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Cannabis is indigenous to Europe and cultivation began during the Copper or Bronze age: a probabilistic synthesis of fossil pollen studies

By May 5, 2018No Comments

Vegetation History and Archaeobotany

, Volume 27, Issue 4, pp 635–648| Cite as

  1. John M. McPartland | 1. University of VermontBurlingtonUSA
  2. Geoffrey W. Guy        |2. GW PharmaceuticalsCambridgeUK
  3. William Hegman       |3. Department of GeographyMiddlebury CollegeMiddleburyUSA
Review
First Online: 05 May 2018

Abstract

Conventional wisdom states Cannabis sativa originated in Asia and its dispersal to Europe depended upon human transport. Various Neolithic or Bronze age groups have been named as pioneer cultivators. These theses were tested by examining fossil pollen studies (FPSs), obtained from the European Pollen Database. Many FPSs report Cannabis or Humulus (C/H) with collective names (e.g. Cannabis/Humulus or Cannabaceae). To dissect these aggregate data, we used ecological proxies to differentiate C/H pollen, as follows: unknown C/H pollen that appeared in a pollen assemblage suggestive of steppe (Poaceae, Artemisia, Chenopodiaceae) we interpreted as wild-type Cannabis. C/H pollen in a mesophytic forest assemblage (Alnus, Salix, Populus) we interpreted as Humulus. C/H pollen curves that upsurged and appeared de novo alongside crop pollen grains we interpreted as cultivated hemp. FPSs were mapped and compared to the territories of archaeological cultures. We analysed 479 FPSs from the Holocene/Late Glacial, plus 36 FPSs from older strata. The results showed C/H pollen consistent with wild-type C. sativa in steppe and dry tundra landscapes throughout Europe during the early Holocene, Late Glacial, and previous glaciations. During the warm and wet Holocene Climactic Optimum, forests replaced steppe, and Humulusdominated. Cannabis retreated to steppe refugia. C/H pollen consistent with cultivated hemp first appeared in the Pontic-Caspian steppe refugium. GIS mapping linked cultivation with the Copper age Varna/Gumelniţa culture, and the Bronze age Yamnaya and Terramara cultures. An Iron age steppe culture, the Scythians, likely introduced hemp cultivation to Celtic and Proto-Slavic cultures.

Keywords

Cannabis sativa Humulus lupulus European Pollen Database Europe GIS Pleistocene Holocene 

Introduction

springer-logo-fw-siteLinnaeus (1737) knew Cannabis sativa as a cultivated plant in Europe, so he assumed its centre of origin (CoO) was elsewhere. He suggested a CoO in India Orientali (encompassing the Indian subcontinent, southeastern Asia, and the Malay Archipelago), Japonia (Japan), or Malabaria (the Malabar coast of southwest India). Most scholars concur with de Candolle (1883) who offered Central Asia as the CoO of C. sativa. He collated linguistic, historical, archaeological, and palaeontological data, as well as “in what country it grows spontaneously and without the help of man”. He proposed that C. sativa expanded to Europe under the aegis of human transport around 1500 bce, and he implicated the Scythians.

Some botanists placed the CoO of C. sativa in Europe (Thiébault de Berneaud 1835; Keppen 1886), or a CoO spanning Asia and Europe (Herder 1892; Vavilov 1926). Lamarck (1785) recognized two species, C. sativa and C. indica. He suggested that C. sativa grew croît naturellement in Persia and presque naturalisée in Europe, whereas C. indica originated in India. Many botanists treat these taxa as subspecies, C. sativa ssp. sativa, and C. sativa ssp. indica (Small and Cronquist 1976). Winterschmidt (1818) recognized two species, C. sativaand C. chinensis, with their CoOs in Russia and Ostindien, respectively.

De Candolle limited his palaeontological evidence to print fossils (impressions of leaves or fruits in rocks), and not fossil pollen (accurately, subfossil pollen). Reports of print fossils of C. sativa have not been convincing (four reported world-wide), with one exception (Palamarev 1982). Conversely, hundreds of fossil pollen studies (FPSs) across Eurasia have identified Cannabis pollen. Researchers have utilized this database to track the cultivation history of C. sativa in Europe and Asia. Dörfler (1990) analysed 77 European FPSs conducted by himself and others. He concluded that hemp cultivation began in west-central Europe by the Iron age. Clarke and Merlin (2013) reviewed 133 European FPSs. They concluded that C. sativa diffused from Asia to Europe during the Bronze age. Long et al. (2017) synthesized 46 FPSs in their pan-Eurasian study. They also concluded that C. sativa diffused from Asia to Europe during the Bronze age.

All three meta-analyses (Dörfler, Clarke and Merlin, Long et al.) corroborated FPS data with archaeobotanical evidence—seed, fiber, cordage, textiles, and pottery impressions or pseudoliths of the same. We have also examined archaeobotanical evidence, published separately (McPartland and Hegman 2017). Robust archaeobotanical evidence identified the Bronze age Yamnaya culture as an early adopter of C. sativa, in southeastern Europe.

The purpose of this study is to revisit European FPSs, by accessing an expanded database, and using a different pollen identification method. Unlike previous studies (Dörfler 1990; Clarke and Merlin 2013; Long et al. 2017), we included FPSs that analysed strata anterior to the Holocene epoch. As de Candolle (1883) stated, “conditions anterior to our epoch determined the greater number of the facts of the actual distribution of plants”. Examining pollen from earlier epochs will address the question of C. sativa endemicity in Europe, prior to its transport by humans.

