Humans smoked cannabis as part of ritualised burial ceremonies 2,500 years ago, according to the latest evidence unearthed at a burial site on the Pamir Plateau in western China.
But before you imagine ancient stoners passing around a primitive bong, the reality was probably quite different, archaeologists report in the journal Science Advances today.
The international team, led by Meng Reng of the Chinese Academy of Sciences, excavated 10 wooden pots, or braziers, containing burnt stones from tombs at the Jirzankal Cemetery.
When the team analysed the pots and burnt stones they found traces of cannabis.
What's more, the cannabis contained higher levels of tetrahydrocannabinol, or THC — the main psychoactive compound in cannabis — than generally found in wild cannabis plants.
The fact that the traces of THC was found in the pots and on the stones suggest that cannabis was used for ritual purposes, said Nicole Boivin of the Max Planck Institute for the Science of Human History.
Smoking cannabis in ancient times was very different to today.
There's no clear evidence of smoking pipes existing in Central Asia before the modern era, instead people would either eat the plant or inhale the smoke or vapours when it was burnt.
"We clearly see that cannabis is being burned on hot rocks, which must have produced smoke and psychoactive effects on those present in an enclosed space," Professor Boivin said.
The high THC levels and burning of cannabis in a ritual context fit with the description of the use of a drug thought to be Cannabis sativa in a text written by the fifth century BC Greek historian Herodotus, she added.
The discovery paints a vibrant picture of what these past burial rituals might have looked like, said Alison Crowther, an archaeobotanist from the University of Queensland, who was not involved in the study.
"Rites such as these would have had really important social functions for ancient communities just like funerals do today."
Further work at the site has shown that some of the people buried there were not local to the area.
"So we can start to then question or wonder about how these rituals played out in bringing communities together and helping bind people in this very dynamic social and cultural environment," said Dr Crowther.
Preliminary analysis of the human bones found at the cemetery have identified skull perforations and fatal cuts and breaks in several bones, which the researchers interpreted as signs of human sacrifice, although they caution more research is needed to verify this claim.
Cannabis has long been cultivated as a crop, but its early history remains unclear.
Previous discoveries of cannabis have been based on identification of parts of the plant, which doesn't tell us how it has been used, or historical accounts, which can be difficult to interpret, the researchers said.
"The debate over early cannabis use in Central Asia has been lively and filled with speculation," said Robert Spengler, a co-author on the paper who is also at the Max Planck Institute.
"There are a few key archaeological accounts that have been discredited outright, and some speculations about references to plants in historical accounts that are not exactly clear," he said.
Understanding when people started to use cannabis is challenging because the preservation of cannabis in an archaeological site generally requires some special circumstances.
And even when it was preserved it's still difficult to work out what the tiny fragmentary plant remains were used for.
"Cannabis has many uses, it's not just as a recreational drug which is what most people today would associate it with. It has uses as a medicine, as food, as fibre," Dr Crowther said.
"What's exciting about this particular study is that they've got that direct evidence of how the cannabis was used, because they've got the residues on the artefact."
The researchers can't say for certain whether the cannabis being burnt was a cultivated variety, or was being selected from wild populations that had naturally higher levels of THC.
"However, increasingly robust evidence for [cannabis] use in different parts of Central Asia by this time may well suggest the cultivation and spread of higher THC varieties," Professor Boivin said.
"By this time period, trade routes across Eurasia were extensive and wide-ranging, and many species of plants and animals were being moved around to create more diverse repertoires of crops, herd animals, foods, delicacies, spices and medicines throughout the Old World from Europe to China."