Chapter 1: Natural Origins and Early Evolution of Cannabis
Introduction
Basic life cycle of Cannabis
Ecological requirements of Cannabis: Sunlight, temperature, water and soil
Cannabis origin and evolution studies
Central Asia: Vavilov and the origins of Cannabis
Cannabis and Vitis
Theories for South Asian origin of domesticated Cannabis
Model for the early evolution of Cannabis
Summary and conclusions
Introduction
Where and when did humans first come into contact with Cannabis? And how and why did people begin to employ these extremely useful plants? In order to support our hypothetical scenario of early human interactions with Cannabis presented in the introduction, we need to identify when and where the species originated. What were our planet's environmental and biotic conditions during Cannabis' early evolution? What are the environmental conditions in which it grows naturally without human help? Can we realistically understand how Cannabis evolved? And if so, where and how did it evolve? To answer these questions we must investigate the basic life cycle and ecological requirements of Cannabis. After understanding the botany and ecology of Cannabis, including an identification of its closest botanical relatives, an analysis of the processes through which it reproduces and an application of these parameters to ancient vegetation and climate reconstructions, then we can begin to comprehend its geographical and evolutionary origins (see Chapter 11 for discussions of climate reconstruction, refugia, and species formation).
Many avenues must be explored during a thorough study of the origin and evolution of any cultivated plant. We have utilized diverse sub-disciplines of botany, along with archaeology, paleontology, history, linguistics and geography to provide useful insights into the origin and dispersal of Cannabis (also see Chapter 3). The data gleaned from each approach have been filtered and then assembled carefully in order to gain the clearest possible view of the plant's antiquity. Several great plant geographers studied Cannabis and we investigate their theories of origin in Central and South Asia. Even so, the information is often incomplete for a particular region or era, and it is important to resist temptation to indulge in idle conjecture without sufficient facts to back up hypotheses. With this in mind, we have attempted to assemble enough data to support the hypotheses for the natural origin and early evolution of Cannabis presented here. However, we have constructed only the skeleton of our model and additional, more rigorous, biosystematic analysis is still needed to flesh out more detail. [Figure01.01.a-j]
Basic life cycle of Cannabis
Typically, Cannabis is a medium to tall, erect, annual herb, but environmental influences strongly affect the growth habits ofindividual plants throughout its range. Provided with an open sunny environment, light well-drained soil and sufficient nutrients and water, Cannabis can grow to a height of five meters (16 feet) in a four to six month growing season. Exposed riverbanks, lakesides and agricultural lands are ideal habitats for Cannabis since they normally offer good sunlight, moist and well-drained soil and ample nutrients. When growing in arid locations with negligible soil nutrients, Cannabis develops minimal foliage and may mature and bear seed when only 20 centimeters (eight inches) tall. When planted in close stands on fertile soil, as in fiber hemp cultivation,plants do not branch but grow as tall, slender and straight stalks. If a plant is not crowded (e.g., when cultivated for seed or drug production) limbs bearing flowers will grow from small axial meristems (growing points) located at the nodes (intersections of the petioles or leaf stalks) along the main stalk.
In temperate environments, seeds are sown outdoors in the springtime and usually germinate in three to seven days. The first true leaves arise about ten centimeters (four inches) or less above the cotyledons (seed leaves) as a pair of oppositely oriented single leaflets. Subsequent leaves arise in opposing pairs, and a variously shaped leaf sequence develops with the second pair of leaves having three leaflets, the third five, and so on up to nine, 11 and even 13 leaflets. In some warm sunny climateswith favorable soil conditions Cannabis can grow taller by as much as 10 centimeters (four inches) a day. The rapidly elongating stalks produce a strong bast (bark) fiber used for cordage and woven textiles.
Cannabis exhibits a dual response to day length. During the first two or three months it responds to increasing day length with more vigorous vegetative growth, but later in the season Cannabis requires shorter autumn days to flower and complete its life-cycle. Cultivated Cannabis produces flowers when it is exposed to a day length (photoperiod) of 12 to 14 hours, which varies with the strain, and all varieties have an absolute requirement of a minimum number of short day lengths (or, more accurately, long nights) that will induce fertile flowering. Dark (night) cycles of 10 to 12 hours must be uninterrupted by light periods in order to induce flowering.
Cannabis is normally dioecious, which means that unisexual male or female flowers develop on separate plants, although co-sexual monoecious or hermaphrodite examples with both sexes produced on one plant occasionally do occur (see Chapter 9). Cannabis is anemophilous (wind-pollinated) and relies on air currents to carry pollen grains from male plants to female plants.
The first sign of flowering is the appearance of undifferentiated floral primordium along the main stalk at the nodes, one behind each of the stipules (leaf spurs), with one on each side of the base of each leaf's petiole. Before flowering, the sexes of Cannabis are indistinguishable except for general trends in growth habit-in less crowded conditions female plants tend to be shorter and produce more branches than male plants. When flowering is initiated, the male flower primordium can be identified by its curved, crab's claw shape. This is soon followed by the differentiation of dense clusters of round, pointed flower buds each having five radial segments. The female primordium can be identified by the enlargement of a tapered curved tubular bract (floral sheath). In both sexes, when flowering begins, the pattern of increasing numbers of leaflets reverses, and as flowering progresses, the number of leaflets per leaf decreases until only a small single leaflet appears below each pair of flowers. The phyllotaxy (leaf arrangement along the main stalk) also changes from opposite to alternate (and remains alternate) throughout flowering regardless of sexual type.
Development of branches bearing flowering organs varies greatly between males and females. Female plants are leafy to the top with many small leaflets subtending the flowers tightly crowded within erect compact clusters while male plants have only a few small leaves growing sparsely along the elongated flowering limbs. Male flowers hang from long, multi-branched, loose clusters formed of small (approximately five millimeters or 1/5 inch long) individual flower buds along an axis up to 30 centimeters (12 inches) long. Tightly clustered female flowers have two long white, yellowish or pinkish stigmas (female sexual organs receptive to pollen) protruding from each bract. The bract measures two to eight millimeters (1/12-1/3 inch) in length and adheres closely to the single ovary, completely surrounding it. The bract is covered with hundreds of glandular trichomes or plant hairs. These glands and their resinous secretion may protect the reproductive organs from excessive transpiration and may also repel pests (Clarke 1981). It is this aromatic resin that contains the psychoactive properties that have attracted human attention for millennia (Pollan 2001, also see Merlin 1972, Clarke 1981).
The differences in flowering patterns of male and female plants are expressed in many ways. Soon after pollen is shed, the male plant dies. The female plant may mature for up to five months after viable flowers are formed if little or no fertilization occurs and if it is not killed by frost, pests or disease. Compared with female plants, male plants show a more rapid increase in height as well as a more rapid decrease in leaf size and leaflet number approaching the single leaflets that accompany the flower clusters.
