The mysteries of Cannabis domestication

Cannabis shares a mysterious history with humans stretching back more than a dozen millennia. That's why its domestication is a fascinating topic for many. This weedy plant played a fundamental role in the development of human civilization, providing food, fibers and entheogenic substances, embedding itself deep into our ancestral cultures.

However, because humans have spread and modified the plant for millennia, it is difficult to determine its original geographical range, discern migratory patterns and reconstruct what happened during the domestication process.

Although an aura of mystery still pervades humans and Cannabis intertwined history, luckily for us, in the last years, challenges in disentangling such a complex relationship have become more manageable thanks to the coupling of molecular, pollen fossils and archaeobotanical data.

Cannabis speciation and diffusion

The time of the first appearance of Cannabis as a species was estimated using DNA molecular phylogenies calibrated with fossils of related genera. This method allowed the estimation of Cannabis and Humulus divergence at 27.8 million years ago (McPartland et al., 2019). Previous estimates place the speciation event more recently at 24.8 Mya (Wu et al., 2018), 18.2 Mya (Zhang et al., 2018) and 21 Mya (Zerega et al., 2005). Therefore, these studies can tell us that around 20 million years ago, Cannabis was a newly evolved species. These estimates are indeed supported by the finding of the oldest known pollen compatible with Cannabis in 19.6 Million years old rocks from the NE Tibetan Plateau (currently Ningxia, China). For these reasons, McPartland and colleagues propose this area as the center of origin.

Moreover, pollen and seed fossil records show that wild Cannabis expanded from its center of origin throughout Europe and East Asia well before modern humans inhabited these regions (~2 Mya). While subsequent variations in the habitat range during glacial and interglacial periods could have contributed to Cannabis diversification before human interaction (Rull 2022).

Cannabis domestication process

Cannabis is an excellent example of a crop postulated to have initially been associated with humans thanks to its "camp follower" characteristics (Anderson, 1954; Schultes, 1970). As the primordial men and women lived hunter-gatherers' nomadic lives, they often traveled between temporary camps. Such anthropic activities created clearings and paths in the vegetation, frequently near lakes or streams. Here the soils would be enriched by organic materials (such as unused remains of harvested animals and plants). The plant likely exploited this camp's niche. In fact, looking at the current habitat of wild Cannabis, we can find the plants growing in clearings, usually caused by anthropic activities where there is limited competition and where soils are mammalian-manured, continuously moist, but well-drained (Small 2015).

It is plausible that a plant useful for multiple purposes naturally growing in the vicinity of camps was a precious resource readily exploited. It is unclear how long this interaction could have lasted (potentially many millennia). Subsequently, as human populations settled and expanded, the natural Cannabis populations in their vicinity were probably depleted. This likely prompted the early farmers to sow seeds actively. With time, seeds to sow were chosen from better plants presenting larger seeds, taller stalks, and/or more resin production. Initiating the process of active domestication (Clarke & Merlin 2016)

Hypothesis on domestication centers

Several hypotheses have been put forward regarding where and when the domestication of Cannabis took place. From pollen fossils, it is clear that wild Cannabis was already widespread throughout Eurasia millions of years ago. Like always, depending on which studies we consider, different hypotheses seem more plausible than others. Moreover, since we are looking back thousands of years, interpretation of the records will most likely give us probabilities rather than proofs (here is where controversies are born). For this reasons, the most parsimonious approach is to look at the big picture and try to piece together the puzzle with a critical eye and an open mind.

European domestication

The earliest evidence of fiber attributed to hemp is associated with the Gravettian culture, around 27000 yr ago, found in the Czech Republic (Pringle 1997). Most likely nomadic tribes already harvested and used wild Cannabis for a long time but didn’t domesticate it until cultivation practices diffused from the Middle East into Europe. Indeed, according to McPartland et al. (2018) and McPartland and Hegman (2018), considering both pollen and archaeological records, Cannabis was present in its wild form throughout most of Europe between 12000-6000 years ago. Cannabis was most concentrated in the regions north and west of the Black Sea. Here the highest frequency of recorded Cannabis-type pollen fossils older than 8000 yr ago is found (Long et al., 2017).

