Plant and Animal Domestication Research Paper

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One of the greatest survival techniques of the human population was the domestication of their environment. Domestication, based on human preference, changed the natural behavior and characteristics of the plants and animals they chose to cultivate. The earliest examples of plant and animal domestication date back hundreds of thousands of years, but the environmental effects of those lifestyle changes are still apparent today.

Domestication of plants and animals is the most fundamental subsistence change in human history. For 100,000 years or more, humans subsisted only by foraging and hunting. Then, near the end of the Pleistocene epoch (1.9 million–10,000 BCE), people in a few regions of Earth independently began to control and breed the species they had hitherto only collected. Between 12,000 and 5000 BCE food producing began to replace food collecting. In the process humanity gained unprecedented control of the natural world, and human populations grew geometrically. But adaptive success was purchased at the price of ecological disruption, new infectious diseases, and accelerated environmental destruction.

The theory that domestication took place in a single limited area and then spread by diffusion throughout the rest of the world has been discredited. It now appears that plant and animal domestication occurred independently and repeatedly within six or more primary “hearths” in southwestern Asia, sub-Saharan Africa, peninsular southeastern Asia, Eastern Asia, Mesoamerica, and Andean South America. Later, more limited domestication took place in secondary “hearths” such as southeastern North America and southern Asia.

Locating these hearths is easier than determining why such profound adaptive changes occurred within them. The ethnographic (relating to the study of human cultures) record indicates that, as a rule, foraging peoples do not farm unless new circumstances make foraging difficult or impossible. Therefore, the transformations of food-collecting systems into food-producing ones at the end of the Pleistocene epoch may have been caused by worldwide climate changes, mammalian extinctions, sea-level rise, and apparent human population increases that also occurred around that time. Archeologist Mark Cohen (1977) argues that, although prehistoric foraging populations grew throughout the Pleistocene epoch, the pressure of population on food resources became critical only when climatic and environmental changes at the epoch’s close undercut existing foraging economies. As environments became increasingly less productive, population pressure forced foragers to intensify and expand their subsistence activities. According to economic historian Frederic Pryor (1983), such intensification might have included taming animals and tending plants in order to (1) reduce the risks arising from overreliance on increasingly unreliable wild foodstuffs, (2) supplement wild foodstuffs available primarily in one season with cultivated foodstuffs available in another, (3) utilize the labor of marginal members of societies who were unable to participate fully in the primary food-collection activities, or (4) simply add variety to the diet.

Where taming and tending were practiced over the long term on suitably malleable species, it likely resulted in domestication. Not all scholars, however, accept the “food crisis theory” and instead favor other causes, including human population growth, differentiation and specialization within human adaptive systems, or creative human manipulation of the environment.

Plant Domestication

Whatever their reasons for doing it, after people began to systematically replant the seeds they gathered, they interposed human selection between the organisms and the natural selective forces of the environment.

—A common procedure at harvest time is for the cultivator to go through his field and carefully select heads to be saved as stock seed for the next planting. The rest of the field is harvested for consumption. In this situation, selection is total. The components of the population that contribute to the next generation are those chosen by the cultivator. The rest are eliminated from the gene pool. (Harlan, De Wet, and Stemler 1976, 8)

As a result, some genetic mutations that would ordinarily be eliminated in the wild come to be favored in the cultivator’s field. In cereal grasses such as wild wheat, barley, oats, or rye, seed-dispersal mechanisms are the first to change under cultivation. Wild grains have excellent means of scattering their ripe seeds because natural selection allows only the genes of plants with efficient dispersal mechanisms to be transmitted to succeeding generations. Although genetic mutations retarding seed dispersal occur in each generation in wild populations, they are fatal to the progeny of the plants that carry them. Yet, the seeds of wild plants with retarded seed-dispersal mechanisms are easier for humans to collect. It is their seeds that are unconsciously selected by collectors. When collectors began to reseed their fields, such mutations began to survive. Reseeding may have first been done by collectors seeking to introduce wild cereals into habitats outside their natural range. Whatever the reason, when collectors began saving and planting some of the seeds they harvested, greater and greater numbers of the dispersal-retarding mutations were concentrated in the grain populations. Generations of unconscious human selection genetically altered the grain and increased its harvestability and, therefore, the productivity per unit of labor invested in its collection.