Many factors contribute to sex determination in flowering Cannabis plants. Under average conditions, with a normal day length of 12 to 14 hours, a Cannabis population will flower and produce approximately equal numbers of male and female plants, their sex determined by X and Y sexual inheritance (see below and Chapter 11). Monoecy (male and female flowers on the same plant) is an aberration used by breeders to create relatively stable monoecious hemp cultivars. Artificial selection under cultivation for sex-related characters such as female flower form and seed size may also result in abnormal sex ratios (see Chapters 2 and 9). However, under conditions of extreme stress, such as nutrient excess or deficiency, mutilation, extreme cold or radically altered light cycles, populations have been shown to depart greatly from the expected one-to-one male-to-female sex ratio, and cosexual individuals of many different phenotypes (observable traits) may arise. Environmental influences and genetic mechanisms affecting monoecious sexual differentiation in Cannabis are poorly understood.
Pollination of the female flower results in browning, shriveling and eventual loss of the paired stigmas and a swelling of the tubular bract inside which the fertilized ovule is enlarging. After approximately three to six weeks the seed matures and after some time is harvested and dispersed by humans or drops to the ground. This completes the normally four to six month life-cycle that may take as little as two months or as long as ten months, varying according to its biotype (group of organisms sharing the same genotype) or ecotype (group within a species sharing similar ecological adaptations) as well as ambient environmental conditions. Fresh and fully mature seeds approach 100% viability, but this decreases with age. For example, usually at least 50% of the seeds will germinate after three to five years of storage at room temperature, but without refrigeration, viability of seeds rarely exceeds 10 years (Clarke personal observation). On the other hand, uninterrupted freezing can preserve seeds for decades. We would expect viability over time to be much lower under most natural conditions.
The mature, achene fruit (seed) is partially surrounded by the bract. The calyx is reduced to a seed coat variously patterned in gray, brown or black. The seed is slightly elongated and compressed, measuring two to six millimeters (1/12-1/4 inch) in length and one to four millimeters (1/24-1/6 inch) in maximum diameter. Seed weights vary from 600 seeds per gram (16,800 seeds per ounce) in wild varieties to very large seeds comprising only 15 seeds per gram (420 seeds per ounce) in cultivated varieties. Larger seeds have long been used as edible grains. Cannabis seed provides an excellent nutritional source of easily digestible protein and essential fatty acids (EFAs, Deferne and Pate 1996, see Chapter 5). Now our focus turns to some of the more significant environmental aspects of Cannabis adaptation, growth and development.
Ecological requirements of Cannabis: Sunlight, temperature, water and soil
Relationships between an individual plant and its environment are complex, and for most plants, poorly understood. Environmental conditions, in association with the genotype, determine what the phenotype will be, i.e. the actual individual organism. We know that a number of different genetic and environmental variables affect the morphology and physiology of a Cannabis plant. Key environmental factors influencing growth and development of Cannabis plants include sunlight, temperature, moisture and soil condition.
Although Cannabis plants are thermophilic (warmth-loving) and heliotropic (sun-loving), they are more tolerant of shade than many crop plants and may survive in shaded areas, but their biomass and production of pollen and seed will be greatly reduced. Cannabis thrives best in exposed places where it does not have to compete with taller plants for available sunlight. Cannabis plants find a suitable habitat for their energy needs in open environments, such as scars in vegetation created by stream erosion, landslides and various forms of human landscape alteration. Disturbed soils are vital for proper establishment of feral populations, which are derived from escapes from cultivation and are comprised of self-sown, naturalized individuals. If seeds cannot find a crevice in the soil in which to sprout safely they will be eaten by birds or small mammals; if they do germinate atop the soil, they will dry out and die if their roots cannot find moist soil to penetrate.
Cannabis can become acclimated to high temperatures if sufficient water and nutrients are available, but it does not tolerate extreme cold. Seedlings and young plants are more frost resistant than plants nearing maturity. At higher latitudes, hemp is traditionally planted in late spring and harvested at the end of the short summer, avoiding the cold temperatures and short day length of the low sun, autumn.This climatic adaptation indicates that Cannabis is probably native to a northern temperate region where it can successfully complete its life cycle between spring and autumn without experiencing lethal frosts.
Impact of temperature variation is regulated by the transpiration rate of the plant, or how fast it loses moisture. In hot dry climates, Cannabis' high transpiration rate makes it very susceptible to wilting. However, the pubescence of glandular trichomes concentrated around the inflorescences, especially of female flowers, helps protect vital reproductive tissues from drying out, slowing water loss by producing a lower surface temperature and slowing transpiration.
Cannabis needs relatively small amounts of water to merely survive, except during germination and establishment. It flourishes on well-drained soils where ample supplies of water are available; on the other hand, stressful arid conditions or waterlogged soil can cause severe stunting and death. Cannabis matures and reproduces under a wide range of moisture regimes, especially in sub-humid to moderately arid conditions. However, water deficiencies negatively affect root proliferation, branch and leaf development, flower formation, seed production and resin secretion.
The Farmer's Cyclopedia (United States Department of Agriculture1914) briefly described the moisture requirements for fiber Cannabis cultivation in the temperate continental United States, providing us with insight into its natural adaptation:
"Hemp requires about 110 days for its growth. It should have a rainfall of at least 10 inches [25 centimeters] during this period. If the level of free water in the soil is within 8 to 10 feet [2.5-3.0 meters] from the surface, as is often the case in alluvial river-bottom lands, and the character of the soil is such that there is good capillary action to bring the water up, hemp will not suffer from drought, even should there be very little rainfall."
Cannabis plants display prominent adaptations to a variety of moisture conditions that relate closely to its differing uses. For example, the long fibrous cells in the stalk are much more durable and flexible when grown under mild humid conditions (Klages 1942). Where moisture stress is high, as in hot and dry environments, these same cells are less well developed and more brittle. This difference in stalk cell formation is an important consideration when the plant is cultivated specifically for strong and flexible fiber.Seed yield and quality are also lower in crops starved for water. Cannabis must have evolved in at least a seasonally moist and temperate region with warm, wet summers, such as areas with continental temperate or sub-tropical climates.
Cannabis plants also need well-drained soils. This is an important ecological requirement, as the roots are attacked by various fungi and cannot tolerate standing water. Cannabis is generally a tall plant growing in open environments, and the extensive root system needs a friable, but nutrient-rich soil to allow proper root growth, adequate drainage and efficient uptake of vital soil minerals. Under natural conditions Cannabis grows best in sandy and loamy alluvial (river valley) soils, and theseedaphic limitations help us determine its original geographical origin.