The previous three meta-analyses utilized pollen grain morphology for pollen identification. Dörfler (1990) encountered problems separating Cannabis from Humulus pollen. Many palynologists, confronted with the morphological similarities between Cannabis and Humulus, resort to collective names, e.g. Cannabis/Humulus, or Cannabaceae. Hereafter we abbreviate these lumped data as C–H pollen. Clarke and Merlin (2013) were flummoxed by FPSs that lumped data as C–H pollen; they understood the complexities regarding grain morphology (Fleming and Clarke 1998). Long et al. (2017) resolved the C–H dilemma by limiting FPSs to studies that explicitly identified pollen as Cannabis—a strategy that excluded a lot of C–H data.

Methods

Search strategy and data analysis

We accessed a larger database by using several internet search engines. The European Pollen Database (EPD, http://www.europeanpollendatabase.net) was searched in April 2013 (McPartland et al. 2013) and again in December 2015. We retrieved fossil pollen studies that included pollen identified as Cannabis/Humulus, Humulus/Cannabis, Cannabis-type, Humulus-type, or Cannabaceae. The FPSs deposited in the EPD are typically linked to publications, but some FPSs are unpublished “grey literature.” Additional FPSs were identified via Web of Science, PubMed, and Google Scholar. The keywords and Boolean operators were: palynology AND (Cannabis OR Humulus). We also used citation tracking—references in retrieved publications were searched for antecedent sources, and these were retrieved.

We excluded FPSs that analysed strata limited to the historic era, because those data did not inform our research questions: the presence of pre-Neolithic endemicity, and the identification of prehistoric hemp cultivators. To map pollen in space and time, retrieved publications had to meet three inclusion criteria: (1) precise geographical coordinates; (2) accurate chronology; (3) a minimal amount of pollen grains.

  1. 1.

    Precise geographical coordinates were localized to within a hundredth degree of latitude and longitude. Some studies not provide geographical coordinates. We obtained coordinates of those sites via Google Earth, which uses the World Geodetic System of 1984 (WGS84) datum. The boundaries of our data collection correspond to the definition of European territory by Flora Europaea (Akeroyd 1993). The problematic eastern border of Europe is delimited by the crest of the Greater Caucasus Mountains, the north western shoreline of the Caspian Sea, the Ural River, and the crest of the Ural Mountains.

  2. 2.

    Accurate chronology was achieved by restricting data to FPSs with calibrated radiocarbon (14C) dates. Many studies in the EPD have been retroactively calibrated, using control points (relative and independent dates) and CLAM software (Giesecke et al. 2014). A scarcity of FPSs from the Pontic-Caspian steppe—a critical region—impelled us to include several uncalibrated studies from that region. Possible dating errors arising from retroactively calibrated data or from uncalibrated data (as well as anomalously old 14C dates due to hard water effects) were minimized by our binning methodology—temporal gaps of 500 years were placed between each time slice (see below).

  3. 3.

    Establishing the minimal amount of C–H pollen grains as an inclusion criterion proved difficult. Palynologists debate the minimal amount of pollen required to determine the presence of a plant species at a study site (versus pollen that arrived there via long-distance transport). The absolute minimum—one pollen grain—may result in “utilization fallacy” (Fuller 2006). Wild-type C. sativa colonizes ephemeral niches, such as flood water-disturbed alluvial soil. This ephemeral niche is consistent with its sporadic appearance in the fossil record. We adopted a unique inclusion criterion: for a retrieved publication to be included in our study, C–Hpollen had to appear in a minimum of five strata within a stratigraphic core. For an elaboration regarding the use of this metric, please read Online Resource 1, Extended Methods.

Pollen identification

Palynologists usually identify Cannabis and Humulus pollen by morphological characters, including grain diameter, exine thickness, and pore protrusion or pore aperture diameter. For example, Mercuri et al. (2002) identified grains ≤ 23 µm diameter as Humulus, ≥ 28 µm as Cannabis, and between 23 and 28 µm as undetermined. Others who tried to differentiate Cannabis and Humulus by grain diameter, even in comparison with reference slides, have considered it “most unreliable” (Punt and Malotaux 1984), “not possible… a suspect activity” (Whittington and Gordon 1987), “a dubious procedure” (Whittington and Edwards 1989), and “not cleanly achieved” (Hauschild 1991). The use of exine thickness, pore aperture diameter, and pore protrusion has also been criticized and contested (Engelmark 1976; French and Moore 1986; Whittington and Gordon 1987; Tweddle 2000).

Discerning Cannabis from Humulus is an old problem. “The pollen in question consists of spherical, structureless, and almost completely transparent vesicles with thin walls” (Fröman 1939). Purkyně (1830) provided the first illustration of C. sativa pollen, but it was the sole species whose identification he questioned, out of 284 species in his book. This old problem remains intractable despite new and improved microscopy. Simply: C. sativa and Humulus lupulus are highly plastic yet homologous species, whose morphological and phytochemical characters are variable and overlap (McPartland and Guy 2017). Botanists have synonymized H. lupulus under Cannabis (Scopoli 1772), and herbarium specimens of C. sativa have been misidentified as H. lupulus.

We collated 12 reference texts or methods papers, and their data regarding mean grain diameters show great variability. For C. sativa they range from 19.0 to > 28 µm, for H. lupulusthey range from 16.2 to < 25 µm (see Online Resource 1). Confronted with these inconsistencies, many palynologists transform their identification into a larger class, e.g. Cannabis/Humulus or Humulus/Cannabis. Or they apply the collective names “Humulus-type,” “Cannabis-type”, or Cannabaceae (sic, which now includes Celtidaceae).

