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Thanks to a long history of domestication, cannabis now comes in an assortment of different varieties, each with its own unique configuration of cannabinoids and other compounds. According to new research in the journal Science Advances, this process of selective breeding may have begun during the early Neolithic period in what is now northwest China, where wild cannabis first appeared.
Previous genetic studies have suggested that cannabis originated on the Tibetan plateau, even though the earliest archaeological evidence for the use of hemp cord comes from China, and has been dated to approximately 12,000 years ago according to the authors of this study. As such, it is widely accepted that the plant probably has its roots somewhere in Asia, yet pinpointing its birthplace has proved to be something of a challenge.
This is largely down to the fact that the wild progenitor of cannabis is now extinct, and therefore can’t be studied directly. In its place are a number of specially cultivated strains of both hemp and drug-type cannabis, as well as some more ancient varieties called landraces. While these aren’t the same as the original cannabis plants, they are less domesticated than modern commercial strains, having been altered over the centuries by natural selection rather than intensive human-led breeding programs.
To retrace the plant’s history, the study authors analyzed the genomes of 110 different cannabis varieties from around the world, including 82 new genomes and 28 publicly available ones. Collectively, these encompassed the entire spectrum of modern strains and landraces as well as wild-growing feral varieties, which arise when seeds from domesticated varieties “escape” and are allowed to grow and breed naturally.
After assessing the genetic relationships between all of these plants, the researchers noted that every modern hemp and drug-type cannabis strain can be traced back to “an ancestral gene pool” that appears to have its roots in Chinese landraces and feral plants.
“Current Chinese landraces and feral plants represent the closest descendants of the ancestral gene pool from which hemp and marijuana landraces and cultivars have since derived,” write the study authors.
“Contrary to a widely accepted view, which associates Cannabis with a Central Asian center of crop domestication, our results are consistent with a single domestication origin of [cannabis] in East Asia,” they continue.
Furthermore, genomic dating revealed that the first domesticated ancestors of modern cannabis plants diverged from this wild progenitor some 12,000 years ago, “indicating that the species had already been domesticated by early Neolithic times.”
Since then, intensive breeding by humans has resulted in the emergence of several distinct genetic groups, with hemp and drug-type plants representing two of these. When comparing these two varieties, the study authors found 134 genes that are specific to hemp and 472 that are unique to psychoactive cannabis.
These genes control characteristics such as branching and flower formation, as well as potency. For instance, genes that code for an enzyme that synthesizes tetrahydrocannabinol (THC) – the main psychoactive component in cannabis – are fully expressed in most drug-type plants but lacking in hemp.