Cannabis origin and evolution studies
Humans have been attracted to Cannabis for a very long time, resulting in its wide distribution and multiple uses. Generally, we assume that the longer people use a plant, the greater the number of applications they will find for it. Cannabis has been used for millennia as a fiber, food and drug plant and ranks among the very oldest of economic plants. Many varieties of Cannabis have evolved through the pressures of natural selection within the diverse environments into which humans have introduced it, compounded by varying human selective pressures to provide hemp fiber, seed or resin. We should point out here that a controversy surrounds the taxonomy of Cannabis, which has been classified either as a monotypic genus, containing only a single species, Cannabis sativa, or a polytypic genus, including up to three species, Cannabis sativa (NLH and NLHA), Cannabis indica (NLD, NLDA, BLD and BLH) and possibly Cannabis ruderalis (PA) which is the taxonomy we support (see the Table of Acronyms and also Chapter 10for a more detailed discussion of Cannabis taxonomy). In any case, we suggest that there are three population types for Cannabis plants based on their natural origins and associations with humans: (1) those that are truly wild, (2) those that are cultivated and (3) those that grow spontaneously in areas associated with (and often disturbed by) humans, either derived from wild populations or from feral escapes from cultivation. We rely on the ecological requirements and reproductive strategies of Cannabis to offer clues as to which regions it inhabited prior to human contact. The present geographical distribution of truly wild and feralpopulations should also provide us with a good indication of the geographical region, or at least the ecological conditions, within which it evolved.
The first criterion when searching for the geographical origin of a cultivated plant is to determine the range of its truly wild growth (de Candolle 1967). At first this may seem straightforward, with relatively easy solutions resulting from a survey of herbarium specimens, biodiversity surveys and guidebooks to native floras. Cannabis, however, is particularly difficult to study in this respect as it was among the very early plants to be cultivated and spread by humans. Consequently it has escaped from cultivation repeatedly and has become naturalized (feral) in a wide range of environments throughout Eurasia and North America.Early Cannabis is characterized as a weedy camp follower, living on nutrient-rich dump heaps associated with human occupation and as such was pre-adapted to cultivation (Anderson 1967, Merlin 1972). Consequently, it is difficult for observers to accurately determine if a self-sown population of Cannabis is truly wild, and therefore indigenous to a region, or if it is growing spontaneously as a feral escape from ancient or recent cultivation.
Cannabis is particularly adept at naturalizing to a range of temperate and sub-tropical climates. The contemporary geographical range of Cannabis in all its biotypes (ecotypes, subspecies or varieties) is immense, and it grows spontaneously or cultivated, or both, in many regions. If a plant was recorded in a region and at a later date has vanished, it may be assumed that it was either not indigenous to that region and only introduced for a time, or was native but became extinct in that part of its original truly wild range. Conversely, because a plant maintains its spontaneous growth in an area does not necessarily mean that it is indigenous to that region; as an introduced species it might, in fact, have found a niche favorable for its continued proliferation and become naturalized and even invasive. For example, Cannabis is found today growing as a weed along streams, drainage ditches and farm fields across temperate continental areas of North America where it was introduced from Europe in the 17th century.
The diversity of Cannabis populations, both in terms of morphology and economic usage, varies from region to region. Areas of rich diversity are often interpreted as probable places of origin, or at least areas with lengthy periods of naturalization, since increased diversity can be a product of increased time during which to diversify. However, great diversity within a region is not always a sign of antiquity. Alien plants often evolve quite rapidly under a new set of natural and/or human cultural (artificial) selection pressures encountered in new habitats and can diversify extensively in a relatively short time.
The famous Swiss botanist Alphonse de Candolle (1967) postulated that agricultural crops in particular are subject to sudden and often radical evolutionary pressures of human selection for a particular plant product such as fiber, food or drug. A cultivated plant varies from its wild ancestor primarily in those economically or culturally valuable characteristics for which it is grown and selected. Feral escapes from cultivation will often vary in these same characteristics. Other characteristics tend to vary much less, as they are of less importance to the farmer, and thus are not as affected by careful scrutiny and selection. De Candolle's basic principles accounting for morphological changes in crop plants during domestication still hold true. However, physiological changes are probably of greater importance as plants adapt to new ecological conditions but are harder to recognize as they leave no direct fossil evidence.
As we have noted, Cannabis grows in a wide variety of areas across distant regions of the world and thrives in temperate continental climates. But because its distribution is often so closely associated with human settlements or trade routes, the original native range is obscured. Today it is widely believed that Cannabis is indigenous to some area in the broad region referred to as Central Asia (e.g., Vavilov 1931, Schultes 1969a/b, Merlin 1972, Damania 1998).
Central Asia: Vavilov and the origins of Cannabis
Parts of Central Asia (from the Caucasus to the Altai Mountains), South Asia (through the foothills of the Himalaya and Hindu Kush Mountains) and East Asia (in the mountainous Hengduan-Yungui region or along the Yangzi River and Huang He (Yellow River) of present-day China) have all been proposed as possible locations for the area of natural origin and/or primary domestication of Cannabis, and all these regions likely played a role in Cannabis evolution at one time or another. Exact geographical origin is unclear today because Cannabis' range shifted repeatedly during glacial-interglacial cycles covering hundreds of thousands of years. Perhaps soon after Holocene warming began about 12,000 years ago, or later during the advent of agriculture, it was spread across Eurasia by humans. In any case, we believe Central Asia offers by far the most plausible location for the primary origin and early evolution of Cannabis.
De Candolle (1967) stated that Cannabis occurs "wild" only south of the Caspian Sea, in Siberia near the Irtysch River, and in the Khirgiz Desert beyond Lake Baikal; he also suggested that it was first cultivated in southern Siberia. The Indian Hemp Drugs Commission Report (1893-94; see Kaplan 1969) identified a broad area encompassing the southern Himalayan foothills from Kashmir through Nepal and northeastern India as the region of spontaneous growth. Currently, the range of self-sown growth extends throughout Eastern Europe into the western and central regions of the former Soviet Union and across northern South and Southeast Asia. Cannabis grows spontaneously, as well, in its introduced ranges in parts of Africa south of the Sahara Desert and in parts of temperate, central North America (Hulten 1970)and as a weed in farm fields and disturbed niches across temperate China where it has escaped from cultivation (International Association of Agricultural Economists 1973).
Fieldwork and theories of the famous Russian botanist, Nicolai Ivanovich Vavilov (1931) added considerably to our understanding of crop plant origins. Vavilov studied phenotypic diversity (variation of observable traits) within Central Asian Cannabis, made many first-hand observations of the genus and usedit as an example of how to differentiate between a genuinely wild plant and a more recent escape from cultivation. Some of these characteristics of domestication are also found in wild cereals. His four criteria for identifying wild Cannabis were as follows:
1 - Germination of seed is slow and irregular,
2 - Seed coat [reduced perianth] persists as an outer husk around the seed, developing a camouflaging pattern,
3 - Seed has oil glands, and these attract various insects that remove and distribute them,
4 - Inflorescence shatters and distributes the seeds.
Vavilov and Bukinich (1929) reported that weedy Cannabis occurred commonly in irrigated parts of Afghanistan. More importantly, Vavilov (1931) also wrote about his 1929 visit to Chinese Turkestan to look for evidence of proposed origins of several wild and cultivated plants. Chinese Turkestan, in present-day Xinjiang province of China, lies north and northwest of the Himalayan Mountains and Qinghai-Xizang plateau, southwest of the Tian Shan Mountains and northeast of the Pamir Plateau; it is separated from the whole of China by the Taklimakan Desert to the east. Vavilov reported numerous thick stands of cultivated Cannabis in valleys of Chinese Turkestan and along the slopes south of the Tian Shan Mountains as well as its occurrence as a common weed throughout the Russian provinces of Irkutsk, Omsk and east to the Amur River. He concluded that the majority of the cultivated plants of the region were predominately imports from China to the east, or Afghanistan and Pakistan to the southwest. However, Vavilov considered Cannabis to be a native crop that originated in Central Asia.
