Domestication: Definition and Overview

Introduction

The domestication of plants and animals represents a key turning point in human history. This first foray into genetic engineering created new varieties of plants and animals that could be grown around the world – most often at the expense of other species that remained outside a domestic partnership with humans. The development of agricultural economies based on domesticates is arguably the central factor in the loss of global biodiversity. It transformed earth’s landscapes and its atmosphere. It fueled a population explosion of agro-pastoralists and has been a cornerstone of increasingly complex societies around the world. Understanding when, where, how, and, above all, why humans and certain plant and animal species began on their journeys into domestication remains an enduring and rewarding area of inquiry of archaeological research.

Answering these central questions requires a solid understanding of just what domestication is – and here there is a decided lack of consensus. Domestication at its heart represents a nexus between the biological world and the cultural. As such it has been a focus of inquiry for biologists, as well as archaeologists and anthropologists, who all bring different perspectives of how to define domestication. Recent advances in our ability to both detect and date the ongoing process of domestication and to trace its impact in both the genotype and phenotype of domesticates point the way to how the cultural and the biological components of this process can be reconciled and brought together in the construction of a definition of domestication that can more effectively guide the pursuit of questions of how, when, where, and why it developed.

Definition

Domestication is defined here as a sustained multi-generational mutualistic relationship in which humans assume some significant level of control over the reproduction and care of a plant/animal in order to secure a more predictable supply of a resource of interest and through which the plant/animal is able to increase its reproductive success over individuals not participating in this relationship (Zeder 2012a).

There are two other terms that require definition here: Agriculture, defined as a subsistence economy based primarily on resources garnered through the propagation and tending of domesticates, and Management, defined as the manipulation of the breeding and/or conditions of growth of a plant or animal, or the environment that sustains it, in order to increase its yield or predictability.

Historical Background

The definition of domestication adopted here represents an attempt to reconcile the different approaches to defining domestication offered by researchers coming at the subject from different perspectives. All definitions, however grounded, recognize that domestication involves a relationship between humans and target plant/animal populations (Zeder 2006). They differ in the degree of emphasis placed on either the human or the animal side of the equation and the degree to which they portray domestication as a culturally driven process or a biological one.

Some approaches, especially those adopted by researchers focusing on animal domestication, cast humans as the dominant partner in a relationship in which humans, deliberately and with forethought, assume complete control over the domesticate’s movement, supply of nutrients, its protection, its distribution, and, above all, its reproduction in order to achieve specific clearly identified goals that service the human community (Clutton-Brock 1994). Many working within this perspective emphasize the degree to which the domesticate becomes incorporated into the socioeconomic dimensions of human societies, either as productive capital (Meadow 1989), or as objects of ownership and vehicles for social relations between human individuals and households (Ingold 1984).

Other approaches, in contrast, define domestication as a symbiotic or mutualistic relationship in which both partners, humans and domesticates, reap benefits. This approach seems to be most common among researchers focusing on plant domestication (Blumler & Byrne 1991), although researchers primarily concerned with animal domesticates have also embraced it (O’Connor 1997). Perhaps the most extreme version of this approach can be found in the work of the late David Rindos (1984), who argued that the mutualism that lies at the heart of the relationship between humans and domesticates is no different from other mutualistic relationships in nature (i.e., that between social insects and a variety of other plants and animals). In fact, Rindos goes so far as to place the balance of power in this mutualism in the hands of the domesticate. According to this view, the domestic relationship confers great evolutionary advantage on the domesticate by raising its evolutionary potential far above other plants and animals outside of a mutualistic relationship with humans. Humans, on the other hand, do not fare as well because they become increasingly dependent on domesticates which offer lower energetic and nutritional returns at higher production and processing costs. This definitional approach, moreover, denies the role of any deliberate or directional intent on the part of humans who become ensnared in a coevolutionary dance with domesticates that effectively lowers their selective fitness.

