However, an excavation of a 2,500-year-old tomb in western China has revealed the earliest clear evidence of humans using cannabis for its psychoactive properties.
Scientists from China and Germany analyzed wooden fragments and burnt stones from pots in the tombs, and the results showed an exact match to the chemical signature of cannabis -- particularly that with a high amount of tetrahydrocannabinol (THC), the most potent psychoactive agent in the plant.
The authors of the study, which was published Wednesday in journal Science Advances
, suggested that cannabis was probably used during burial ceremonies, perhaps as a way to communicate with the divine or the dead.
However, it was unlikely that cannabis was smoked in the same way it is today. More likely, it would have been burnt like incense in an enclosed space to release vapors.
Nicole Boivin, director of the Max Planck Institute for the Science of Human History and an author of the report, said the cannabis was burned on hot stones inside wooden braziers, containers for hot coals.
"This is the only way that cannabis could have been smoked prior to the arrival of pipe technology, which is not until much later, it seems," she said.
The 10 braziers were found in the Jirzankal cemetery in the Pamir Mountains near the Chinese border with Pakistan.
It had long been a mystery when and where varieties of cannabis plants with higher levels of psychoactive compounds were recognized and used by humans, according to the study.
Cannabis plants were cultivated in East Asia for their oily seeds and fiber from at least 4000 BC. However, the early varieties of cannabis, as well as most wild populations, had low levels of THC and other compounds with psychoactive properties, the authors said.
Many historians had placed the origins of cannabis smoking on the ancient Central Asian steppes, but these arguments relied solely on a passage from a single text from the late first millennium BC, written by the Greek historian Herodotus.
The authors said this study offers the first concrete evidence for cannabis smoking in this area, although it is still unclear whether the people buried at Jirzankal cemetery actively cultivated cannabis or simply sought out higher THC-producing plants.
"Finding evidence for ancient drug use is a bit like finding a needle in a haystack, because this kind of evidence is rare due to there being few opportunities for long-term preservation of the remains of activities involving drug use, which is very ephemeral, and doesn't necessarily leave a lot in the way of physical evidence," Boivun said.
The authors said that cannabis plants produce greater quantities of active compounds when they grow at higher elevations, and this may be why more potent plants had been discovered -- and a new use initiated -- by people living in high mountainous regions like the Pamirs.
Although the region is remote today, it may once have sat on the Silk Road, a key trade route.
"Our study implies that knowledge of cannabis smoking and specific high-chemical-producing varieties of the cannabis plant were among the cultural traditions that spread along these exchange routes," Robert Spengler, the lead archaeobotanist for the study who is also at the Max Planck Institute for the Science of Human History, said in a statement.
"Modern perspectives on cannabis vary tremendously cross-culturally, but it is clear that the plant has a long history of human use, medicinally, ritually, and recreationally, over countless millennia," Spengler added.