Vavilov characterized wild and weedy Cannabis populations from Chinese Turkestan and northern Central Asia (1931) as "shattering forms with a horseshoe at the base of the fruit, with seeds of different size, up to the dimensions of the cultivated large-seeded forms." Wild hemp, according to Vavilov, was utilized only occasionally by local people for the manufacture of cordage, and he commented that the people of Central Asia extracted hemp fibers in a most primitive way and without retting-merely pulling the fibers from the dry stalks. Its utilization, however, was especially extensive in the Altai Mountains, and he surmised that it was there that wild hemp could have been a likely candidate for cultivation near settled populations. Vavilov observed what may have been a vestige of ancient hunter-gatherer (by then pastoralist) use of Cannabis, collecting wild hemp fiber in the mountains at the beginning of autumn before moving into lower valleys to avoid the cold winter. Similarly the Nu, an ethnic minority of part-time pastoralists living in Yunnan province, China, will sow hemp seeds along ridges in early summer while grazing their livestock and leave the crop unattended until they return in autumn to collect winter fodder, when they thresh the hemp seeds, strip off the bark and haul it back to town for processing, spinning and weaving (Clarke 1996, personal observation).
Vavilov and Bukinich (1929) generally characterized Afghan Cannabis as short in stature with short internodes and profuse branching from the first node. Eastern Afghan varieties were described as having small leaves with egg-shapedleaflets with their narrow ends toward the base, and extremely small dark-colored seeds that shattered and dispersed easily-characteristic of wild plants. Vavilov termed this wild Cannabis of eastern Afghanistan, which commonly had dark gray seeds with a marbled seed coat pattern, C. indica var. kafiristanica. Hillig and Mahlberg (2004) preserved this name and assigned it to wild and feral narrow-leaf drug ancestor (NLDA) biotypes. A second variety was described with a colorless seed coat and named C. indica variety afghanica. Hillig and Mahlberg (2004) also preserved this name to represent broad-leaf drug (BLD) biotypes, which they called wide-leaf drug biotypes (see also Chapter 10). Vavilov concluded that Afghan Cannabis varieties were entirely different from both wild and cultivated European and Asiatic Cannabis and therefore must be considered as varieties of C. indica Lam. Vavilov also pointed out that "C. sativa L." of the European type was cultivated for hashish in northern Afghanistan. We recognize this as the C. indica ssp. indica narrow-leaf drug (NLD) biotype based on its resemblance to European hemp (tall with narrow leaves) and its high THC content (Hillig and Mahlberg 2004).
Earlier, Russian botanist D. E. Janischevsky (1924) described and published descriptions of a new species, Cannabis ruderalis, growing wild in the Volga River region, Western Siberia and Central Asia. Hillig (2005a/b) recognized C. ruderalis as the putative ancestor (PA) of C. sativa (see Chapter 10). However present-day narrow-leaf hemp (NLH), narrow-leaf ancestor (NLHA) and PA populations overlap in range and traits and there is no clear differentiation between the taxa (Hillig and Mahlberg 2004). Janischevsky's work was part of a large-scale Soviet agricultural research program carried out under the direction of Vavilov during the 1920s and 1930s. Vavilov, with help from a team of experts, conducted an extensive series of expeditions to many continents, collecting information that contributed to identifying and understanding regions of species diversity, which Vavilov argued were the areas of species formation. De Candolle (1967, see above) first used this criterion, although he did not rely so heavily on it, and took a more comprehensive approach in his attempt to determine Cannabis origins, integrating a greater variety of sources than Vavilov.
Based on the work of Janischevsky and others, Vavilov (1949-1951) classified Cannabis as indigenous in three major "centers" of species formation described below. These "centers" we may categorize with hindsight as areas of hybridization induced by relatively intense, directional, artificial selection for desired crop characteristics as well as regions of trade and exchange rather than areas of evolutionary origin. Each of these factors can promote variation within a cultivated species. Under the category of fiber plants, Vavilov placed varieties of"Cannabis sativa"that produce large seeds, which we now consider broad-leaf hemp (BLH) biotypes after Hillig (2005a/b), in his "Chinese Center" of cultivated plants which includes the mountainous regions of central and western China and adjacent lowlands. Under the category of spice plants and stimulants, Vavilov listed"Cannabis indica," which we now consider to be a NLD biotype (Hillig 2005a/b), as originating in the so-called Indian Center, which includes all of the Indian sub-continent except northwestern India, Punjab and the Northwest Frontier (now a part of Pakistan). Finally, under the category of grain crops in his "Central Asiatic Center," Vavilov again listed "Cannabis indica," which we recognize as BLD biotypes, once again after Hillig (2005a/b). This comparatively small area includes northern Pakistan, all of Afghanistan, the Central Asian Republics of Tajikistan and Uzbekistan as well as the western Tian Shan Mountains. We propose that these three centers were more likely areas of early agriculture and selection for specific uses (food, drug and fiber) following the early Holocene dispersal of Cannabis throughout Eurasia.
Although Vavilov's evidence for centers of agricultural crop diversity is convincing, his interpretations of the evidence are not as well accepted (e.g., see Merlin 1972). The use of phenotypic diversity as a key to plant origins has been thoroughly challenged in recent decades. For example, the idea of a "center of origin" might be intellectually satisfying, but it does not always follow as a logical conclusion from an analysis of the data. Although patterns of variation can supply valuable information about the genome of a crop, the question of agricultural and species origins, "is much too complex to be solved by such a simple device, and every scrap of evidence is needed from any source that might be even inferentially pertinent" (Harlan 1971). Phenotypic change upon dispersal away from the area of origin and into an introduced environment is common and has obviously occurred in the case of Cannabis. In addition, evolution, and thus variation, has been greatly accelerated by human selection during domestication.
David Harris (1967) emphasized the significance of crop hybridization with weeds in the evolution of species diversity and argued that a large number of weedy plants "are derivative from, rather than ancestral to, their associated crops, and consequently Vavilov's centers of maximum diversity are not necessarily centers of primary domestication." Edgar Anderson (1967) in his informative book exploring the antiquity of the ongoing important relationship between humans and plants suggested that Vavilov's areas of greater variability are places where flora previously separated came together and hybridized. Indeed the continued existence of primitive varieties of cultivated plants among traditional peoples, often found in remote areas (such as regions of temperate Eurasia where spontaneously growing Cannabis is found today), is probably a result of the basic conservatism of these isolated peoples. Thus Vavilov's ancient centers of species formation may very well be centers of early human cultural and agricultural survival rather than centers of origin. Actually, Vavilov (1931) suggested this possibility when he discussed the origin and distribution of Cannabis in Central Asia. At first, he asserted that Cannabis was most likely one of the few indigenous crops of the area:
"The autochthonic [indigenous] crops of Central Asia are few, but still such ones may be found. Of the field crops the first to be mentioned is hemp. All over [the] northern Tian Shan [Mountains], on its slopes, in the valleys to the north of it, wild growing hemp is of common occurrence. The waste lots of the town of Yarkand in Xinjiang province Chinaare covered with thick stands of hemp. It grows on the ridges of fields, not infrequently forming broad borders along the roads. In ravines, on forest skirts, on marshy ground, on waste land near the villages - weed hemp is the commonest of plants."
