Humans have been getting high on cannabis for a very long time.
Now scientists have found what they believe to be the earliest evidence of people using the plant to get stoned.
They found high levels of tetrahydrocannabinol (THC) in an incense burner inside tombs in the Pamir Mountains, China, dating back 2,500 years.
There is evidence showing the cultivation of cannabis plants from at least 6,000 years ago – but there is little archeological material proving they were used as a drug.
The study – carried out by researchers from the University of Chinese Academy of Sciences, Chinese Academy of Social Sciences and the Max Planck Institute for the Science of Human History – suggests that cannabis was used as part of a funeral ceremony.
"The findings support the idea that cannabis plants were first used for their psychoactive compounds in the mountainous regions of eastern Central Asia, thereafter spreading to other regions of the world," said Nicole Boivin, director of the Department of Archaeology at the Max Planck Institute for the Science of Human History.
The researchers also explained that the evidence shows the ancient inhabitants of the Pamirs were choosing specific varieties of cannabis with high levels of THC.
For Robert Spengler, also at the Max Planck Institute, the study asserts the cultural and historical importance of the region.
"The exchange routes of the early Silk Road functioned more like the spokes of a wagon wheel than a long-distance road, placing Central Asia at the heart of the ancient world,” he said.
“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.”