The broken wooden braziers, unearthed from 2,500-year-old tombs in Western China, contained burned, blackened stones, and the interior of the wooden vessels also looked charred. To find out what had been burned in them, University of Chinese Academy of Sciences archaeologist Yemin Yang and his colleagues used gas chromatography/mass spectrometry to analyze small samples of the charred wood and the residue from the stones.

Their analysis turned up a chemical called cannabinol, or CBN—an unmistakable chemical signature of cannabis. Those ancient chemical traces offer an important clue in the history of human drug use and the domestic history of cannabis.

In around 500 BCE, the ancient Greek historian Herodotus described people near the Caspian Sea gathering in small, enclosed tents to breathe in the smoke from cannabis burned atop a bowlful of red-hot stones. Yang says people did something similar at Jirzankou, probably as part of funeral ceremonies. Archaeologists there also found the remains of a musical instrument called an angular harp, which played an important role in later funeral rites in Western China.

“We can start to piece together an image of funerary rites that included flames, rhythmic music, and hallucinogen smoke, all intended to guide people into an altered state of mind,” wrote Yang and his colleagues.

Where there’s smoke…

Archaeologists have spent years debating when people first domesticated Cannabis sativa as a drug. The plant was first domesticated in eastern Asia around 3,500 BCE, but it was used for its oily seeds and its long, durable fibers. Like modern hemp crops, the earliest domesticated varieties didn’t produce much of the psychoactive compound called THC. Cannabis is a surprisingly versatile plant—so versatile that Yang and his colleagues say ancient people domesticated it at least twice, for very different reasons.

Although cannabis has turned up at other sites, from Western China to the Altai Mountains in Siberia, archaeologists have never found such direct indications that ancient people were lighting it up. Elsewhere, cannabis plants buried with the dead may be a sign that people ate parts of the plant for a similar effect (although brownies wouldn’t be invented for millennia). But without doing a similar chemical analysis on human remains from those graves, archaeologists can’t say for sure. At other sites, like a burial in the Altai Mountains of Siberia where archaeologists found a small tent, a bowl, and a pouch of cannabis seeds, it’s pretty reasonable to speculate that the cannabis involved may have been intended for use as a drug.

“It's hard to judge how ancient people consumed them. Thus, I try to chemically analyze artifacts and human tissues to provide more reliable evidence,” said Yang. Evidence just doesn’t get any clearer than CBN biomarkers in a charred burner.

Xinhua Wu

Xinhua Wu

Yimin Yang

Xinhua Wu

…there are biomarkers

The burned residue in the Jirzankou braziers provides the first direct evidence of people burning cannabis for its smoke, but it’s also the first unambiguous indication of people using the plant specifically for its mind-altering effects. Yang and his colleagues’ chemical analysis found that the cannabis plants burned at the cemetery had been very high in THC, which makes them different from domesticated hemp plants and from most of the wild cannabis that grows on hillsides from the Caucasus to Western China.

Plants that produce more THC tend to produce less CBD, and vice versa. And THC, the psychoactive compound in marijuana, starts breaking down into CBN when it’s exposed to air, heat, or light. The presence of so much CBN (and no CBD) in the charred residue from the Jirzankou braziers suggests that the cannabis used in burial rites was higher in THC than most wild plants. So by 2,500 years ago, people in Western China either knew where to find the most psychoactive wild cannabis or they’d actually started breeding it to suit.

Studies on modern wild cannabis have shown that plants produce more THC in response to low temperatures, exposure to ultraviolet light, and other conditions found at high altitudes. “Humans are always going to be looking for wild plants that can have effects on the human body, especially psychoactive effects, so if there were wild varieties with high THC levels, they would have been readily targeted,” said co-author Robert Spengler, laboratory director at the Max Planck Institute for the Science of Human History.

At the moment, Yang and his colleagues don’t have enough evidence to say whether the plants here were domesticated or gathered, but they can say that by 2,500 years ago, people were smoking cannabis as a ritual drug in Western China. And that offers a clue about how it may have spread to the rest of the world. The Pamir Mountains of Western China sit along the ancient network of trade routes known as the Silk Road, connecting Eastern Asia to Europe and the Middle East. At Jirzankou, the mixture of artifacts buried with the dead—silk from East Asia and glass beads from Southern or Western Asia, for instance—suggests a cultural crossroads.

"Plants were one of the major commodities to move along these trans-Eurasian exchange routes, and in so doing largely reshaped the foods in all of our kitchens today," said Spengler. "I think with this new study, we can now actually place cannabis within that list as well, as being one of these crops that originates on these ancient trade routes."

Science Advances, 2019. DOI: 10.1126/sciadv.aaw1391 (About DOIs).