But then Vavilov reevaluated this assumption in the very same paperand noted that there is also good reason to believe that hemp is not endemic to Central Asia:
"We admit that the introduction of hemp, as of a wild growing plant characterized by a vast area stretching from the southeast of European USSR to the Pacific, has taken place simultaneously, as well as at different times, in different regions. It may as well have taken place in the agricultural districts of Central Asia."
The former Harvard University economic botanist Oakes Ames (1939) referred to scholars of his time who generally believed Cannabis "to be indigenous to the temperate parts of Asia near the Caspian sea, southern Siberia, the Kirghiz Desert and Persia." Ames' student, Richard Evans Schultes, who eventually took Ames' position at Harvard, stated that Cannabis is "...one of the most ancient of cultivated plants [and] is native probably to Central Asia" (Schultes 1969a/b).
Even though the arguments for hemp being endemic to Central Asia are not conclusive and, in fact, the origin and first use of C. sativa and C. indica may have occurred elsewhere, we suggest, with the same cautious reserve as Vavilov, that its natural origin was probably in Central Asia, possibly in the upland valleys of the Tian Shan or Altai Mountains and that very early, if not the first, cultural applications of Cannabis took place in this same general area during the Pleistocene. If Cannabis originated in Central Asia, it would have been ideally situated for migration east into eastern Asia and west into Europe as Pleistocene ice sheets advanced (see Chapter 11).
Cannabis and Vitis
Here it is relevant to briefly review the biological evolution and domestication of common grapevine (Vitis vinifera L.) which was derived from the wild grapevine (Vitis vinifera subsp. silvestris). Evolution of this useful plant, under the processes of domestication, provides us with a model which has strong parallels to that of Cannabis. Vitis is similar to Cannabis in many respects, including its biology, reproduction, geographical origins and human influences that include long-distance seed dispersal followed by localized distribution of select individuals via asexual propagation. The grapevine is much farther down the vegetative domestication path than Cannabis, and therefore an understanding of grapevine history and its interaction with humans may help us predict the future of Cannabis evolution.
Vitis vinifera is the only member of the grape genus indigenous to Eurasia, possibly originating in the Near East (e.g., see Myles et al. 2010), with evolutionary origins dating back to around 65 million years ago (This et al. 2006). Following the last glacial maximum (LGM) about 18,000 years ago, grape populations began spreading northward from the Italian Peninsula and also westward from the Caucasus, resulting in some admixture in central Europe (Grassi et al. 2008). Presently, the truly wild form, V. vinifera subsp. silvestris, is relatively rare. It is occasionally found in environments from sea level up to 1000 meters (about 3300 feet) in elevation all the way from the southern Atlantic coast of Europe to the western Himalayas, and from Portugal in the west to Turkmenistan in the east, and the Rhine River valley in the north to Tunisia in the south, it grows as a vine on the surrounding tree canopy (This et al. 2006). The common, domesticated grapevine is one of the oldest fruit crops; it is cultivated extensively worldwide and is of great economic importance in its use for table fruit, raisins, sweet preserves, juice and wine. Its domestication, occurring between 9000 to 7500 years ago in the Near East, with the earliest archaeological evidence for this in northern Iran, Georgia and Turkey, was coincident with discovery of wine (This et al. 2006, also see McGovern 2003). Domestication brought many changes to grape's agronomic traits including greater fruit yield and sugar content. Truly wild grapevines are dioecious and wind-pollinated with bird mediated dispersal (similar to Cannabis) while domesticated grapevines are self-pollinating hermaphrodites (Grassi et al. 2008). How did this change come about?
Selection for higher yield, more sugar content and determinant maturation resulted in changes in berry color, berry and bunch size as well as a crucial change from dioecious to hermaphrodite sexuality; this eliminated the need to maintain male plants as pollinators and allowed the self fertilizing of mutant phenotypes. By 5500 to 5000 years ago early domesticates were spread by humans to Egypt and Lower Mesopotamia, followed by dispersal into several Mediterranean cultural realms, especially the Roman Empire, eventually reaching as far as China and Japan by 200 CE. In the process, humans shaped the diversity of grape cultivars extant today. Long-range transport was facilitated by seeds, new cultivars arose from sexual crosses between seedlings, and unique offspring with favorable characteristics were multiplied asexually, producing populations of identical clones (This et al. 2006). The domestication process presently followed in indoor drug Cannabis production is much the same-sexual crossing of pollen and seed parents to produce genetically diverse seeds which are transported to new environments, sown and grown with only a few select female plants reproduced asexually through rooted cuttings, thereby fixing the selected traits and allowing no further evolution.
As a result of centuries of exchange of genetic material (seeds and cuttings), it is difficult to determine the original home of widespread domesticated plants such as grape and hemp. Wild-growing grape populations are documented from many regions of Europe, but it is often unclear if they are truly undomesticated silvestris rather than vinifera cultivars growing as feral escapes from cultivation or possibly hybrids resulting from crosses between wild and cultivated plants (This et al. 2006). This situation must also be rectified in Cannabis before its evolutionary pathways can be deciphered. Are there any truly wild progenitor populations of Cannabis extant today?
Theories for South Asian origin of domesticated Cannabis
South Asia also presents another possible location for the origin and/or early domestication of Cannabis. Used in preparation of a ritual drink known as bhang, Cannabis was referred to in the ancient Indian Atharva Veda or "Science of Charms" (written sometime between 4000 and 3400 BP) as one of the "five kingdoms of herbs....which release us from anxiety" (Abel 1980, see also Booth 2003 and Chapter 6). Carolus Linnaeus (Carl von Linné), the "Father of Taxonomy," who first used the Latin binomial Cannabis sativa believed it to be native to India although he never collected or categorized specimens from this area. The great diversity of Cannabis varieties and usages in northern India and Nepal along the foothills of the Himalayas may indicate that this region was one of the first areas where Cannabis was extensively utilized, most likely for mind-altering purposes.
Sharma (1979, 1980) used phenotypic diversity as a major criterion in his conclusion that Cannabis originated in the valleys along the southern slope of the Himalayan Mountains from Kashmir through Nepal and Bhutan to Burma. He noted that wild (or nearly wild) populations occur in relatively unpopulated areas throughout the Himalayan region, and that significant variation can be measured between glandular trichome characteristics and epidermal (leaf surface) patterns of populations from differing climates. He did not, however, offer evidence that leaf surface traits are sufficient taxonomic criteria to determine races of Cannabis. Furthermore, like Vavilov, Sharma assumed that an area with the greatest diversity within a species is also the area in which the natural origin of the species occurred, rather than recognizing that such variation may be derivative instead of ancestral. In other words, if Cannabis was introduced to the southern slopes of the Himalayan Mountain range and then intensively cultivated, the evolution of many varieties through artificial selection and hybridization by humans, in conjunction with substantial ecological variation along steep elevation gradients, may have occurred subsequent to its introduction. In addition, it should be remembered that perceptions of significant variation are often subjective. Whether it was Vavilov or Sharma who observed the "most" variation in spontaneously growing populations they investigated (in Central Asia and the Himalayan foothills respectively) is impossible to determine by studying their reports, and neither traveled in the study area of the other.