Evidence of Cannabis smoking in Europe was provided by a record of a clay vessel with carbonized seeds (bracts surrounding the seeds contain the highest concentration of cannabinoids) from a tomb in Romania dated 5300-4300 yr ago (Merlin 2003). At the same time, hemp fibers were found well preserved in a Varna site (Bulgaria) dated 4200 yr ago. Interestingly, the first evidence of Cannabis cultivation in Europe was detected as a spike of Cannabis pollen in lake Varna (Bulgaria) between 7000 and 5000 yr ago. From this area, Cannabis cultivation expanded across the entire European continent between 4500 and 2300 years ago (McPartland et al., 2018). McPartland and Hegman, therefore, concluded that Cannabis dispersed from Asia to Europe in its wild form during the Pleistocene. Archaeological records suggest that hunter-gatherer communities exploited wild Cannabis as a source of food, fiber and entheogen during Mesolithic and Neolithic times. Whilst pollen fossils indicate that around 7000-5000 years ago, cultivation and probably domestication occurred around the Black Sea.

Asian domestication

On the other hand, other authors have proposed that Cannabis was domesticated in Central Asia (presently Mongolia and northern China) around 12000-10,000 years ago (Vavilov 1992; Crawford 2006), making Cannabis one of the oldest domesticated plants. This hypothesis is the most cited and generally accepted.

In a recent molecular study, Ren et al. (2021) estimated that early ancestors of hemp and drug types diverged from wild Cannabis between 15,728 and 6458 yr. ago (95% confidence interval). The researchers used coalescent simulations to analyze the molecular data and reconstruct the domestication process. The analysis suggests a multistep domestication process with a continuous post-domestication introgression of wild/feral genes into both hemp and drug types populations.

Archaeological records in East Asia show sites dating between 7000 and 5000 yr. old where Cannabis seeds and fibers were recovered (Zhang & Gao, 1999; Zhou 1980; Yin, ,2003). In northern China, Zhou et al. (2011) recovered seeds at a site associated with the Yǎngsháo culture (7000–5000 yr. ago), showing traits of domestication (seed enlargement, loss of abscission zone). Not surprisingly, a marked increase in seed records is observed in the following millennia (5000-4000 yr. ago).

The genomic analysis of Ren et al. 2021 shows that only a small number of early domesticated Cannabis populations expanded to form hemp and drug types around 4000 years ago.

This is a time when substantial archaeological evidence of fiber artifacts appears in East Asia, the Middle East and Europe. For these reasons, the authors proposed an early domestication in East Asia (around 10000 years ago) followed by a spread of the domesticated plants around 4000 yr. ago toward the middle East and Europe.

Can we draw a conclusion?

From the pollen fossil studies and the archaeological evidence, it is clear that wild Cannabis was widespread throughout Eurasia millions of years ago. At the same time, Asian and European archaeological records as early as 10000 yr. ago are probably a reflection of a much earlier use of Cannabis. Indeed, domestication might have been just a natural step in the process of coevolution due to the spread of agricultural practices and a shift from nomadic life to more fixed settlements. Cannabis growing in the vicinity of camps in its wild form might have been a natural commodity easily exploited for its different purposes and perfectly adapted to the nomadic hunter-gatherer communities.

Unfortunately, researchers sometimes simplify Cannabis domestication into a straightforward process, while it is clear that we are dealing with a long history characterized by multiple layers of complexity.

Indeed, cultivation might have happened in waves linked to different cultural groups inhabiting the interested areas, with Cannabis populations cycling through cultivated and feral status, as suggested by molecular analysis.

Moreover, although Ren et al. 2021 align with the single Asian domestication hypothesis, their recent genomic data might instead reveal a migratory pattern of Asian-domesticated cultivars toward the Middle East and Europe. Indeed, we cannot exclude that domesticated cultivars of European origin were probably lost and/or hybridized millennia ago.

Furthermore, the molecular data of Ren et al. 2021 helped estimate domestication time, but not place. The current wild Cannabis populations distribution provides the researcher with a probable area of domestication, yet this might be misleading. In fact, the domestication center of a crop does not necessarily match its modern center of biodiversity, as documented in studies of Asian rice (Fuller 2011).

Nonetheless, the idea of at least two domestication centers, one in Europe and one in Asia, is not only intriguing but also supported by archaeobotanical studies using pollen, seeds and fibers (Long et al., 2017; McPartland et al., 2018).

Although authors, when publishing an article, want to provide exhaustive evidence regarding a theory, it is clear that in the case of a complex topic such as Cannabis domestication and in light of the recent data, the discussion is more open than ever.

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