Harvesters also unconsciously favored uniform rates of seed maturation. Wild cereal seeds do not germinate at the same time but rather tend to sprout throughout the growing season. Some even remain dormant for years. Cultivated cereals are harvested all at once. Individual plants that produce the most seeds at harvest time thus make the greatest contribution to the next generation. In this way, harvesting narrows seed germination ranges and reduces prolonged seed dormancy.

A trend toward larger seeds is also common, although not entirely universal, in cereals under cultivation. The increased size of domesticated seeds is partly due to conscious human selection of the largest seeds for replanting, but larger seeds also result from seedling competition. Seed size is closely related to seedling vigor, and the seedlings that sprout first and grow fastest in the seed bed are the ones most likely to contribute genes to the next generation.

In addition to these unconscious selective behaviors, a variety of others is imposed by the cultivators’ conscious preferences in taste, color, ease of processing, and storage. As a general rule, the part of the plant most used by humans is the part most modified by human selection. Consequently, little resemblance exists between the ears in maize, the tubers in yams, or the pods in peas and the comparable organs in the wild ancestors of these plants. Domesticated plants have been sculpted by human selection. As the process continues through time, plants lose, to a greater or lesser degree, their capacity to survive without direct human intervention in their life cycles.

Domestic wheat, a cereal grass of the genus Triticum, has proven to be the most significant cultivated plant in human history. Yet, such minor genetic differences separate wild and domestic wheats that they might readily be included in the same species. This genetic proximity suggests that the domestication of wheat was both simple and rapid. Recent experiments in the measurement of domestication rates done by archeologist Gordon Hillman and plant scientist Stuart Davies (1990) indicate that wild einkorn and emmer wheats could have become completely domesticated in twenty to two hundred years. By contrast, the domestication of maize or corn appears to have taken upward of two thousand years.

Maize (Zea mays spp.) evolved from the wild grass-like perennial teosinte in Mesoamerica and northern South America. The earliest evidence of what is probably maize comes from a pollen profile dating to 5100 BCE. The earliest definite archeological maize remains were recovered at Guila Naquitz, a dry rock shelter in Oaxaca, Mexico. These tiny cobs date to about 4200 BCE and came from plants in an early stage of domestication. Maize does not appear to have been fully domesticated until about 3000 BCE. From that time forward, it was widely cultivated in North and South America. The slow pace of maize domestication is striking considering that squash (Cucurbita spp.), another New World cultigen (an organism of a variety or species for which a wild ancestor is unknown), was first cultivated between 8000 and 6000 BCE.

Although maize produces more foodstuffs per unit of cultivated land than any of the other Native American crops, it has nutritional disadvantages. Most notably, certain of its amino acids are difficult for humans to process. Native Americans learned to enhance the nutritional quality of maize by boiling dried kernels in a lime solution before grinding them into flour. Beans (Phaseolus spp.) supply many of the nutrients missing in maize, and Native Americans also learned to grow and consume the two crops together.

Although a vast array of vegetables is grown in eastern Asia, dry and wet rice (Oryza spp.) are the focus of cultivation there. In the world at large, only wheat exceeds rice in dietary importance. Although it has been suggested that rice domestication began about 15,000 BCE, the earliest archeological evidence of domesticated rice, recovered at sites from northern Thailand and the well-watered, tropical regions of southern China, is no earlier than about 5500 BCE. Rice cultivation spread to—or emerged independently in—northern China shortly after this time.

Millet is the common name for a number of drought-resistant cereal grasses widely used as human food, animal forage, and hay. Most species tilled today were apparently domesticated in northern China. As one moves from south to north in China, rainfall levels and mean annual temperatures decline. Not surprisingly, the plants domesticated in northern China contrast markedly with those in southern China. Foxtail (Setaria italica), pearl millet (Pennisetum americanum), broomcorn millet (Panicum miliaceum), and other species were probably domesticated on the upper and middle reaches of the Yellow River. After millet’s domestication, various millet races spread throughout semiarid mainland Asia and arrived in Taiwan between 4000 and 2500 BCE.