Although we have argued that Cannabis evolved naturally in Central Asia, if Cannabis did originate in northern South Asia, it most likely would have evolved along or relatively near streams in the Himalayan foothills. According to our Holocene dispersal scenario Cannabis arrived in this region early on as it expanded westward from the Hengduan Mountains and Yungui Plateau in southwestern China. Much later, traders could have carried Cannabis west to the Middle East. NLD varietieseventually spread westward by sea traders to the east coast of Africa and eastward through Burma into Southeast Asia. Following this scenario further, NLH would have evolved at higher latitudes from South Asian NLD varieties and spread farther north into southern Russia and then west into Europe. Some varieties could have migrated so far north that the summer season was too short to produce psychoactive levels of THC and evolved into fiber or seed varieties under human selection. Under this scenario, the PA, C. ruderalis, collected during the 20thcentury, in turn, would likely have evolved from C. sativa as this species spread farther north into the Central Asian region formerly known as Turkistan. However, there is little evidence to support this scenario and several reasons to doubt it.
The massive Himalayan and Hindu Kush Mountains, which have proven such a mighty barrier to plant and animal dispersals (including humans), lie between the origin regions proposed by Vavilov and Sharma. No examples of crop plant co-origin both north and south of the Himalaya and Hindu Kush Mountains have yet been reported (Simmonds 1976, Smartt and Simmonds 1995). In addition, genetic data does not reveal any links between South Asian NLD and European NLH populations except those resulting from more recent hybridization influenced by cultivation and breeding (Hillig 2005a/b). Although Cannabis now grows spontaneously throughout Eurasia, but not necessarily as a native plant, it seems unlikely to us that the genus originated both north and south of these mountain ranges. However, human migrations spread Cannabis throughout the Himalaya and Hindu Kush Mountains early in prehistory, probably starting sometime after the beginning of the Holocene, but possibly much earlier as anatomically modern humans (AMHs) first began their advance across Eurasia.
Models of early use and domestication (see Chapter 2), archaeological data (see Chapter 3) and historical records (see Chapters 4 through 8), in conjunction with evolutionary studies involving reproductive strategies and geography (see Chapter 11), lead us to conclude that Cannabis originated somewhere in Central Asia, rather than South or East Asia, although these regions may have served as glacial refugia where speciation occurred. China and India were both regions of early Cannabis evolution under domestication and foci for later diffusion, resulting in the broad diversity of phenotypes selected for various uses appearing across both East and South Asia. In the following discussion we evaluate the, at times, seemingly contradictory data and opinions in a temporal framework, then rectify many of the discrepancies and propose a hypothetical model for the early evolution of Cannabis.
Model for the early evolution of Cannabis
How long ago did Cannabis originate, and when did AMHs begin their association with these useful plants? We know that the earliest angiosperms (flowering plants) probably evolved more than 140 million years ago (e.g., see Soltis et al. 2008); early humans appear to have evolved into Homo sapiens in Africa about 200,000 years ago (e.g., University of Utah 2005, McDougall et al. 2005), and AMHs began extensively colonizing the Middle East during the Upper Paleolithic about 45,000 to 40,000 years ago reaching the steppes of Central Asia and highland southern East Asia by about 35,000 years ago (Wells 2002, Finlayson 2005). Migrations then radiated outward reaching Europe and South Asia by about 30,000 years ago and northeastern Asia by around 15,000 to 20,000 years ago (Meltzer 2009, Kunzig 2004, also see Wells 2002). The ice age of the LGM reached its peak about 21,000 to 18,000 years ago (Soffer and Gamble 1990, Otto-Bliesner et al. 2006) and the warming Holocene epoch began about 12,000 years ago (Roberts 1998). Early farming commenced relatively soon after the end of the Pleistocene and spread widely from a series of centers in the Old and New Worlds (Bellwood 2005). The timing and location of the earliest cultivation of Cannabis, as with most plants, may never be completely ascertained, and although we do not have archaeological evidence for very early cultivation of Cannabis in Central Asia (probably due to the lack of sufficient research in that general region), we do know that hemp was planted quite early on in China and most likely much later in Europe (see Chapter 3 for a full discussion of cultural spread and early farming of Cannabis, and Chapter 11 for a detailed look at climate change and glacial refugia).
When and where along this continuum did (1) family Cannabaceae appear, (2) Cannabis and Humulus diverge and (3) Cannabis' species evolve? When was the natural evolution of Cannabis first affected by human contact? How did the various subspecies, biotypes and ecotypes evolve? Answering these questions will allow us to advance our hypothesis for the early evolution of Cannabis. In the absence of pre-Holocene Cannabis seeds, limited ancient pollen (which may be hard to identify with certainty, see Chapter 3)and without fossils of a clearly identified Cannabis progenitor, it is difficult to determine with any accuracy when Cannabis evolved into the biotypes we see today. The survey of reproductive strategies presented above indicates that Cannabis, an herbaceous, sun-loving, short-day flowering annual, most likely evolved somewhere in temperate latitudes of the northern hemisphere, and data from published research favors Eurasia, especially Central Asia, as its region of origin. Future DNA research and additional forms of molecular genetic investigation may help to more accurately determine the original home of Cannabis.
In the meantime, a review of evidence for the origin and prehistoric dispersal of Cannabis offered by the disciplines of palaeoclimatology, archaeology and taxonomy supports our model for the evolution of Cannabis. During the last interglacial period (approximately 135,000 to 110,000 years ago) the northern hemisphere, including the vast region of Eurasia, was relatively warm and humid; it is somewhere within this huge area that the ancestors of modern Cannabis and Humulus would have found environmental niches suitable for their evolution and proliferation. Around 50,000 years ago, AMHs began migrations northward out of Africa into middle Eurasia where their populations thrived and multiplied, eventually spreading both west and east to occupy vast areas of the earth's landmass (Kunzig 2004). This middle Eurasian cauldron of human evolutionary and cultural change lay within the natural range of Cannabis, and Pleistocene early humans would have been attracted to its readily apparent attributes.
