way north into Mexico, in some cases skipping the areas in between (a few variants show little to no presence in Colombia and Ecuador).

That said, the genetic samples collected from the area that is now Ecuador, in the center of the cacao region, not surprisingly have the greatest number of diversity of cacao variants and cross-breeds. That suggests Ecuador might have been a hub for trade across the region 3,000 to 5,000 years ago.

This study teaches us important things about ancient civilizations, most notably the strength of their agricultural practices and the extent to which trade was widespread and active across all of these communities. For "early" and "ancient" civilizations, they were remarkably advanced in some ways.

But this study also points out that DNA is a critical tool for anthropologists. We already knew that, a bit: Scientists already use human DNA to reconstruct the migration patterns of people over tens of thousands of years. But studying the DNA of other species that were consumed or otherwise used by humans can tell us so much more that has otherwise been lost to time, including (as we see in this case) their economic and societal practices, how far they traveled, and which other societies they were interacting and trading with.

It's also interesting and notable that this study relied so heavily on cultural artifacts in museum collections as a source for DNA samples. It's easy to think of them as dead objects that we can look at and appreciate for their artistic, cultural, and historical relevance. But it turns out that some of them still have secrets to offer up — if we know how to look for them.

A revisited history of cacao domestication in pre-Columbian times revealed by archaeogenomic approaches

Featured image via Narong Khueankaew/Shutterstock.com.