Research published in the journal Science Advances shows that mankind's interest in cannabis and by extension THC -- tetrahydrocannabinol, the primary psychoactive constituent of cannabis--dates back thousands of years.
    The study focuses on the Pamirs mountain range. Most of the Pamirs are within Tajikistan, but fringes penetrate Afghanistan, China and Kyrgyzstan. In this case, inhabitants of the Pamirs in China selected plants with higher levels of THC and burned them as part of mortuary rituals, according to researchers from the Max Planck Institute for the Science of Human History, the Chinese Academy of Sciences and the Chinese Academy of Social Science.
     The psychoactive compounds had been preserved in 2,500-year-old funerary incense burners. "This is the earliest clear evidence to date of cannabis being used for its psychoactive properties," the Planck institute reports.
     The story of humans' interest in the plant goes back further than thought. While cannabis plants were grown in East Asia from at least 4000 B.C., early varieties had low levels of THC and other compounds with psychoactive properties. Researchers in this study were hoping to understand the function of ancient wooden burners discovered by archaeologists from the Chinese Academy of Social Sciences, the institute recounts.  
      When they isolated and identified compounds preserved in the burners, they found "the chemical signature of the isolated compounds was an exact match to the chemical signature of cannabis."
     Evidence suggests that burning cannabis was part of a ritual commemorating the dead.

     The research is: "The origins of cannabis smoking: Chemical residue evidence from the first millennium B.C. in the Pamirs," by Meng Ren et al., published in Science Advances, June 12, 2019.

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