A Pamir brazierXinhua Wu

Marijuana can linger in the human system for a few months at most, but cannabis residue will stick to other surfaces for millennia. High up in the Pamir Mountains, in what is now western China, archaeologists were excavating the tombs of Jirzankal Cemetery when they came upon a set of braziers and asked themselves what purpose the tools served. After analyzing the residue, a team of researchers found that it not only came from cannabis, but contained unusually high levels of THC—the compound that gives cannabis its psychoactive, or mind-altering, qualities.

Indeed, these braziers, or wooden incense burners, mark some of the earliest, most robust physical evidence of humans burning cannabis specifically for its psychoactive effects. Researchers from China and Germany described their findings in a study published today in the journal Science Advances.

One of the most compelling pieces of evidence pointing to the use of psychoactive cannabis in ancient Central Asia is purely textual: a section in Herodotus’s Histories about the Scythians, who, after funerals, would “throw the seed … upon the red-hot stones” and “shout for joy” as the vapor rose. But archaeological evidence has been harder to come by. While excavating temples in Turkmenistan, the famed archaeologist Viktor Sarianidi reignited interest in the region’s ancient drug culture when he claimed to have found ritual plant remains inside ceramic vessels, which could have been used for drinking the hallucinogenic soma. Similar ceramics, however, have since been identified elsewhere in the region as cheese strainers, and later tests found that the ceramics did not actually hold remains of the ingredients originally reported. In fact, they did not contain plant remains at all—only plant impressions. And while a major 2006 study confirmed the presence of cannabis seeds in another ancient Chinese tomb, there was no evidence of burning or smoking the plant. The debate surrounding ancient drug use in Central Asia has been “extremely … I guess lively is the best way to put it,” Robert Spengler, a co-author of the new study and an archaeobotanist at Germany’s Max Planck Institute for the Science of Human History, told reporters.

The team identified the chemical traces clinging to the burners using a technique that articulates a sample’s chemical signature. By vaporizing the sample, separating its components, and recording their differences in mass, researchers can identify the relative levels of the chemicals they’re looking at. “To our excitement, we identified the biomarkers of cannabis,” says Yimin Yang, another co-author of the study and a researcher at the University of the Chinese Academy of Sciences. And not just cannabis, but a strain bursting with CBN, the compound that forms after THC metabolizes. (These Jirzankal Cemetery samples contained, however, noticeably low levels of CBD—a medicinal, nonpsychoactive compound favored by some cannabis users.) Higher than what are typically found in regional, wild cannabis plants, the CBN levels suggest that the ancient grave keepers deliberately sought out these mind-altering varieties, and potentially even domesticated them.

Those elevated levels are what make this discovery so exciting and unique next to other confirmed examples of ancient cannabis, according to Mark Merlin, a professor of botany at the University of Hawaii at Mānoa. With those findings, says Merlin, a co-author of Cannabis: Evolution and Ethnobotany, determining the cannabis’s function was harder, as the samples weren’t so clearly tilted toward the psychoactive end of the spectrum. These results, meanwhile, are rather more persuasive, and suggest that ancient humans were lighting up to honor the dead long before that became a stoner’s cliché.

The excavations dug up more than just THC residue. Notably, analysis of human bones found at Jirzankal revealed that not all of the cemetery’s tenants had been born locally. This hint of ancient immigration supports the idea that the high-elevation Pamirs were once part of the Silk Road, along which goods and traditions passed between geographically distant communities. Spengler told reporters that these findings suggest that cannabis, and ideas regarding its various uses, may well have been among the items exchanged along the Silk Road. Another discovery supporting this idea—a grave in northwestern China laid with large cannabis plants—was described in 2016 by the archaeologist Hongen Jiang, another co-author of the new paper.

This new investigation of ancient cannabis use, says Patrick E. McGovern, the scientific director of the Biomolecular Archaeology Project at the Penn Museum, “is a much-needed contribution to our scientific understanding of the vast expanse of central Asia—some 4,000 miles from the Caucasus Mountains through the Pamirs and across the forbidding Taklamakan Desert. It provides yet another piece in the archaeological puzzle of the ‘abiding mystery of Central Asia’ and its impact on human cultural and biological development through the millennia.” But much more remains to be learned about the ways cannabis might have been used (perhaps as a medical additive or in fermented beverages) and the ways ideas about fermentation and the related domestication of plants moved across this area, without leaving their trace on artifacts. “We are still very much in the dark about the underlying dynamics of the transfer of fermented beverages and their mind-altering additives”—including cannabis—“from oasis to oasis along the prehistoric Silk Road and back into the Central Asian hinterland,” McGovern wrote in his book Uncorking the Past.

As tempting as it is chuckle at the thought of ancient drug use, Merlin says, viewing it as recreational is too simple, no matter how high the THC levels may be in these samples. What many see as the “recreational” nature of psychoactive drugs could have been a spiritual practice: a vessel for ushering the deceased safely into the afterlife, or for altering the mind in order to facilitate a closer conversation with the gods—much like a priest aims to do. Another indication of cannabis’s spiritual connotations is the centuries-old Chinese practice of wearing hemp—which is not psychoactive—while in mourning. (Merlin also posits that cannabis may have been used simply to deodorize corpses, though that’s likely not all it was doing at Jirzankal, given the THC levels.)

Further research could help give a clearer view of the plant’s ritual significance, but the traces of cannabis left on the braziers from Jirzankal Cemetery are enough to begin “to piece together an image” of cannabis-inflected funerary rites, the new paper’s authors write. The rituals, they think, might have included “flames, rhythmic music, and hallucinogen smoke, all intended to guide people into an altered state of mind.” Call it what you will, but that’s certainly something more than recreational.

We want to hear what you think about this article. Submit a letter to the editor or write to letters@theatlantic.com.