To dissect these aggregate data, we used ecological proxies as an inferential, probabilistic method to differentiate Cannabis and Humulus pollen. Wild-type C. sativa flourishes in steppes, an open, treeless habitat (de Candolle 1883; Herder 1892; Vavilov 1926). Temperate steppe is dominated by Poaceae and Artemisia species. Desert steppe is dominated by Artemisia spp. and Chenopodiaceae. Phytosociologists and other field botanists report wild-type C. sativa cohabitating with Poaceae, Artemisia, and Chenopodiaceae; hereafter abbreviated PAC (for attestations see Online Resources 1).

Palynologists have extended these associations into the past. The BIOME Project designates Cannabis fossil pollen as a “botanical marker” of the “grassland biome”, along with PAC pollen and the relative absence of tree pollen (Tarasov et al. 1998, 2000). Cannabis fossil pollen may indicate a xerophytic steppe—hot and arid (van Geel et al. 2004; Kuneš 2008) or a mesophytic steppe—moderately warm and wet (Riehl and Pustovoytov 2006). “Dry tundra” describes a Pleistocene-early Holocene biome that was cooler than steppe, and drier than contemporary tundra. In this biome assembly, Cannabis and Artemisia pollen are categorized as “arctic forbs” (Binney et al. 2017) or cryophytes/xerophytes (Bolikhovskaya 2007). Dirksen and van Geel (2004) commented, “The synchronous fluctuations of Artemisia, Chenopodiaceae and Cannabis ruderalis pollen curves are remarkable”.

In contrast, H. lupulus requires trees to climb. Wild H. lupulus flourishes in mesophytic deciduous forests. Phytosociologists and other field botanists report H. lupulus associating with alder (Alnus spp.), willow (Salix spp.) and poplar (Populus spp.), hereafter abbreviated ASP. Palynologists have extended these associations into the past. Fries (1958, 1962) first noted synchrony in pollen counts of Humulus and Alnus. He said C–H fossil pollen did not appear in Sweden until Alnus trees migrated into the region during the early Holocene. BIOME project palynologists characterize Humulus as a drought-intolerant climber of trees (Ni et al. 2010), and assign Humulus pollen as a botanical marker of deciduous broadleaved forests (Zhou et al. 2007), or tropical evergreen forests (Lee and Liew 2010).

Arboreal/nonarboreal ratio

Firbas (1937) reasoned that Neolithic humans had to clear forests to grow crops. He detected this as a decrease in tree pollen (arboreal pollen, AP) and an increase in pollen of grasses, sedges, and forbs (nonarboreal pollen, NAP). Some palynologists argue that the AP-to-NAP ratio has limited predictive value as an indicator of landscape openness. Others find significant correlation between NAP percentages and percentage cover of open herb vegetation. AP and NAP percentages are oppositional—when one goes down, the other goes up. Similarly, palynologists have shown that Alnus and PAC demonstrate oppositional characters in studies using multivariate analyses, such as PCA, RDA, and NJ methods.

Pollen of cultivated C. sativa

Scholz (1957) distinguished between wild-type and cultivated Cannabis by grain diameter, 15–25 µm and 25–35 µm, respectively. No palynologists have adopted grain diameter as a criterion to distinguish between wild-type and cultivated Cannabis. Many palynologists interpret C–Hpollen as that of cultivated hemp when its pollen count surges or becomes a continuous curve in synchrony with pollen from other crop plants (e.g. Fries 1958, 1962; Godwin 1967; Wilson 1975; Maher 1977; van Zant et al. 1979; Whittington and Edwards 1989; Dörfler 1990; Peglar 1993; Hall et al. 1995; Fleming and Clarke 1998).

The other crop plants in these studies include Avena (oats), Hordeum (barley), Secale (rye), Triticum (wheat), and Cerealia-type (undifferentiated cereal pollen). Pollen curves of two weed species, Centaurea cyanus and Scleranthus annuus, have also been used to interpret C–Hpollen as that of cultivated hemp (Whittington and Jarvis 1986; Gaillard and Berglund 1988; Edwards and Whittington 1990).

An algorithm was developed to differentiate C–H pollen (Fig. 1). We interpreted C–H pollen as that of cultivated Cannabis when it appeared de novo along with crop pollen, or increased at least twofold over earlier pre-Neolithic counts. To differentiate C–H pollen in pre-agricultural strata, we used ecological proxies. When C–H occurred in a pollen assemblage where the AP-to-NAP ratio ≤ 2 (i.e. ≤ 33%/66%), dominated by steppe vegetation (PAC), we interpreted it as wild-type Cannabis. When C–H occurred in a pollen assemblage where the AP/NAP ratio ≥ 2 (i.e. ≥ 66%/33%), dominated by ASP, we inferred it to be Humulus. In some ambiguous FPSs, pollen counts of PAC and ASP rise and fall in near-synchrony, and the AP/NAP ratio approaches 1:1 (i.e. 50%/50%). At these sites, we classified C–H pollen as unresolved C/H.

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Fig. 1

Algorithm for the identification of C–H pollen

Readers are invited to explore Online Resource 1 for: (1) the morphological identification of Cannabis and Humulus pollen; (2) phytosociological studies linking C. sativa with PAC spp., and H. lupulus with ASP spp.; (3) debates regarding AP/NAP ratios and pollen signals indicative of crop cultivation; (4) methods of data extraction and binning for algorithm application.