An effective argument against this more extreme position comes, paradoxically perhaps, from entomologists drawing parallels between attine ant-fungal mutualisms and the relationships between humans and domesticates (Schultz et al. 2005). While these researchers highlight many features of the sustained multigenerational coevolutionary relationship between ants and their fungal gardens that resemble those between human farmers and their crops, they also identify a basic, essential difference that distinguishes the human/domesticate partnership from other mutualistic relationships found in nature. Mutualisms in nature, they note, are shaped by selection operating on random mutations in both partners that are passed on, through sexual reproduction, to their immediate descendants. Humans, on the other hand, have the ability to spontaneously invent and adopt new behaviors, or modify old ones, to meet consciously conceived goals. Humans, through social learning, are also able to pass on those innovations judged effective not only to their own offspring but also to others in their immediate group and beyond. The human ability to spontaneously, and with deliberate intent, invent new behaviors to meet consciously conceived goals and to pass these behaviors on through cultural transmission quickly shifts the balance of power in the developing mutualism between humans and developing domesticates far beyond the slowly evolving, balanced mutualisms between ants and fungus, as well as those found within other coevolutionary relationships in the natural world.

The definition offered here recognizes domestication as a form of symbiotic mutualism that, quite aside from questions of the quality of life of either partner, enhances the fitness of both. At the same time it acknowledges the role of intent-driven human behavior at the heart of the process, making this an essentially asymmetrical form of mutualism in which humans become the dominant partner. Humans may not have foreseen the adaptive responses of target plants and animals to their efforts at promoting the biotic potential of economically useful species, nor could they have envisioned the development of agricultural economies based on the genetically modified varieties of plants and animals that developed as a result of these efforts. But they could, consciously and with intent, decide to influence the distribution, feeding, and reproductive cycle of these species in order to achieve consciously perceived proximate goals of enhancing the supply and, above all, the predictability of these important resources. And it is this driving element of purposeful human behavior that is a central distinguishing feature of domestication.

Portraying domestication as a sustained multigenerational mutualism between humans and target plants and animals shaped by both biological and cultural components requires that we think of domestication as a process that operates on multiple axes on both the plant/animal and the human side of the equation (Fig. 1). On the plant/animal side, a sustained relationship with humans results in genetic changes and their phenotypic expression that take the plant or animal from its wild phenotype and transform it into its domestic one. While directed, or artificial, selection for specific traits is most often thought of here as the major vector of genetic change in domesticates, other less directed factors probably played a greater role in moving plants and animals along this axis. Specifically, both the relaxation of pressures that operated on the free-living plant or animal and the introduction of a whole new range of selective pressures as it came into closer association with humans are likely major shaping factors during the earliest stages of the process. Reproductive isolation and genetic drift also play major roles in moving the target species along this axis as humans prevent the plants and animals under their care from interbreeding with free-living populations, often by moving these managed resources away from the natural habitats of their progenitors. As the target domesticate precedes along this axis, it may also progress along another axis, moving from a free-living state to one in which some degree of feralization is possible should it leave the domestication relationship, to a plant or animal that is entirely dependent on humans for its reproduction and development.

Domestication: Definition and Overview, Fig. 1

Multiple axes of domestication

On the human side of the equation lies an axis that involves the increasing investment of labor in the propagation and care of the target species as humans engage in progressively more intensive activities directed at increasing the yield or predictability of the species. These activities can be classified under the general rubric of management, as defined above. The degree of dependence on these managed resources forms another axis on the human side of the relationship as the human group moves from utilizing only free-living resources to the increasing incorporation of managed/domesticated resources into their subsistence economy, culminating in the adoption of an agricultural economy, also defined earlier. Another possible axis on the human side of the relationship might involve the degree to which the target species becomes woven into the socioeconomic fabric of the group, here moving from a state in which there are no notions of individual, or group, ownership over a resource or resource territory, to increasingly more defined and individualized notions of rights over resources and the means of producing them.

