Historical biogeography of Cannabis in the Iberian Peninsula: palynological evidence

The tempo and mode of colonization of the Iberian Peninsula (IP) by Cannabis sativa, its further internal spreading and the potential cultural and environmental factors involved remain unknown. The available continental-wide European meta-analyses using pollen and archaeological evidence account for only a few IP sites, insufficient for a sound assessment. This paper presents a nearly comprehensive database of almost 60 IP sites with palynological evidence of Cannabis and analyzes the corresponding spatiotemporal patterns. The first scattered records of this pollen type, likely corresponding to wild Cannabis, date from the Middle and Upper Paleolithic (150 to 12 ky BP) and would have entered the IP by maritime Mediterranean or terrestrial continental pathways, or both. A first burst of introductions, probably in a cultivated form, would have occurred during the Neolithic (7-5 ky BP) using similar paths. Human participation in this Neolithic acceleration remains unclear but cannot be dismissed. A period of reduced Cannabis arrivals (mostly via MP) occurred between the Chalcolithic and the Roman Epoch (4.5-2 ky BP), when the innermost parts of the IP were colonized (Late Bronze). A second, likely anthropogenic, introduction acceleration took place in the Middle Ages (1.5 ky BP onward) using the MP and CP. Maximum cultivation and hemp retting activity was recorded during the Modern Ages (16th-19th centuries), coinciding with the increased demand of hemp fiber to supply the Spanish royal navy for imperial expansion and commerce. A potential link between Cannabis colonization/introduction bursts and climatic warmings has been observed that should be tested with future studies. Regional moisture variations seem to be less influential. Further efforts to enhance and improve the database used in this study are encouraged. The results of this paper should be compared with archaeological and historical evidence to clarify the role of human migrations and cultural changes in the historical biogeography of Cannabis in the IP.