Stratigraphical data were binned into six time slices, with temporal gaps of 500 years between each time slice, to minimize errors from dating inaccuracies:

  1. 1.

    18,500–15,000 cal bp, including the latter half of the Last Glacial Maximum and the onset of deglaciation, when dry tundra and steppe plants colonized northern Europe, and mesophytic tree species were limited to glacial refugia, mostly in southern peninsulas.

  2. 2.

    14,500–10,500 cal bp, including the Late Glacial and onset of the Holocene, an unstable period of deglaciation, punctuated by short-lived warming and cooling episodes, such as the Bølling, Older Dryas, Allerød, and Younger Dryas.

  3. 3.

    10,000–7,500 cal bp, including most of the Early Holocene (Walker et al. 2012), a period of improved climate, re-emerging forests, and increased anthropogenic impact on landscapes, prior to Neolithic cultures penetrating Europe beyond the Mediterranean coastline and the lower reaches of the Danube River.

  4. 4.

    7,000–5,000 cal bp, including most of the Middle Holocene, a period including the Mid-Holocene Climatic Optimum, when agriculture spread across much of low-altitude Europe, and when copper smelting began.

  5. 5.

    4,500–2,300 cal bp, onset of the Late Holocene, a period that began with the Bronze age, spanned the Iron age, and ended with the earliest recorded European history. This period includes archaeological evidence of hemp cultivation.

  6. 6.

    2,000–800 cal bp, a period that began when Europe beyond the Roman Empire was still in the prehistoric Iron age. The period ends with the beginning of hop cultivation. Our analysis stops there, in agreement with Wilson (1975), who hypothesized that pollen from cultivated hops may impact pollen diagrams (and thereby conflict with part “B” of the pollen identification algorithm).

GIS mapping

Latitude and longitude of each FPS was plotted, using geographic information system (GIS) software (ArcGIS 10.3). FPS sites were plotted on six maps, corresponding to the six binned time slices. Each FPS site was marked with a symbol indicating pollen interpretation—either wild-type C. sativa, cultivated C. sativa, H. lupulus, or unresolved C/H pollen. To better illustrate data trends over time, the six bins are grouped together as separate maps with the same scale and extent (aka presented as small multiples). Some authors of FPSs did not discuss the archaeological contexts of their sites. The locations and time slices of FPSs were compared with eight maps that delineated territories of archaeological cultures: Early Neolithic (ca. 6800–3500 bce), Late Neolithic and Copper age (ca. 5500–2700 bce), Early Bronze age (ca. 3200–1800 bce), Middle Bronze age (ca. 3000–1500 bce), Late Bronze age (ca. 1700–500 bce), Early Iron age (ca. 1100–400 bce), Middle-to-Late Iron age (ca. 800 bce–100 ce), and End-stage Iron age cultures beyond the Roman Empire (ca. 100–500 ce). These time intervals overlapped because the Neolithic, Copper, Bronze, and Iron ages spread at different rates across Europe. See Online Resource 2 for maps and methodology.

Results

The search strategy identified 603 FPSs that included C–H pollen from the Late Glacial/Holocene. Eighty studies did not meet our inclusion criteria, and another 44 studies reported duplicate data. The remaining 479 FPSs were tabulated, each with an accession number, study location, details regarding application of the algorithm, and duplicate reports. Excluded studies were also tabulated. Included and excluded FPSs appear in Online Resource 3, Table S1 and Table S2, respectively.

Geographical locations of FPSs, with pollen interpretations, distributed over six times slices, are presented in Figs. 2, 3, 4, 5, 6, 7. Locations of all the 479 FPSs, plotted on a map of Europe, are provided in an interactive map, available at http://arcg.is/2jK9uAv (also see Online Resource 2, Map S1, for a non-interactive version). Interactive functionalities include FPS site queries (click on each individual site to obtain its accession number and other data), pan and zoom, and changing the basemap (for topography, vegetation type, etc.). Note that some FPSs are deep-water cores, so they are located in the Black, Adriatic, and North Seas. Data from the 479 FPSs were binned and mapped:

Bin 1 (18,500–15,000 cal bp, Fig. 2). Only eleven FPSs in this time slice reflect a paucity of pollen sediment during the Late Glacial Maximum. In many areas of Europe, the landscape was dominated by nonarboreal (NAP) pollen and steppe species (PAC). The algorithm interprets C–H pollen in this assemblage as wild-type Cannabis. However, pioneer trees, especially Betula, occasionally blanketed landscapes with up to 85% arboreal pollen (AP). C–H pollen in these landscapes was interpreted as unresolved C/H pollen.

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Fig. 2

Bin 1 (18,500–15,000 cal bp). Background base map by Natural Earth, free open-source map data, http://www.naturalearthdata.com

Bin 2 (14,500–10,500 cal bp, Fig. 3). NAP, PAC, and C–H pollen inferred as wild-type Cannabisoccurred throughout Europe. Pollen from all three Humulus associates—ASP—began spreading from glacial refugia. At some FPS sites, the percentage of ASP pollen equalled PAC pollen, which made the determination of C–H pollen uncertain. At other FPS sites, pollen counts of AP + ASP clearly overtook those of NAP + PAC that dominated in Bin 1; C–H pollen at those sites was interpreted as Humulus in forested landscapes.

