Cannabis was one of the first plants domesticated by humans, and has a long history of being used in textiles, food, and oilseed as hemp, the study said. But this history has been difficult to research, due to modern
legal restrictions around the plant's use and accessibility.
To trace the origins of cannabis, a team led by researchers in Switzerland and China compiled 110 whole genomes, covering a spectrum of different wild-growing feral plants, domesticated varieties and modern hybrids of hemp and psychoactive forms of the plant, commonly known as marijuana, with higher levels of the compound Tetrahydrocannabinol (THC).
Their genomic analysis separated the samples into four major groups, including basal cannabis, which is found in China and the United States. The analysis suggested that early domesticated ancestors of hemp and marijuana -- two of the other major groups -- diverged from basal cannabis about 12,000 years ago -- indicating the species may already have been domesticated by then.
This coincides with the dating of ancient artifacts including pottery from southern China, Taiwan, and Japan from the same period, the study added.
"Our genome-wide analyses corroborate the existing archaeobotanical, archaeological, and historical record, and provide a detailed picture of the domestication of Cannabis and its consequences on the genetic makeup of the species," said the study.
Modern Chinese landraces -- cultivated plants that have been altered through agriculture -- and feral cannabis plants are now the closest descendents of the ancestors of hemp and marijuana, the study said. Meanwhile, the pure wild ancestors of Cannabis sativa have likely gone extinct.
After its domestication in the Neolithic period, the plant slowly spread across Europe and the Middle East in the Bronze Age, archaeological evidence shows. The first record of Cannabis sativa appeared in India about 3,000 years ago, when the species was likely introduced from China along with other crops, said the study.
Cannabis then traveled to Africa in the 13th century, spread to Latin America in the 16th century, and arrived in North America at the beginning of the 20th century.
The study sheds some light on the longstanding mystery of where and when varieties of cannabis with higher levels of psychoactive compounds were recognized and used by humans.
Although they have been cultivated for centuries, early varieties and wild populations had low levels of THC and other compounds with psychoactive properties.
Many historians had placed the origins of cannabis smoking on the ancient Central Asian steppes, but these arguments relied solely on a passage from a single text from the late first millennium BC, written by the Greek historian Herodotus.
In 2019, the excavation of a 2,500-year-old tomb in western China revealed the earliest clear evidence of humans using cannabis for its psychoactive properties. Wooden fragments and burnt stones from pots in the tomb showed the chemical signature of cannabis, specifically that with a high amount of THC -- suggesting the plant may have been used during burial ceremonies.
However, it was unlikely that cannabis was smoked in the same way it is today. More likely, it would have been burnt like incense in an enclosed space to release vapors.