It is important to recognize that all of these axes are in play during the process of domestication and that the pace and direction that humans and their plant/animal partners progress along any of them varies from case to case. Movement along these axes is shaped by the biological potential and constraints of the target species, the cultural context in which the process takes place, and even accidental happenstance that brings potential domesticates together with receptive human groups at particular times and places. Arrival at all, or even any, of the endpoints of these various axes is not inevitable. In fact, human management of resources most often does not result in the development of phenotypically new varieties (or species) unable to survive outside the domestication relationship. Sustained management of certain resources may not, in fact, result in any genetically driven changes in behavior or morphology that, by some definitions, would qualify the resource as a domesticate. Management activities may remain stable at fairly low levels, without intensifying, for hundreds, if not thousands, of years. There are also many examples of long-lived low-level food-producing economies that are reliant on a diverse range of resources – free living, managed, and fully domesticated – in which domesticates remain minor components, serving more as buffers against leaner times than as major subsistence staples. These economies may persist for millennia without ever becoming full-fledged agricultural economies (Smith 2001). This approach, then, views resource management, domestication, and agriculture as three independent, though related, phenomena that are not linked in a unidirectional way of inevitable progression from one to another. All domesticated resources are managed, but not all managed resources become domesticates. By the same token, agricultural economies by definition focus primarily on the production of domesticated resources, but not all subsistence economies that utilize domesticates become agricultural economies.

When, then, can a managed resource be considered a domesticated one? Is there a clear threshold or line of demarcation that separates the wild from the domestic? Some researchers reserve the mantel of domesticate for those plants and animals which, over successive generations under human management, have undergone changes in their genetic makeup that preclude them from being able to survive and reproduce outside of the domestic relationship with humans (De Wet & Harlen 1975; Blumler & Byrne 1991). Strict adherence to such a hard-line position would, as most would admit, exclude many clearly domesticated plants and animals that, although they do not revert to the pre-domestic form or behaviors, can survive apart from humans as feral escapes from domestication. Others take a slightly softer line, but still require the domesticate to have undergone genetically driven changes in either morphology or behavior (Meadow 1989) and that it be genetically isolated from populations of its wild relatives (Uerpmann 1996).

Researchers who focus more on the cultural side of the equation are generally less concerned with the genetic or phenotypic impacts on the plant or animal or with defining threshold where wild becomes domestic, emphasizing instead a continuum in human relationships with plants and animals that range from collecting to farming and from hunting to herding (Ingold 1984). Some have argued that the term domestication should be done away with altogether and replaced by a less biologically deterministic one like “cultural control” (Hecker 1982) or by defining a species as domesticated “whenever another species knows how to harvest it” (Terrell et al. 2003).

Once again the definition offered here seeks to chart a middle path through these contrasting approaches. It explicitly does not specify the degree of genetic and phenotypic change required for a managed plant or animal to qualify as a domesticate, nor does it indicate the degree of human investment in the management of the resource, its importance in the subsistence economy, or the extent to which it becomes an object of proprietary ownership needed for a resource to be classified as a domesticate. At the same time it acknowledges domestication as a unique form of human-driven mutualism. By spotlighting the sustained multigeneration relationship between humans and target domesticates, it distinguishes domestication from more casual episodic human-plant/animal interactions, like taming, periodic transplanting or sowing, or harvesting, that do not lead to the long-term coevolutionary relationship that defines domestication. By focusing on this process, rather than any particular threshold or defining criteria that separates the wild from the domestic, it directs attention to the full span of the evolving relationship on both the plant/animal and the human side of the equation and requires that all the different axes of change involved in this process be taken into consideration when trying to trace the course of domestication in any particular instance.

Key Issues/Current Debates

Always a major focus of archaeological inquiry, research on the domestication of plants and animals has seen a special resurgence in recent years thanks to the development of new analytical tools that provide a much more detailed understanding of when, where, and how different plant and animal species were taken into domestication. The development of markers (both biological and cultural) capable of tracing the domestication process is a particularly active area of current research and debate. In part as a result of the more robust understanding of the trajectory and timing of the domestication of different species in different parts of the world gained by the development of these markers, there has also been renewed energy directed toward answering the question of why people and target plants and animals set out on these various pathways to domestication.