In tropical and subtropical regions of the world, cultivation traditionally centered on asexually or vegetatively reproduced root crops such as arrowroot (Maranta spp.), yams (Dioscorea spp.), potatoes (Solanum tuberosum), taro (Colocasia spp.), cassava or manioc (Manihot spp.), and fruit trees such as bananas (Musa spp.). Seeds are not necessary for their reproduction. These plants all have the capacity to regenerate tissues and structural parts, either naturally or through human cutting or grafting. By reproducing plants asexually, farmers are able to preserve desired characteristics from generation to generation. Intensive human vegetative husbandry has caused some species to lose their capacity for sexual reproduction altogether. The ease of this manner of horticulture suggests that root crop domestication by vegetative reproduction may have preceded the domestication of cereal grains and other seed crops. Domesticated root crops were present in Panama by 5000 BCE.

Animal Domestication

It is generally assumed that, confronted by humans, wild animals instinctively either fight or flee. But zooarcheologist Charles Reed (1986) contends that most, in fact, are easily tamed. In his view, the major impediment to their domestication was human hunting. Subsistence systems that minimized hunting were the most likely to have initiated the domestication process; those that specialized in hunting were the least likely to have done so. Recognizing the signs of domestication in skeletal remains from archeological sites is not often easy. Fortunately, in recent years the analysis of DNA has proved a helpful supplement to archeology.

The skeletons of dogs (Canis familiaris) dated to about 12,000 BCE and recovered from hunting camps in Palestine constitute the earliest definite archeological evidence of animal domestication of any kind. One can see why dogs were the first to enter the human orbit. Canines are willing scavengers; their wolf ancestors must have skulked around the edges of human camps since time immemorial. Eventually such skulkers would have become semidomesticated. Full domestication—and human recognition of the utility of dogs in the hunt—no doubt came later. But how much later? A recent comparison of wolf and dog DNA sequences suggests the two species diverged more than 100,000 years ago. Few prehistorians are prepared to accept such an early date for the onset of domestication. Genetic analysis of a large sample of contemporary dogs worldwide suggests that the domestic dog is most likely to have originated in eastern Asia fifteen thousand years ago and to have spread from there. Related DNA analysis suggests that New World dogs originated in the Old World and then, in company with peoples crossing the Bering Strait land bridge, entered North America near the end of the Pleistocene epoch.

Ovicaprines—sheep (Ovis spp.) and goats (Capra spp.)—were the next animal species to enter the human orbit. Archeological evidence suggests that their domestication probably occurred as early as 9000 BCE in the mountainous regions of eastern Turkey and western Iran. Wild ovicaprines were “preadapted” for life with humans; easy to tame when young, they can digest a wide range of plants that humans find inedible. More importantly, their alpha (dominant) males neither defend individual territories nor assemble exclusive harems. Ovicaprines thus can be kept in sociable, mixed-sex herds. Between about 7000 and 6000 BCE domestic goats and sheep became the principal source of meat and hides for the earliest farmers of southwestern Asia. Their dispersal throughout Eurasia and Africa after that time appears to have been rapid.

Domestic pigs are descended from the wild boar (Sus scrofa), a species originally found throughout broad-leafed, deciduous (leaf-dropping) forests from Eurasia to northern Africa. Although adult boars are dangerous, wild piglets are easily tamed. Archeological evidence suggests that pig domestication began slightly later than ovicaprines and independently in southwestern and eastern Asia.

The placid domestic cow of modern times, Bos taurus, is a descendant of Bos primigenius, the Old World wild cow or auroch (plural aurochsen). Aurochsen became extinct in the seventeenth century. Their behavior, however, can be inferred from accounts of Spanish longhorn cattle that reverted to the wild in frontier Texas and California. Longhorns were fierce, quick, willing to attack without provocation, and hard to kill. If aurochsen, which often reached 2 meters at the withers, behaved like longhorns, they must have been among the most formidable creatures encountered by prehistoric people. This leaves researchers wondering what possessed human forebears to domesticate them. Perhaps only religious motivation would have induced people to do so. Aurochsen might have been driven into natural enclosures from which individual animals were extracted for sacrifice. Eventually herds thus impounded might have been kept and bred. Whatever the motive for it, cattle domestication began about 7000 BCE in Anatolia, Greece, and parts of southwestern Asia. Significantly, archeological evidence of bull worship at around 6000 BCE has come to light at early sites such as Catal Huyuk in Turkey and elsewhere in the eastern Mediterranean. Once domesticated, cattle spread quickly. Bos taurus bones dating to about 4500 BCE have been recovered in northern Africa at Capeletti. Cattle herders expanded south into the Sahara desert during a moist period there between about 5500 and 2500 BCE. Rock paintings at Tassili in the central Sahara depict long-horned, humpless cattle of apparent Bos taurus stock. The desiccating Sahara pushed herders farther south. Domesticated cattle, presumably taurines, were present in eastern Africa by about 3000 BCE. The expansion of herders throughout sub-Saharan Africa was well underway after that date. However, recent DNA evidence suggests the possibility that native African cattle were already domesticated in sub-Saharan Africa prior to the arrival of taurine stock.

