The earliest clear evidence of people smoking cannabis for its psychoactive properties has been discovered in what is now Western China. They were using the drug to get high at least 2,500 years ago, according to a new study in the journal Science Advances. While cannabis has been cultivated in East Asia for thousands of years as a fiber and oil-seed crop, the authors describe the discovery of 10 incense burners in ancient tombs that contain the earliest known evidence of smoking cannabis for consciousness alteration anywhere in the world.

The wooden braziers were found in the Jirzankal Cemetery, 3,000 meters (9,850 feet) up in the Pamir mountains, which is believed to date back to the middle of the first millennium BCE. Researchers used a technique called gas chromatography-mass spectrometry to analyze burnt organic material retrieved from the pots, and found that it matched the chemical signature of cannabis.

While the cannabis plant is known to have originated in the region, most ancient strains contained very low amounts of the psychoactive compound tetrahydrocannabinol (THC), and were therefore used mainly for textiles or food. However, the material obtained from the Jirzankal tombs contained a much higher concentration of THC than is typically found in wild cannabis plants, suggesting it had been selected for its psychoactive effects.

This discovery raises interesting questions about the timeline of cannabis cultivation, indicating that people in this part of China may have been actively breeding high-THC strains for smoking as early as 2,500 years ago.

However, it is also possible that high levels of ultra-violet light and other environmental factors associated with high altitudes caused the wild cannabis plants in the Pamir mountains to naturally produce increased THC.

Regardless of how these strong weed strains came to exist, study author Nicole Boivin claims that “[these] 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.”

Radiocarbon analysis has put the burials at around 500 BCE. As well as the wooden braziers, items such as harps and wooden plates and bowls were found alongside the 34 bodies, of which nearly a third were migrants, suggesting that they were probably members of the Sogdian culture. Occupying territories in China and Tajikstan, the Sogdians were long-distance traders who subscribed to the Persian faith of Zoroastrianism. Interestingly, the consciousness-altering power of cannabis was later revered in some Zoroastrian sacred texts.

According to the researchers, smoking was likely to have occurred during burial rituals, possibly as a means of accessing other realms in order to communicate with the dead or the divine. Over time, these psychoactive cannabis strains made their way to other parts of Asia and Europe via trading links such as the legendary Silk Route.