Documenting Domestication

Some of the most exciting advances in documenting domestication have come in the form of genetic analyses of both modern and archaeological plants and animals (see Zeder et al. 2006 for a summary and extensive references). Phylogenetic analyses of neutral or noncoding genes have been widely used to (1) determine the wild progenitor species of a current day domesticates, and (2) identify the likely place (or places) of initial domestication. One of the more interesting outcomes of this work is the discovery that for almost all animal domesticates, and many domestic plants, more than one lineage of its wild progenitor was brought under domestication. There are, for example, six domestic lineages of domestic goats, five to six in cattle, and as many as five or more in pigs. The degree to which these different “domestication events” are truly independent of one another, geographically and culturally, is unclear (Dobney & Larson 2006), but this discovery clearly shows that the process of domestication was more widespread and less centralized than originally thought. Much of this work has been done using modern representatives of domestic crops and livestock and their wild progenitors, where as much as 10,000 years or more of movement, directed breeding, hybridization, and introgression between wild and domestic forms may obscure the picture of the initial process of domestication. Increasingly, however, successful extraction of ancient DNA from archaeological plant and animal remains is allowing more direct insight into the history of domestication for a number of different plants and animals (e.g., Larson et al. 2007).

Especially with the advent of next generation genomic techniques, the specific genes and coding loci that control individual domestic traits are being discovered, giving new insight into the genetic responses of domesticates to human manipulation. These discoveries have been especially impressive in plants where the loci responsible for domestication traits such as the development of the non-shattering rachis in cereals like wheat, barley, and rice, or branching architecture in maize have now been identified. Progress in this direction has been slower in animals, perhaps because identification of genes responsible for the behavioral attributes most involved in animal domestication has proven more elusive, though there has been some progress in this area as well. Moreover, genetic studies of archaeological plant remains are beginning to be able to trace the timing and the trajectory of the development of these domestication traits in crops like maize (Jaenicke-Després et al. 2003), signaling a very promising area for future research.

There have also been significant advances in the development of archaeobiological markers of domestication. Until relatively recently, archaeologists looked to a few archaeologically detectable morphological changes in plant and animal remains as gold standard markers of initial domestication (see Zeder et al. 2006 for discussion and references). We now know that many of these markers may not, in fact, be leading edge indicators of domestication. The appearance of a few cereal grains from non-shattering seed heads can no longer be interpreted as evidence of domestication, since this trait has been documented to occur, in low levels, in wild stands. The dominance of non-shattering cereal rachis in archaeobotanical assemblages, moreover, may reflect a change in harvesting practices that may occur hundreds of years after people began actively sowing harvest seeds from cultivated fields (Tanno & Wilcox 2006). The reduction in overall body size once thought to be an almost instantaneous artifact of animal management has been shown instead to be attributable to a shift in the demographics of prey populations reflecting the different harvest strategies of hunters and herders. In fact, we now know that archaeologically detectable changes in the morphology of domesticated animals may only manifest themselves hundreds of years, or more, after humans begin active management of an animal species. There is increasing awareness that significant genetic changes in both plants and animals under human management (i.e., lowering of seed dormancy rates or changes in the timing of maturation, selection for reduced wariness, or changes in coat color in animals) may not manifest themselves in ways that are detectable in the archaeological record. As a result, archaeobiologists are actively looking for new markers capable of detecting these crucial initial stages of the process – changes in the demographics of harvested animals mentioned above, changes in feeding patterns of managed animals detectable through isotopic analysis, epigenetic changes in bones and teeth reflective of reproductive isolation of managed animals, architectural evidence for the keeping of animals or storage of crops, artifacts or residues that point to the use of animal products, the presence of diagnostic plant microfossils (phytoliths and starches) in soils and on artifacts, modifications of landscape to accommodate the needs of transplanted crops, weed assemblages characteristic of cultivated fields, and plastic responses of seeds and other plant structures to artificial watering.