Southern Asia was a third center of independent cattle domestication. It was a primary hearth for the domestication of the water buffalo (Bubalus bubalis) and humped Zebu cattle (Bos indicus). Zebu cattle from the Indian subcontinent appeared in eastern Africa sometime after about 1000 BCE. Infusion of such stock must have improved African herds. Zebu cattle generally need less water and forage than taurines and gain back weight more quickly after droughts. Crossing Zebu with local breeds may also have produced more disease-resistant cattle capable of greater milk yields.

Cattle were the earliest pack transport and traction animals, but, following their domestication, horses (Equus caballus), donkeys (Equus asinus), and camels (Camelus spp.) performed both tasks more efficiently. Although all three were likely domesticated for their meat, hides, and milk, the subsequent mastery of riding of these species revolutionized human trade, transportation, and warfare. The donkey, apparently the first of the horse family (Equidae) to be domesticated, appears in the archeological record of Mesopotamia about 4000 BCE. DNA analyses of modern horses suggest that their domestication occurred in many centers in Eurasia. Archeological remains indicate that horses were certainly being raised on the steppes north of the Black Sea by 3000 BCE. Archeological evidence of domesticated camels is poor, but these animals are probably present in southwestern Asia by 3000 BCE.

The late Pleistocene extinctions that swept through the New World eliminated wild species, such as horses and giant camels, which might otherwise have been domesticated. The key grazing animal species to survive—pronghorn antelope (Antilocapra americana) and American buffalo (Bison bison)—make poor domesticants. Although social like ovicaprines, pronghorns differ from them in the amount of body contact they will tolerate. They are not “contact animals” but rather maintain distance between each other. Adult male pronghorns have a well-developed sense of territory. They strive mightily to keep females inside their territory and other males outside of it. These traits render domesticating pronghorns diffi- cult and herding them nearly impossible. Bison also exhibit behavioral traits that make handling difficult even for contemporary herders. Not surprisingly, both species remained wild into modern times. Thus, the number of potentially domesticable animals in the Americas was limited. Compared to the extraordinary array of New World domesticated plants, the array of domesticated animals was paltry indeed. It included only dogs, Muscovy ducks (Cairina moschata), turkeys (Mele agris gallopavo), Guinea pigs (Cavia porcellus), and various forms of New World camelids (Lama spp.) such as the llama, vicuna, guanaco, and alpaca. This dearth of domesticated animals had fatal consequences for New World peoples—a fact dramatically illustrated in Mexico and Peru by the rapid military triumphs of mounted sixteenthcentury Spanish conquistadores. But the low number of pre-Columbian domesticated animals had a positive side: it reduced Native American exposure to deadly diseases passed from animals to humans.

Silent Domestication

Domestication of plants and animals in the visible world was accompanied by domestication of organisms in the invisible microbial world. Many of these microbes were benign or useful. Yeast, for example, makes fermentation possible. Systematic fermentation is impractical, even impossible, in the absence of sealable pottery jars. Yeast domestication must therefore have been an unintended consequence of ceramic manufacture. Fermentation processes dependent upon yeast include pickling, cheese making, bread making, culturing, and producing vinegars, sauces (especially fish sauces), and lactic acid fermentations of various vegetables such as sauerkraut. Fermentation can preserve food, remove toxic components, utilize food wastes, and supply essential secondary food constituents. Fermented sauces and relishes improve flavor or disguise its defects.

The fermentation of wine and beer must not be forgotten. Early wine was made from honey, palm sap, dates, figs, raisins, apples, pomegranates, and numerous other fruits. Beer, fermented chiefly from wheat and barley, made an important contribution to the nutrition of the grain farmers who consumed it. Cereal porridge lacks vitamins of the B-complex. Happily, most beers are rich in B-complex vitamins. But human adaptation proceeds on the mental plane as well as the material. Beer and wine help here, too; English poet A. E. Housman wrote, “And malt does more than Milton can / To justify God’s ways to man” (1988).