Our assertion that Central Asia was both the original Pleistocene home and center for evolution and dispersal of Cannabis within the past 50,000 years is supported by our reconstruction of the climatic conditions across Eurasia during past geological periods. In order to further explore the human-Cannabis relationship, it is also important to determine in which regions early people may have lived nearby Cannabis populations. This can be ascertained by present-day human genome analysis combined with palaeoclimate reconstructions. Adams and Faure (1998), in their survey of plant and animal remains from various time periods and geographical locations, made correlations with the environmental requirements of extant species' relatives and produced a map series of reconstructed world vegetation. Gepts (2004) listed Cannabis as originating in the temperate steppes biome. Possible habitats conducive to the growth and spread of Cannabis during the early Holocene are based on ranges of vegetation zones supporting feral growth today. These coincide with three palaeoenvironmental classifications: (1) cool, temperate, deciduous broad-leaved and coniferous forests with a fairly open canopy, (2) semi-arid temperate woodland or scrub and (3) herbaceous forest steppe with clumps of trees in favorable locations. These vegetation zones existed at each time period reconstructed by Adams and Faure, but their ranges shifted between different time periods and they occurred in different geographical regions than today.
Archaeological sites provide physical evidence that bands of hunter-gatherers were living in these regions during the Upper Paleolithic (50,000 to 10,000 BP) many millennia before the LGM (e.g., see Madeyska 1990). As climate cooled leading up to the LGM and early humans migrated southward, they could have taken Cannabis seeds with them. After PA populations migrated southward and diverged geographically, two populations may have survived in two separate isolated locations and evolved into two new species-in temperate foothills of southern and southeastern European mountain ranges the putative hemp ancestor (PHA) and progenitor of modern C. sativa and in temperate mountain valleys of southern East Asia the putative drug ancestor (PDA) the progenitor of modern C. indica. After several millennia, as northern latitudes began to warm and the Holocene commenced, early humans could have returned northward carrying the progenitors of modern Cannabis taxa across much of Eurasia from their twin origins. Following the LGM, and throughout the early Holocene, the Magdalenian and Gravettian cultural complexes of central Europe expanded to the northeast onto the northern European plains and into the steppe regions of Eastern Europe, while the Solutrean cultural complex spread across Mediterranean southern Europe to the Black Sea (Bar-Yosef 1990). By this time, Paleolithic cultures were well distributed across modern-day China, Korea and Japan, and early Huang He and then Yangzi River farming cultures soon began to radiate across East Asia (Chen and Olsen 1990, also see Xue et al. 2006).
However, the divergence of C. sativa and C. indica likely occurred during much earlier glaciations and AMHs encountered Cannabis much later as it began to spread from its most recent refugia following the LGM. Speciation occurred during an earlier glacial period when advancing ice sheets pushed ancestral populations of plants and animals into more southerly refugia, C. sativa evolving in refugia in southeastern Europe and C. indica in southern East Asia, where their respective ranges were likely reduced in subsequent glacial periods leading up to the LGM. If the PA (C. ruderalis) exists today it must have survived at low population density in cryptic refugia at more northern latitudes than C. sativa or C. indica. During interglacial warming, Cannabis populations evolved naturally as they expanded northward recolonizing niches for which they were pre-adapted, only to be restricted to temperate refugia during a subsequent glacial cold period. During the LGM, European C. sativa NLH populations likely found refuge in the foothills of the Caucasus Mountains and on the Balkan Peninsula while Asian C. indica ssp. chinensis (broad-leaf hemp or BLH) populations survived within the Hengduan Mountain-Yungui Plateau region of present-day southwestern China and possibly also in coastal northeastern China, Korea and Japan; C. indica ssp. indica NLD populations may have survived in the Hengduan Mountain-Yungui Plateau region or along the Himalayan foothills, while C. indica ssp. afghanica BLD populations evolved in the foothills of the Hindu Kush Mountains. It is unlikely that any Cannabis populations survived the LGM outside of refugia, which may have been more in number than we can presently identify. After the LGM, Cannabis populations expanded once again, and their dispersal and introduction into newly disturbed niches was often aided by humans migrating from their temperate refugia. During rapid migration into new niches, the Cannabis genome narrowed from founder effects. It then diversified by ecological adaptation to each new niche. Meanwhile, populations remaining within upland refugia with varying topography likely remained genetically diverse due to individual adaption to differing microclimates within a small geographical range. Variation extant today at the subspecies and biotype levels results from relatively recent post-LGM expansion with human assistance and building upon a much more ancient evolutionary foundation. Human imposed geographical isolation and selection have proved sufficient to preserve species integrity while increasing biotype diversity.
Putative progenitor populations (PA, PHA and PDA) are the "missing links" in our model of early evolution and are very likely extinct. Due to the high probability of intercrossing with neighboring feral or cultivated populations in more recent times, it is unlikely that any genetically pure ancestral populations survive today even in remote regions of Central Asia. It is even more unlikely that there would be any relict populations remaining in the regions where the hypothetical progenitor populations of hemp and drug Cannabis (PHA and PDA) originated, as Cannabis has been cultivated for at least two millennia across Europe and much longer in East Asia (see Chapters 3 through 8). If specimens tentatively identified by Hillig (2005a/b) and others as C. ruderalis do not represent relict populations of the original PA, then how did they arise and how do they differ genetically from other extant taxa? It seems likely to us that Central Asian populations studied during the 20th century and perceived as putative ancestors were products of mixed heritage (PA introgressed with NLHA and NLH) combined with lack of human selection and ecological adaptation to marginal environments. Without human selection, Cannabis has a tendency to revert to atavistic (ancient ancestral) genetic combinations quite rapidly and atavistic traits would be expressed frequently, especially when populations are genetically isolated and subjected to increased inbreeding. Naturally growing and seemingly wild populations that could be interpreted as descendants of putative ancestors have also been observed in Kashmir (Watson personal communication 1978) as well as Shandong and Yunnan provinces in China, lowland Nepal and northeastern India (Clarke personal observations 1993, 1995, 2006, 2009 respectively).
By 8000 years ago, large tracts of northern Eurasia had a suitable temperate climate for supporting climax broad-leaf and coniferous woodland vegetation cover and allowing Cannabis to proliferate. We assume that humans spread Cannabis easily via their hunting and gathering activities and eventually introduced it into their new agricultural settlements where and when these became established during the Holocene. Responding to a constantly changing natural environment and early unconscious human selective pressures, the NLH ancestor (NLHA) slowly evolved through intermediate populations into the C. sativa interbreeding complex (NLHA-NLH) extant today in Europe and Western Asia. Uniformity of surrounding climate and vegetation and restricted latitudinal spread within a relatively homogenous cultural setting, may account for lack of genetic diversity within C. sativa. The present-day range of C. sativa NLH includes Europe and North America, yet is relatively small in comparison to the world-wide ranges of C. indica biotypes BLH, BLD and NLD (see Table of Acronyms).
