An international team of researchers have found evidence that people smoked marijuana, also known as cannabis, for its psychoactive properties as part of rituals or religious activities in western China at least 2,500 years ago.

Cannabis is one of the oldest cultivated plants in East Asia. It has been planted as an oil-seed and fiber crop. But the levels of psychoactive compounds in these cannabis plants are low.

It has been a mystery for researchers of when, where and how the plant was selected and cultivated for higher THC, the psychoactive compound used to "get high."

The new study was conducted by researchers from the Chinese Academy of Sciences, the Chinese Academy of Social Sciences and the Max Planck Institute for the Science of Human History in Germany.

From 2013 to 2014, archaeologists from the Chinese Academy of Social Sciences excavated the Jirzankal Cemetery in the eastern Pamir Plateau in northwest China.

The tomb, dating to approximately 2,500 years ago, has apparent religious characteristics and patterns of rituals in its construction. Black and white stone strips run across the site's surface. Circular mounds of earth cover the tombs with one or two rings of stones underneath.

Several wooden burners or braziers, containing stones with obvious burning traces drew attention from the researchers.

They analyzed the chemical residue on the braziers with a technique known as gas chromatography mass spectrometry (GC-MS) and found biomarkers of cannabis in samples taken from the inside of the braziers.

They reported in the journal Science Advances that they found cannabinol (CBN), an indicator that THC is present. It indicates that the plants burned in the braziers were cannabis plants with a higher level of psychoactive compounds.

The researchers said the findings are the earliest directly dated and scientifically verified evidence for ritual cannabis smoking. It is likely that people selected cannabis plants with high THC levels and smoked them as part of a ritual or religious activity during burials, perhaps to communicate with the deceased or the divine.

They also noted that high-elevation mountain passes of Central and East Asia, including the Pamir region, hosted trade routes of the early Silk Road, linking China with West Asia and Europe,

"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," said Robert Spengler, an archaeobotanist for the study from the Max Planck Institute for the Science of Human History, in a press release.

"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," he said.

Spengler noted it is clear that cannabis has a long history of human use, medicinally, ritually, and recreationally over millennia, although modern perspectives on the plant vary tremendously cross-culturally.

Nicole Boivin, director at the Max Planck Institute for the Science of Human History, said given the modern political climate surrounding the use of cannabis, archaeological studies like this can help people to understand the origins of contemporary cultural practice and belief structures, which, in turn, can inform policy. (Xinhua)