The achievement of early farmers in harnessing the microbes of fermentation is impressive indeed. Their practical understanding of the ferments is especially remarkable because it was gained by trial and error in a prescientific context. In addition to yeasts and bacteria, early farmers domesticated numerous lower plants such as mushrooms, fungi, and algae. Cultivation of yeasts, bacteria, and lower plants offers three advantages: short production cycles, modest space needs, and low water requirements. Further, the relative simplicity of their husbandry requirements allows the character of the final product to be precisely controlled.

Unfortunately, silent domestication had a more sinister aspect. By living intimately with their domestic animals, humans altered the transmission patterns of existing animal pathogens (agents of disease) and stimulated the evolution of new ones. Humans came to share the diseases of the creatures they exploited. Old World examples include measles, tuberculosis, and smallpox (derived from cattle) and influenza and pertussis (from pigs and perhaps ducks and dogs). There are many others, some of which spill back from domestic animals to wildlife that come in contact with them.

By making life more sedentary, early farming brought people into sustained contact with fecal wastes, both human and animal. Absent a clear recognition of the relationship between feces and disease, farmers were not overly careful about the disposal of human and animal waste and seemed to have made little systematic effort at preventing their food and water from becoming contaminated by sewage. Such contamination provides ideal environments for the transmission of diseases such as diarrhea (particularly dangerous for small children), typhoid, and cholera. Intestinal parasites such as worms and protozoa also spread readily in this environment.

This brings the discussion once more to beer. To a certain extent, these diseases can be avoided by not drinking the water and making do with beer and wine instead. Alcoholic ferments were thus an important cultural adaptation to the new conditions of disease and health created by the silent domestication processes active in early farming life.

Weeds

Microbes were not the only set of unintended domesticants. Another set, weeds, was found on the margins of the cultivators’ fields. Weed is a folk, not a scientific, term. It refers to plants that rapidly colonize soils disturbed and stripped of vegetation by fires, landslides, windstorms, floods, and so forth. Before the advent of agriculture, there were relatively few of these plants. However, land clearance for agriculture dramatically expanded the habitat of such pioneers and, eventually, turned them into unintended cultivars (organisms originating and persisting under cultivation) dependent upon human environmental disturbance and subject to human manipulation. In this way, new varieties of weeds were produced as the by-products of cultivation. These new varieties emerged as hybrids when wild and domesticated races of cereals and other plants were grown together in the same fields. Some weed species were ultimately domesticated. Both rye and oats were European weeds that entered the repertoire of Old World domesticates when farmer-colonists began farming the interior of that continent. But most weeds remain semidomesticated pests and bedevil cultivators to this day.

The spread of farming carried these weedy pests into new territories. The best evidence for the arrival of agricultural peoples in northern Europe is the appearance of such characteristic Mediterranean weeds as ribwort plantain (Plantago lanceolata), stinging nettle (Urtica dioica), and various kinds of sorrels (Rumex spp.). The later spread of European agriculture into the New World, Australia, and the islands of the Pacific introduced these Old World weeds with devastating effect.

Human-Caused Environmental Destruction

By late prehistoric times few of the Earth’s terrestrial ecosystems had not felt the impacts of humankind. Not all impacts were of the same magnitude, however. Hunting-and-gathering peoples had a comparatively modest effect, whereas farmers radically changed natural conditions wherever they went. By clearing, burning, and killing wild predators and later by plowing and irrigating, they overrode local constraints and created new, relatively homogeneous environments. Agriculture represents an ecological simplification of human subsistence. It narrows human dependence to a limited range of plant and animal species largely controlled by the farmers themselves. When tended by humans, this array of domesticated species can adapt to a wide range of environments. The adaptability of their animals and plants, combined with their efforts at transforming the environment, meant that agriculturalists were less obliged to learn the ins and outs of their new ecologies and cared little for preserving wild food resources. By altering their environment and increasing the size of the human population supported by it, farmers made it difficult for hunting-and-gathering peoples to subsist. Along with agricultural peoples and their animals came deadly zoonoses (diseases communicable from animals to humans). As prehistory drew to a close, continued population growth generated by agriculture stimulated ever-more-intensive forms of cultivation and husbandry such as dairying, wool raising, irrigation agriculture, and its twin, fully nomadic pastoralism. Inexorably, growth and intensification led to the rise of cities, states, and civilization. From then on, there was no going back.

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