Other authors propose two centers of domestication, one in the Caucasus , where the ancestor of hemp fiber domesticated biotypes evolved, and another in south-eastern China, the place where the precursor of narcotic domesticates originated . The idea of at least two domestication centers situated in Europe and Asia is supported by archaeobotanical studies using pollen, seeds and fibers . The human-mediated diffusion of Cannabis outside Eurasia occurred relatively recently and is well documented historically. After a thorough review of the available archeological and historical evidence, Clarke and Merlin subdivided anthropogenic diffusion of Cannabis dispersal beyond Eurasia into six phases: 1) primary dispersal across Eurasia , 2) spread into Africa and SE Asia , 3) diffusion into the Americas from Europe , 4) diffusion to the Americas from Europe and Asia , 5) expansion after World War II , and 6) proliferation of industrial hemp . In the first phase, Paleolithic nomadic peoples – who were expanding their range during the postglacial ice retreat – could have contributed, either consciously or accidentally, to the dispersal of the wild hemp and drug precursors from their corresponding distribution areas located in the Caucasus region and southeastern China, respectively . This was the onset of differentiation of the four major dry rack cannabis biotypes. NLH evolved from the PHA via an intermediate form known as the NLH ancestor , whereas NLD and BLD originated from the PDA through the intermediate ancestral form called the NLDA. BLH could also have evolved from isolated PDA populations in northeast China.
The second phase coincided with the expansion of the Arab Empire into Africa and the Indian Empire into SE Asia. These cultures used Cannabis mainly as a drug and introduced the NLD biotype in their dominions, primarily through adventurers and traders. After the initial introductions, migrants and traders spread the NLD biotype across much of Africa and Asia, whereas the hemp biotype remained restricted to Europe until the colonization of the Americas. Diffusion into the New World occurred during European colonization and consisted mainly of the production of hemp for cordage, cloth and seed . It is speculated whether Vikings could have carried hemp to North America in pre-Columbian times, but there is no evidence for a sound assessment. In addition to domestic uses, hemp cultivation was stimulated by European governments of the colonizing countries to provide their sailing ships with ropes and sails. During this phase , drug biotypes remained in the Old World, where the African expansion of NLD continued. It was not until the middle 1800s CE that Asian hemp was introduced in North America through the Pacific Ocean to replace the inferior European hemp in terms of fiber quality. The first psychoactive biotypes were introduced from India into the Americas by indentured laborers following the abolition of slavery in 1834. After World War II , Cannabis cultivation and use were prohibited at the international level, which signified the end of legal diffusion of this plant and initiated the fifth spreading phase . The illegal traffic of marijuana for recreational use did not stop but has experienced a significant increase since the 1960s.
Most producers of clandestine Cannabis were from Africa, Asia and Latin America, which inadvertently exported seeds to the main consuming centers from Europe and North America, where local cultivation attempts began. In the 1970 s, Asian varieties of BLD were introduced into North America and western Europe, where the cultivation of NLD × BLD hybrids flourished. During the last three decades , Cannabis prosecution has generated new cultivation and dispersal modes to avoid legal detection. At the same time, renewed economic interest in hemp fiber has been sparked, which has promoted the legal industrial cultivation of hemp. Medical applications have also increased by artificial selection toward varieties with increased THC and other cannabinoids. New varieties have been created by hybridization of NLH and BLH biotypes aimed at expanding hemp cultivation to equatorial areas where European NLH does not grow well. Regarding drugs, the production of NLD/BLD hybrids has also increased and disseminated worldwide, and vegetative indoor cultivation of these hybrids has proliferated. As a result, the global genetic diversity of both hemp and drug forms experienced a significant reduction in comparison with the 1970s and 1980s, when the different biotypes were cultivated by traditional farmers in isolated geographic habitats. According to the single center of domestication hypothesis, European Cannabis would have been carried by humans from Asia already in a cultivated form. However, the finding of Pleistocene Cannabis pollen across Europe suggests that this plant would had arrived to this continent before its domestication, and therefore, the plant could have also been domesticated in Europe. Using the assemblage approach for the identification of C. sativa pollen explained above , McPartland et al. performed a meta-analysis on nearly 480 sites and demonstrated that this pollen was widespread across Europe during the Pleistocene prior to human agency .
According to the same authors, no evident signs of Cannabis cultivation have been found either in the pollen or in the archeological record for most of the European Neolithic. The first palynological evidence for potential Cannabis cultivation appeared in present-day Bulgaria during the late Neolithic-Copper age and expanded during the Bronze and Iron ages . Cannabis cultivation expanded across the entire continent between 2000 and 800 yr BP during the Roman Empire and the Early Middle Ages. These results were supported by a further meta-analysis of archeological evidence from almost 140 sites . These authors concluded that Cannabis dispersed from Asia to Europe in its wild form during the Pleistocene and was domesticated in situ during the Copper and Bronze ages. This challenges the single-site domestication hypothesis, according to which Cannabis would have been domesticated in Asia and expanded worldwide as a cultivated plant. The possibility of an European domestication of Cannabis is consistent with the former view of Clarke and Merlin , who proposed that the precursor of the European NLH biotype – the wild PHA, related to C. sativa subsp. sativa – would have been centered in the Caucasus region and expanded to western Eurasia and eastern Europe during the Pleistocene . Chronologically, this possibility also seems plausible, as C. sativa subsp. sativa would have diverged from its Asian sister C. sativa subsp. indica in the Middle Pleistocene, approximately 1 Ma , and the European records of wild Cannabis are younger than this divergence. In addition, the ages of these wild European records show consistent westward and southward dispersal patterns during the Middle and Late Pleistocene, with younger postglacial records situated in the west and south .
