By sequencing genetic samples of the plant, they found that the species had most likely been domesticated by the early Neolithic period. They said their conclusion was supported by pottery and other archaeological evidence from the same period that was discovered in present-day China, Japan and Taiwan.
But Professor Purugganan said he was skeptical about conclusions that the plant was developed for drug or fiber use 12,000 years ago since archaeological evidence show the consistent use or presence of cannabis for those purposes began about 7,500 years ago.
“I would like to see a much larger study with a larger sampling,” he said.
Luca Fumagalli, an author of the study and a biologist in Switzerland who specializes in conservation genetics, said the theory of a Central Asian origin was largely based on observational data of wild samples in that region.
“It’s easy to find feral samples, but these are not wild types,” Dr. Fumagalli said. “These are plants that escaped captivity and readapted to the wild environment.”
“By the way, that’s the reason you call it weed, because it grows anywhere,” he added.
The study was led by Ren Guangpeng, a botanist at Lanzhou University in the western Chinese province of Gansu. Dr. Ren said in an interview that the original site of cannabis domestication was most likely northwestern China, and that the finding could help with current efforts in the country to breed new types of hemp.
To conduct the study, Dr. Ren and his colleagues collected 82 samples, either seeds or leaves, from around the world. The samples included strains that had been selected for fiber production, and others from Europe and North America that were bred to produce high amounts of tetrahydrocannabinol (THC), the plant’s most mood-altering compound.
Dr. Fumagalli and his colleagues then extracted genomic DNA from the samples and sequenced them in a lab in Switzerland. They also downloaded and reanalyzed sequencing data from 28 other samples. The results showed that the wild varieties they analyzed were in fact “historical escapes from domesticated forms,” and that existing strains in China — cultivated and wild — were their closest descendants of the ancestral gene pool.