People have been smoking pot to get high for at least 2,500 years. Chinese archaeologists found signs of that when they studied the char on a set of wooden bowls from an ancient cemetery in western China.
The findings are some of the earliest evidence of cannabis used as a drug.
"We've known that cannabis is one of the oldest cultivated plants in East Asia, primarily for making oil and hemp," says Mark Merlin, a botany professor at the University of Hawaii at Manoa who reviewed the study published in Science Advances. "Now we know the ancients also valued the plant for its psychoactive properties."
The set of 10 wooden braziers come from eight tombs at the ancient Jirzankal Cemetery on the Pamir Plateau in what is now China's Xinjiang region. Researchers found chemical evidence of cannabis in residue on nine of the braziers, which held small stones that were apparently heated and used to burn the plants.
Yimin Yang, an archaeologist at the University of Chinese Academy of Sciences and a co-author of the study, says the location of those artifacts suggests cannabis smoke was "being used during funeral rituals, possibly to communicate with nature or spirits or deceased people, accompanied by music."
The findings are consistent with what's known about cannabis use across Central Eurasia, Yang says, "possibly reflecting some kind of community of shared beliefs in the Eurasian mountain foothills." There is evidence of ancient cannabis use at funerals from frozen tombs in Russia's Altai Mountains, and fifth century B.C. Greek historian Herodotus wrote of the Scythians using cannabis smoke as a post-burial cleansing rite in The Histories.
The Pamir Plateau was a key trade area in antiquity that was "the locus of very long-distance exchanges for a very long time," says Jade d'Alpoim Guedes, an archaeobotanist with the Scripps Institution of Oceanography who focuses on China but was not involved in this study. "Horses were brought to China around the 2nd millennium B.C., and so were wheat and barley; foxtail, broomcorn, millet moved west."
The research team's findings, she says, "shows cannabis may have been one of the things moving along the routes early on."
The researchers aren't sure where the ancient stoners sourced their cannabis. Wild plants tend to have low levels of THC, the main psychoactive compound in cannabis. This raises questions about "whether there was a wild population with higher THC levels that humans somehow discovered and specifically targeted, or whether some processes lead to the increased production of these chemicals in the plants," says study co-author Robert Spengler of the Max Planck Institute for the Science of Human History.
He suggests the cemetery's high-mountain environment, with more solar radiation and colder temperatures, could have influenced certain cannabis plants to produce more THC.
Food- and fiber-producing strains of cannabis are among the oldest cultivated plants in East Asia, but the strains that cause drug effects are different. The researchers say their findings show that use of cannabis as a drug likely originated somewhere else, on the high plains of Central Asia.
Pien Huang is NPR's Reflect America Fellow, helping to bring more diverse voices to air and online.