Notably, the European domestication of Cannabis estimated from pollen and archeological records occurred after the European Neolithic and much later than Asian domestication , as estimated by calibrated phylogenetic trees.The Iberian Peninsula is a key biogeographic region due to its high biodiversity and endemism levels, along with its transitional character between contrasting biogeographical regions, which results in a very peculiar biota in the European context. The IP is one of the main centers of Mediterranean plant diversity, together with the Anatolian, the Balkan and the Italian peninsulas . These peninsulas have had a fundamental role not only as biodiversity cradles but also as glacial refugia, especially during the Last Glacial Maximum.It has been estimated that roughly a quarter of the LGM refugia have been located in the IP and the Balearic Islands.The present Iberian vascular flora is especially rich – 6276 species , distributed in 1278 genera and 189 families – and shows a high degree of specific endemism . This represents more than 50% of the European flora. The main Iberian centers of diversity and endemism are situated in the mountain ranges and the richest groups are the Compositae, the Leguminosae and the Gramineae, whereas the Gymnosperms and the Pteridophytes are the less represented . Traditionally, the IP has been subdivided into two major biogeographical domains characterized by contrasting bioclimatic features: the Mediterranean and the Eurosiberian regions. The Mediterranean region is characterized by the occurrence of Mediterranean macrobioclimates, with at least two consecutive arid months during the warmest season , roll bench and dominates most of the IP.The Eurosiberian region is restricted to the N and NW sectors of the peninsula and is characterized by wet Temperate macrobioclimates, without a period of two or more consecutive with summer aridity.
The Mediterranean region is dominated by species distributed across southern Europe and northern Africa, whereas the Eurosiberian region is characterized by species distributed across central and northern Europe . Land use is also strongly influenced by bioclimatic and biogeographic patterns. In the Eurosiberian region, land use is comparable to the rest of Atlantic Europe, where the traditional agrarian economy is based on cattle raising for milk and meat, combined with small-scale crops such as maize, potatoes and other vegetables, as well as fruits such as apples and chestnuts. The resulting landscape is a mosaic of small fields, woodlands and pastures, combined with larger areas of heathlands, forests and grasslands, especially in higher elevations. The Mediterranean Iberia is dominated by a totally different landscape resulting from land-use practices adapted to the occurrence of the climatic summer drought. In these conditions, crops not needing irrigation are favored. Annual crops, especially cereals, largely dominate the central IP, which is characterized by huge medium-elevation flat terrains. In the southern half, where climates are not so cold, typical Mediterranean crops such as vineyards and olive grows dominate the landscape. In the Mediterranean region, extensive crops are the norm and animal husbandry occupies a subordinate position, except in some low-mountain areas, where extensive parkland landscapes dominate.The oldest human remains found in the Iberian Peninsula date from approximately 1.2 Ma and correspond to Homo antecessor, an autochthonous species related to H. erectus.The Neanderthals entered the IP by 200 kyr BP and the modern humans did the same by 40 kyr BP. The Neolithization of the IP began at approximately 5500 BCE , when the former hunter-gatherer nomadic societies were replaced by more stable agricultural-based cultures.During the Bronze Age and the Iron Age , a diversity of cultures developed on the IP, which were influenced by Indo-European migrations, from the north and the Phoenicians, arriving by the Mediterranean coasts during the 11th century BCE.
The autochthonous Iberian culture fully developed during the 7th-6th centuries BCE but colonizations and invasions from the Mediterranean area continued with the Greeks , the Carthaginians and the Romans, who arrived by 220 BCE and occupied the IP for almost 600 years. After a transitional phase in which the IP was occupied by the Germanic Visigoths, coming from the north, the next long-lasting invasion was that of the Muslims, who entered the IP from Africa and dominated Iberia for almost eight centuries . The Middle Ages signified the progressive expansion of Christian cultures and the unification of monarchies, followed by the consolidation of the modern geographical and political configuration. After the Middle Ages, Spain and Portugal, the major countries of the IP, shifted from colonized to colonizer countries – favored by the expansion of their naval industry, which largely benefited from the development of local the hemp industry – and expanded their dominions all over the world, with emphasis on the American continent, a situation that remained until the 19th–20th centuries . This brief summary shows that the IP is a strategic spot also in cultural and historical terms, mainly due to its geographic location, which has promoted the interaction of a diversity of cultures over prehistoric and historic times, favored by numerous and varied land and sea connections. Therefore, it might be expected that the IP would also be a relevant place for the study of a plant such as Cannabis, which has been subjected to intensive and extensive natural and human-mediated dissemination. It should be noted, however, that the IP is far from any proposed center of origin for Cannabis, and the wild form of this plant seems to have reached the peninsula only in postglacial times , after a long journey from the Caucasus region .