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Fig. 3

Bin 2 (14,500–10,500 cal bp). Background base map by Natural Earth, free open-source map data, http://www.naturalearthdata.com

Bin 3 (10,000–7,500 cal bp, Fig. 4). More NAP-to-AP turnover occurred, including many FPSs where PAC = ASP, possibly representing the coexistence of H. lupulus and C. sativa in forest-steppe communities. Crop plant pollen was detected at several FPS sites, but these sites lacked C–H pollen in this time slice.

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Fig. 4

Bin 3 (10,000–7,500 cal bp). Background base map by Natural Earth, free open-source map data, http://www.naturalearthdata.com

Bin 4 (7,000–5,000 cal bp, Fig. 5). AP + ASP expansion peaked in this time slice, when C–Hpollen interpreted as Humulus blanketed Europe. Data consistent with wild-type Cannabispollen were limited to the Mediterranean coast (Greece, Spain) and the Pontic steppe (Ukraine, Bulgaria). FPS sites with crop pollen did not show signals interpreted as cultivated Cannabisaccording to the algorithm, with one exception (see discussion).

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Fig. 5

Bin 4 (7,000–5,000 cal bp). Background base map by Natural Earth, free open-source map data, http://www.naturalearthdata.com

Bin 5 (4,500–2,300 cal bp, Fig. 6). Humulus flourished in heavily-forested Europe. Only 13 FPSs showed pollen signals consistent with wild-type Cannabis. Thirty-five FPSs showed C–Hpollen interpreted as cultivated Cannabis, mostly north of the Alps.

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Fig. 6

Bin 5 (4,500–2,300 cal bp). Background base map by Natural Earth, free open-source map data, http://www.naturalearthdata.com

Bin 6 (2,000–800 cal bp, Fig. 7). The pollen signal of Humulus was replaced by that of cultivated Cannabis across Europe. Fourteen FPSs showed high levels of NAP, PAC, and crop pollen, but no clear surge of C–H pollen. This pollen signal is not addressed by the algorithm. It may be interpreted as weedy C. sativa that escaped cultivation, or sampling sites that were too far away from cultivation sites to register a clear surge in pollen.

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Fig. 7

Bin 6 (2,000–800 cal bp). Background base map by Natural Earth, free open-source map data, http://www.naturalearthdata.com

In addition to the above, we identified 36 FPSs with C–H pollen that predated 18,500 cal bp. The studies are few in number, and span a large swath of uncalibrated time, from thousands (kya) to millions (mya) of years ago. They were tabulated rather than mapped, in Table 1.

Table 1

Interpretation of C–H pollen in studies predating 18,500 cal bp

C–H pollen consistent with Cannabis sativa

C–H pollen undetermined (Cannabis–Humulus)

C–H pollen consistent with Humulus lupulus

Last Glacial Maximum, OIS 4–2, 70–18.5 kya

 Bulgaria (n = 2), Russia (2), Estonia, Greece, Ukraine

Weichselian cooling trend, OIS 5d-a, 115–70 kya

 Italy, Poland

Poland, Ukraine

Eemian/Mikulino interglacial, OIS 5e, 130–115 kya

Russia

France, Italy, Netherlands, Poland (5), Russia, Sweden

Penultimate glaciation, OIS 6, 191–130 kya

 Russia (2), Italy

Zhizdra cooling event (OIS 7, 243–191 kya), Sanian 2 glaciation (OIS 12, 420–400 kya), or Gremyache interglacial (OIS 19, 770 kya)

 Poland (2), Russia (2)

Eopleistocene (1.8–1.2 mya) or Early Pliocene (5.32–3.6 mya)

 Russia, Romania, Kazakhstan

Late Miocene (11.6–5.8 mya) or Oligocene (33.9–23.0)

Bulgaria

Russia, Georgia, Bulgaria, Italy, Czech Republic

Discussion

This study has several limitations. As with any pollen-based method, the algorithm (Fig. 1) is challenged by inherent assumptions and biases. It assumes that ecological niches of modern Cannabis, Humulus, PAC, and ASP can be extrapolated to past populations (the “nearest living relative” paradigm). To wit, natural plant communities break down in landscapes altered by anthropogenic activities. Synanthropic communities, however, often include C. sativa and PACspecies; we cite examples in Online Resource 1. The results generated by our algorithm are best characterized as “probabilities, not proofs.”

The dispersal of FPS datapoints in Figs. 2, 3, 4, 5, 6 and 7 reflects the clumped distribution of palynologists, not the distribution of pollen. Some regions of Europe are dense with FPSs (e.g. UK, Switzerland, Czech Republic), whereas few studies have been conducted in Ukraine and southeast Russia. The paucity of studies in southeast Russia is unfortunate, because Flora Europaea states that “native” populations grow there (Akeroyd 1993).

In Fig. 7, copious pollen from cultivated Cannabis no doubt swamped some of the signal coming from wild Humulus. Conversely, woodland clearance would have enhanced potential habitats for Humulus. Today, cultural landscapes provide artificial surfaces for wild Humulusgrowth—forest edges, hedges, walls, and fences (Lisci and Pacini 1993). These situations could result in a surge of Humulus pollen, potentially misinterpreted as cultivated Cannabis. See Online Resource 1 for elaborations on these and other criticisms of our methods.

In general, when extracting data from FPSs, our dichotomizing proxies were quite robust: NAP vs. AP, and PAC vs. ASP. This made the inference of C–H pollen as either C. sativa or H. lupulus rather straightforward. Other authors have noted that pollen counts of Alnus and PACdemonstrate an oppositional character (Hicks and Birks 1996; Kuneš et al. 2008; Conner 2011).