In response to entirely different sets of natural and human selective pressures, PDAs also adapted and evolved as they migrated into regions that were both climatically and culturally diverse, became isolated and were exposed to a far wider range of selective pressures than PHAs. In response, C. indica evolved into three biotypes or subspecies. BLH landraces likely evolved in China very early on, in close association with the expansion of Chinese agriculture, and relatively soon spread to Korea and Japan where additional BLH populations may already have been growing if they survived the LGM; escaped feral populations can presently be found in several regions across China, Korea and on Hokkaido Island, Japan. Although the closest relatives of BLH are the highly psychoactive BLD and NLD biotypes, East Asian hemp varieties are relatively low in THC. Since there was little traditional psychoactive use following the rise of Confucianism, BLH landraces were only rarely selected for drug content in the past two millennia. BLD varieties evolved under extremely arid conditions in an isolated mountain range within present-day Afghanistan, were eventually used for producing hashish and are the most morphologically distinct of the Cannabis taxa (see Chapter 10). NLD biotypes are also high in drug content. Along the Himalayan foothills in northern South Asia NLDA populations introgress with NLD cultivars to form an interbreeding NLDA-NLD complex similar to that of the NLHA-NLH complex of Europe and western Asia. According to our taxonomy (following Hillig 2004a/b, 2005a/b), C. indica cultivars are the most geographically widespread and most widely utilized biotypes today, growing on all continents and used for recreational and medicinal drugs as well as fiber and seed production while C. sativa cultivars are presently grown only for fiber and seed on limited acreage in Europe and North America.
Summary and conclusions
Glacial ice sheets advanced and retreated many times during the earth's history, and species have either moved or perished as they advanced; survivors recolonizing their previous homelands as the climate warmed and glaciers retreated. During Quaternary glaciations Cannabis'range would have been highly restricted to two or more isolated refugia (located in distant parts of southern Eurasia or possibly within smaller cryptic refugia at more northern latitudes) with climatic conditions similar to those favored by Cannabis today. Isolation of populations during times of glacial advance could have led to speciation within genus Cannabis. There were several series of Quaternary glaciations during the past two million years as well as many Tertiary glaciations before them, and Cannabis would have moved southward during times of cold and back northward during warm periods several times during its evolution; adaptive radiation during the Holocene is only the most recent cycle of expansion. Today, possible refugia are represented by favorable microclimates where Cannabis survived to later disperse and re-enlarge its range. It is more difficult to determine both areas of origin or endemism and potential refugia in organisms such as Cannabis with widespread ecological ranges and partial fossil records, and even more difficult to determine in plants with ancient human relationships.
Feral Cannabis populations are found today growing in temperate climates at northern latitudes. These are usually characterized as warm continental regions with spring and early summer rains, followed by a dry cool autumn and accompanied by the widely fluctuating day length (photoperiod) afforded by more northern latitudes; indeed, feral Cannabis only flourishes in this narrow climate niche. The vegetation cover most favorable for Cannabis is temperate-climate upland open woodland growing in valleys with alluvial soil deposits and slopes for drainage with sufficient sunlight and summer rainfall. Suitable regions would have had moist temperate conditions during glaciations without being so near the equator as to lose short-day flowering response and cold hardiness. Humans created many favorable open habitats, but Cannabis thrived in more or less these same conditions long before we entered the scene.
Equally important in determining Cannabis'prehistoric range are the conditions itdoes not tolerate such as extreme heat, cold, aridity or humidity, heavy or waterlogged soils and permafrost. Many presently warm and humid tropical equatorial regions were arid deserts during the LGM. In addition, Cannabis could not survive too much humidity; today Cannabis does not become feral in subtropical monsoon regions. During glacial periods Cannabis'range would not have included semi-tropical and tropical regions as this is not where natural wild or feral Cannabis populationsflourish today. Mediterranean climates with cool wet winters and hot dry summers are also not conducive to the natural growth of Cannabis because it requires summer rain. Some Cannabis populations became extinct while some survived in suitable microclimates providing additional chances for isolated populations to evolve independently within their refugial ranges. Topography within large southern montane refugia is complex and local microclimates abound. Each river valley offered isolation from neighboring populations and a unique suite of selective pressures, an ideal setting for genetic divergence and speciation. Several regions of ancient Eurasia presented likely locations for Pleistocene Cannabis refugia. We propose that such favorable LGM refugia for C. sativa could have existed within the Caucasus Mountains with another for C. indica in the Hengduan Mountains and Yungui Plateau and possibly also along the Himalayan foothills as well as on the Shandong and Korean Peninsulas and the Japan Archipelago.
Most plant species have very limited distributions, so why and how has Cannabis become so widespread and abundant? Animals including humans are more mobile than plants and can migrate away from advancing ice sheets. Plant populations are much more sedentary moving no farther spatially than their propagules. During glacial advances plant populations do not move so much as just die off as the climate becomes less favorable and their range becomes more restricted. During times of glacial advance, ice sheets would expand southwards encroaching upon the expanded range of Cannabis. In populations adapting to changes nearest the ice sheets, female plants would drop their seeds nearby at the end of the season, but male plants could spread their adaptive success via windblown pollen deep into the extant population. It is only during interglacial warming that Cannabis would have expanded from its reduced refugial range. Today Cannabis is widely distributed around the world largely as a consequence of the human-Cannabis relationship but may also have been endemic in several regions of Eurasia prior to human contact. Some plants are aided in long distance transport of their seed by migrating birds and hoofed mammals, which also could have played a part in Cannabis'earlydistributional changes although humans have certainly had the greatest effect since the Holocene began. However, because Cannabis seed is not regularly disseminated by animal, water or wind vectors and most seeds remain near the seed plant, the post-glacial range of Cannabis would have expanded much more slowly without the assistance of humans.
Cannabis likely originated millions of years ago in northern Eurasia andmoved ahead of climate changes, migrating (likely without human assistance) southward during glaciations to escape unfavorable conditions. During a glacial maximum, Cannabis populations were forced into refugia in southern Europe and southwestern East Asia, possibly leading to speciation events giving rise to European C. sativa and Asian C. indica. During this time, C. indica evolved enhanced biosynthetic capacity to produce THC. Early humans utilized both C. sativa and C. indica for fiber and seed, but only C. indica has a history of drug use. Cannabis thrived during the early Holocene as the earth warmed, and with human assistance its range expanded around the world. Range expansion continues, although genetic diversity has decreased, also as a result of human influence. Self-sowing feral Cannabis presently occupies a restricted ecological belt extending around the world.
Cannabis' annual life cycle and its ecological requirements for open environments, ample water and well-drained soils favor origin in moist riverside environments. Studies of the reproductive strategies of Cannabis indicate probable evolution in northern temperate latitudes. Early researchers such as de Candolle (1967, originally published in 1882) and Vavilov (1931) favored Central Asia as the likely region of origin, in which case, Cannabis was advantageously positioned for dispersal throughout Europe, southern Asia and the Far East. More recent studies indicate that if primordial Cannabis naturally evolved in Central Asia prior to contact with humans, it must have moved to warmer, more southern latitudes several times before, and again during, the LGM, possibly carried by early human migrants and then was redistributed throughout Eurasia by humans migrating northward as climate warmed during the Holocene. Cannabis was pre-adapted for successful growth upon its return from southern refugia as it originated farther north many millennia earlier.Present-day C. ruderalis,the putative ancestor of extant Cannabis taxa, grows throughout Central Asia and most likely represents a degenerate, inbred and unselected hybrid blend of various Cannabis gene pools that survived as feral escapes, rather than direct descendants of the now long extinct ancestral population in its original home.
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