Indigenous European C. sativa

Global studies of climate change indicate that climate variability began to cycle into stadials (glacial periods) and interstadials (warm periods) during the Pliocene Epoch (Ehlers and Gibbard 2007). The oldest C–H pollen consistent with Cannabis in Europe appeared during the Olduvai cold stage, beginning 1.8 mya (Danukalova 2010). Older C–H pollen from the late Miocene, 6.1–5.3 mya (Ivanov et al. 2007), appeared in an ambiguous milieu (AP/NAP ~ 1:1, PAC = ASP), which the algorithm classified as unresolved C/H. However, another site in Bulgaria, also from the late Miocene, reported a similar mixed flora—and yielded a fossil fruit of Cannabis: carefully analysed, illustrated, and convincing (Palamarev 1982).

Studies that predate 18,500 cal bp (Table 1) and Bin 1 (18,500–15,000 cal bp, Fig. 2) support our hypothesis that Cannabis expanded from Asia to Europe prior to human agency. This should not be surprising, because her sister genus—Humulus—also expanded from Asia to Europe prior to human agency. European print fossils of Humulus date back to the Miocene (Collinson 1989). Several FPSs in Fig. 2 fall within regions traditionally considered glacial refugia of tree species—in Iberia, Italy and Greece. Nevertheless, the pollen in these tree refugia was dominated by NAP, PAC and C. sativa; they may represent “refugia within refugia” (Feliner 2011).

It may seem like a climactic improbability that northern Europe served as C. sativa habitat during the Last Glacial Maximum (Fig. 2) and subsequent deglaciation (Fig. 3). But recall, C. sativa has been classified an “arctic forb” (Bolikhovskaya 2007; Binney et al. 2017). In fact, C. sativa can tolerate climatic conditions north of 68°N (Schübeler 1875). In perspective, our maps here extend up to 63.5°N. Furthermore, northern Europe underwent post-glacial isostatic rebound. This caused land uplift, nearly 300 m in places (Berglund 2004). Uplift coupled with glacial melt created alluvial ravines cutting through dry tundra and steppe—a disturbed landscape perfect for C. sativa.

The dominance of NAP + PAC pollen in Bin 1 is gradually replaced by AP + ASP pollen in Bins 2 through 5. Thus by proxy, C–H pollen interpreted as Cannabis in Bin 1 is gradually replaced by Humulus in Bins 2 through 5. Our estimates of PAC and ASP distribution and abundance across time and space broadly agree with isopollen maps by Huntley and Birks (1983). They present separate maps of P, A, C, and A, S, P pollen across Europe, in time slices corresponding to Bins 2 through 5, in a meta-analysis of 423 FPS. Our estimates also broadly agree with an updated study by Brewer et al. (2017), who used pollen percentage symbols for spatial visualization in a meta-analysis of 828 FPSs. Their maps show the distribution and abundance of P, A, (no C), and A, S, P pollen. Our estimates of Alnus and Salix expansion during the Holocene parallel results in other studies (King and Ferris 1998; Alsos et al. 2009; Douda et al. 2014). Also in agreement with our results, BIOME studies show steppe landscapes contracting from 18 to 6 kya, and rebounding at 0 kya (Prentice et al. 1996; Tarasov et al. 2000).

Aggregating many FPSs enables the separation of collective signal from the noise within individual FPSs. A trend emerges from the collective data: C. sativa colonized Europe during stadials, and was largely replaced by H. lupulus during interstadials. This trend parallels cycles reported for Artemisia (Blyakharchuk and Amel’chenko 2012; Liu et al. 2013; Cao et al. 2013). Thus C. sativa and Artemisia retreated to “interstadial refugia” (sensu López-García et al. 2010), diametrically opposite to tree species, which retreated to refugia during stadials.

Our study shows a nadir of C–H pollen interpreted as Cannabis pollen in Bin 4 (Fig. 5), following the Mid-Holocene Climatic Optimum (MHCO). Cannabis was limited to two interstadial refugia: the Pontic steppe (Ukraine, Bulgaria) and the Mediterranean coast (Greece, Spain). Akeroyd (1993) reports a single modern refugium of “native” C. sativapersisting in the Pontic steppe (southeastern Russia). Consistent with our MHCO findings, Pokorný et al. (2015) demonstrated that steppe landscapes underwent a MHCO bottleneck in the Czech Republic. Their pollen diagram showed Poaceae and Artemisia persisted through the MHCO, but no Cannabis pollen appeared during that interval. We hypothesize that this bottleneck contributed to allopatric segregation between two recognized subspecies—the European C. sativa ssp. sativa, and the Asian C. sativa ssp. indica (Small and Cronquist 1976).

No cultivated C. sativa in Neolithic Europe

C–H pollen consistent with Cannabis largely disappeared from Europe by the time farmers and/or agricultural technology arrived from the Fertile Crescent. West Asian “founder crops” diffused into Europe. This crop package included flax, Linum usitatissimum L., a fiber and seed oil crop (functionally analogous to C. sativa). We hypothesize that people from three Neolithic cultures had the potential to domesticate wild-type C. sativa. This hypothesis was generating by constructing maps that delineated the territories of Neolithic cultures (Online Resource 2, Maps S2, S3). We compared these culture maps with locations of C–H pollen consistent with wild-type Cannabis in Figs. 4 and 5. Those locations fell into the territories of Neolithic Greece, the Cardium Pottery culture, and the Bug-Dniester culture. For scholars interested in citations, see Online Resource 3, Table S1 (Neolithic Greece studies #89, 282, 284, 288, 289; Cardium Pottery: #94, 196; Bug-Dniester culture: #237).

However, no pollen signals consistent with cultivated Cannabis arise within these cultures, or any early Neolithic cultures (e.g. Starčevo-Körös-Criş, Linearbandkeramik, early Cucuteni-Tripolye, see Map S2). This agrees with archaeological studies of these cultures, which lack evidence of C. sativa (Cârciumaru 1996; Bogaard 2004; Kreuz et al. 2005; Conolly et al. 2008). When it comes to Cannabis, these farmers either “missed the boat,” or their cultivation of hemp was a “false start” (meaning its cultivation was quickly abandoned).

Rimantienė (1979) identified Canabinaceae (sic) pollen at Šventoji in Lithuania, a Narva culture site with an uncalibrated 14C date of 4,190 bp. She identified it as C. sativa, alongside a grain of “millet” pollen. Our algorithm interpreted the pollen as Humulus. Consistent with this, Rimantienė (1992) stated that Alnus glutinosa surrounded the Šventoji lagoon of 4,190 bp. We analysed 24 fossil pollen studies in the Baltic region (Table S1, studies #12–30, 246–250). All C–H pollen from the Baltic Neolithic was interpreted as Humulus or C/H (undetermined). The earliest C–H pollen suggestive of cultivated Cannabis in the Baltic region appeared 500 bce(study #12). Piličiauskas et al. (2017) recently deconstructed Rimantienė’s identification of Cannabis and the entire concept of Subneolithic farming in the Baltic region.

C. sativa during the Copper age

We compared the territories of Copper age cultures (Map S3) with locations of C–H pollen consistent with Cannabis in Fig. 5. This suggests that two Copper age cultures had the potential to domesticate wild-type C. sativa: the Greek Chalcolithic (Table S1, studies #89, 94, 284, 288), and the Cucuteni-Tripolye culture (study #237). C–H pollen consistent with cultivated Cannabis occurred at one site in Bulgaria (#274). This site may correspond to the Varna culture or Gumelniţa culture. However, pollen at five other Varna and Gumelniţa sites was interpreted as Humulus (#276), or undetermined C/H (#70, 71, 273, 275). Archaeological studies of Gumelniţa and Cucuteni-Tripolye sites have found C. sativa seeds and less-robust evidence—pottery seed impressions (Clarke and Merlin 2013; Long et al. 2017; McPartland and Hegman 2017).

C. sativa during the Bronze age

Eight Bronze age cultures had potential: C–H pollen consistent with wild-type Cannabis in Fig. 6 appeared within the boundaries of several Bronze age cultures (Maps S4–S6). These include the Netted Ware culture (Table S1, study #6), Ezero culture (#271), Yamnaya culture (#237, 427), Corded Ware culture (#305), Bell-Beaker culture (#198), Terramara culture (#347), Aegean Bronze age (#282, 284, 288), and Mycenaean Greece (#282, 284, 288).

C–H pollen interpreted as cultivated C. sativa appeared in four studies: One study in Yamnaya territory (#274) agrees with archaeological studies, which have recovered C. sativa seeds or pottery seed impressions (Clarke and Merlin 2013; Long et al. 2017; McPartland and Hegman 2017). Two study sites are associated with the Terramara culture (#344, 349). However, pollen in 11 other studies at Terramara sites suggested Humulus or indeterminate C/H pollen. One FPS in France (#436) was likely contaminated by taphonomic processes, as admitted by its authors.

Collectively, FPSs show evidence of hemp cultivation during the Copper and Bronze ages. This poses a larger question: was European C. sativa domesticated autochthonously, separate from its domestication in eastern China? This complex issue is discussed in our sister publication (McPartland and Hegman 2017). Basically, we agree with Vavilov (1926), “It is probable that the cultivation of hemp arose simultaneously and independently in several places”.

C. sativa during the Iron age

C–H pollen interpreted as cultivated Cannabis appeared within territories and time slices occupied by several Iron age cultures (Maps S7–S9). By the Iron age, the Proto-Indo-European language diverged into several language groups. We organized this section by language groups, including Iranian, Greek, Celtic, Slavic, and Baltic languages.

The Iranian-speaking Scythians migrated to the Pontic steppe from Central Asia. Three FPSs in Scythian territory showed C–H pollen consistent with wild-type or cultivated Cannabis (Table S1, studies #243, 244, 427). At the dawn of European history (ca. 440 bce), Herodotus said Open image in new window “grows both wild and cultivated” in the land of Scythia (Herodotus 2007). No less than a dozen Scythian sites in Europe have recovered seeds, pottery seed impressions, cordage, or textiles (McPartland and Hegman 2017).

Herodotus introduced Open image in new window with Open image in new window “there is, there exists”, a verb he joined to a noun he assumed was unknown to the reader. Pollen suggestive of wild-type Cannabis occurred in Greece prior to Herodotus’ time, but no archaeological evidence suggests the Iron age Greeks recognized its utility. After Herodotus’ time, pollen consistent with cultivated Cannabisappeared in Greece or Greek colonies (studies #84, 361, 362). Pollen consistent with cultivated Cannabis appeared at one FPS site prior to Herodotus, ca. 525 bce (#287). However, that site borders Thrace. Herodotus described the Thracians making textiles from Open image in new window fiber. Surprisingly, no other studies within Thracian territory showed pollen signals of cultivated C. sativa in the bce era.

The Iron age Celts (Hallstatt phases C and D, and La Tène culture) grew hemp, as attested by plentiful macroscopic evidence (Clarke and Merlin 2013; Long et al. 2017; McPartland and Hegman 2017). C–H pollen consistent with cultivated Cannabis was found in France (#184, 189, 190, 191, 193, 366, 369, 371–374, 380, 436), Germany (#126, 128, 130, 132, 134, 135), Switzerland (#334, 336, 442, 443), Great Britain (#201, 390), Hungary (#95, 97), Czech Republic (#114, 451), northern Italy (#340), and Spain (#386). This plethora of evidence partially reflects the density of palynologists in these countries.

Studies in Proto-Slavic territories showed pollen signals consistent with hemp cultivation, associated with the Pomeranian culture (#43, 45, 265), Przeworsk culture (#38, 39, 44, 47, 48, 51, 262, 263, 266), and Zarubintsy culture (#239). Three Proto-Baltic cultures also showed C–H pollen consistent with cultivated Cannabis: the West Baltic Barrow Culture (#12, 13, 28, 256, 474), Milograd culture (#245), and Bogaczewo culture (#12, 13, 16, 28, 253, 256). Proto-Germanic cultures showed evidence of hemp cultivation: the Jastorf culture (#317), Nordic Pre-Roman Iron age (#230), and Nordic Roman Iron age (#232, 233, 299, 397, 457), as well as the Proto-Germanic-Slavic Wielbark culture (#34, 259, 260).

The question arises whether Celtic, Proto-Slavic, and Proto-Baltic cultures began cultivating C. sativa autochthonously, or as a result of interacting with Scythians. The evidence of Scythian cultivation is old. In Central Asia, Cannabis pollen at Proto-Scythian Tagar culture sites arises 900 bce (McPartland and Guy 2016). After 900 bce the Scythians moved to the Pontic steppe, where signals of hemp cultivation preceded their arrival, back to the Bronze age.

The Scythians impacted deeply on the Celts, in the realms of art, animal husbandry, military strategy, language, and even clothing. The oldest evidence of Scythian–Celtic interactions that we could find was a 7th century bce burial in Bulgaria, which combined elements of Scythian culture along with a Hallstatt vessel (Braund 2015). Scythian artifacts in Hallstatt-occupied Hungary first appear around 550 bce (Bartosiewicz and Gál 2010). A Hallstatt burial at Vix in France from 525 bce contains items and motifs inspired by Scythian culture (Megaw 1966).

These data collectively suggest a conservative date of 550 bce as the terminus post quem for Scythian contact with the Celts. Only three sites in Celtic territory showed pollen signals consistent with hemp cultivation prior to 550 bce (#114, 135, 340). To wit, the oldest ones (#135, 340) had problems with dating. In contrast, 28 FPSs in Celtic territory showed pollen signals of hemp cultivation arising post-550 bce, after their contact with the Scythians.

The Scythians also impacted Proto-Slavic cultures. The Scythians left a trail of burned-out settlements built by the Proto-Slavic Lusatian culture around 600 bce (Bukowski 1977). A horde of Scythian artifacts found at Witaszkowo in Lusatia dates to 550 bce (Furtwängler 1883). Only two pre-550 bce sites in Slavic/Baltic territory showed signals consistent with hemp cultivation, and they occurred in the southeast, towards the Scythian homeland (#245, 580 bce; #265, 570 bce). Ralska-Jasiewiczowa and van Geel (1998) linked the appearance of Cannabispollen in Poland with Scythian incursions. The Scythians appear to be responsible for the spread of Cannabis amongst several Iron age European cultures.

Moving into Bin 6 (Fig. 7), C–H pollen interpreted as cultivated Cannabis blankets much of Europe. Dörfler (1990) cites macroscopic and pollen findings of hemp cultivation spreading into new regions in synchrony with Roman invasions.

Pollen evidence suggestive of hemp cultivation appears in new places after the Romans arrived (Map S9), such as the Italian Alps (#171, 173, 174), Spain (#197, 386), Switzerland (#169), Austria (#309), Great Britain (#201, 203, 207), and Greece (#282, 286).

We plan to compare these results with linguistic data, by examining European cognates for hemp in Indo-European, Finno-Ugric, Caucasus, and Semitic language families. We also plan to extend our fossil pollen research into Asia.

Notes

Acknowledgements

Felix Bittmann, Anna Maria Mercuri, Mark Merlin, and two anonymous reviewers greatly improved this manuscript with their suggestions. Funding was provided by GW Pharmaceuticals.

Supplementary material

334_2018_678_MOESM1_ESM.pdf (986 kb)

Online Resource 1. Extended methods. Inclusion criteria with a critique of pollen threshold percentages, morphological methods of pollen identification, phytosociological studies of C. sativa and H. lupulus, AP/NAP ratios and pollen signals indicative of crop cultivation, methods of data extraction and binning for algorithm application (PDF 985 KB)
334_2018_678_MOESM2_ESM.pdf (9.8 mb)

Online Resource 2. Maps. This file contains nine maps. Map S1: Locations of 479 fossil pollen studies, plotted on a map of Europe. Maps S2-9: Territories of archaeological cultures (PDF 10080 KB)
334_2018_678_MOESM3_ESM.pdf (1.3 mb)

Online Resource 3. Tables. This file contains three Tables. Table S1: Fossil pollen studies with C-H pollen from the Late Glacial and Holocene. Table S2: Fossil pollen studies excluded from analysis. Table S3: Fossil pollen studies predating 18.5 kya (PDF 1319 KB)

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