As promising as these new developments are in the genetic, archaeobiological, and archaeological documentation of domestication, it is crucial that those developing and employing these new approaches be able to explicitly link the marker they are using to some aspect of the process of domestication and to explain why the marker in question is an artifact of this evolving mutualism between human and a plant/animal species. There are plenty of examples of markers that have been linked to domestication in a post hoc way after they were observed in the archaeological record in assemblages thought to represent initial domestication, only to be discovered later to be attributable to entirely different factors unrelated to domestication (i.e., overall body size reduction in animals). There is still a tendency to engage in this kind of after the fact rationalization of how something observed in the archaeological, or genetic, record might be an artifact of domestication. Continuing development of these promising biological and archaeological techniques for documenting domestication can only make headway if there is an active dialectic that allows us to link our understanding of the process of domestication to the markers we use to trace this process.

Explaining Domestication

The more robust record of domestication produced by these advances in our methodological ability to document transformations in animal and plant species has energized efforts at answering the key question of why humans and plants and animals embarked on these various pathways to domestication (see Zeder & Smith 2009 for summary and references). Recent years have seen a resurgence of models featuring climate change as a major driving force behind plant and animal domestication, either as a response to climate deterioration during the Younger Dryas or as a response to the amelioration and stabilization of climate that followed. Models have been offered that feature the role of domesticates in promoting the ambitions of individuals who seek social promotion, that spotlight religion as key motivating force behind domestication, or that see the seeds of domestication as deeply buried within human cognitive abilities and their use of symbols to navigate and transform the world around them.

Perhaps the most prominent explanatory models offered in recent years are those that characterize domestication as a response to resource depression, usually caused by human demographic expansion, but sometimes due to environmental change. Framed within the precepts of optimal foraging theory, which was originally developed to account for the foraging behavior of animals, the adoption of plant and animal domesticates, with their purportedly lower energetic yields, higher processing costs, and delayed returns, can only be considered less than optimal, lower ranked resources resorted to only when higher yield, more optimal resources are no longer available. This resource depression model has been applied globally to account for the adoption of domesticates and the diversification of the resource base that often precedes it in early and middle Holocene context around the world (see Zeder 2012b for discussion and references).

Recently, however, archaeologists have begun to turn to another school of thought in evolutionary biology, niche construction theory, for an alternative explanatory model for the origins of plant and animal domestication (Smith in press; Zeder 2012b). This model emphasizes the role of humans in manipulating environments and resources within them to increase overall biotic potential. To many it is a better fit with an increasingly better-resolved archaeological record, in which resource diversification and follow-on domestication of certain plant and animal species seem to occur more often in the context of resource abundance and low population levels than in the context of resource depression and population packing.

Regardless of which model proves in the long run to be more useful in accounting for this transition, the richer empirical understanding of how domestication came about in different areas of the world makes it increasingly difficult to posit single lever, prime mover explanatory scenarios for initial domestication. Instead, it is becoming increasingly clear that multiple interacting factors – environmental, demographic, social, and biological – are responsible for driving this process and shaping its trajectory as it played out in different areas of the world with different plants and animals and human societies.

International Perspectives

Twenty years ago, the prevailing view was that domestication and agricultural economies based on them arose only in a handful centers around the world. The majority of work on origins of domestication and agricultural emergence focused primarily on two world areas – the Near East, primarily in the eastern and western arms of the Fertile Crescent region that stretches from southern Israel to southern Iran; and in Mesoamerica, primarily based on the small soundings in a handful of caves central and southern Mexico. In recent years the number of documented centers of independent domestication has skyrocketed (Fig. 2). Evidence of independent domestication has been documented on essentially every continent, save Antarctica, and the variety of domesticates under study has exploded from a handful of annual crop plants (wheat, barley, corn, rice, pulses) and livestock species (sheep, goat, cattle, pigs) to include a rich array of perennials, tree crops, tubers, mammals, birds, and even fish and shellfish. Moreover, the more fluid understanding of the process of domestication and the role of deliberate human manipulation of plants and animals and their environments has further broadened the global reach of domestication studies to all areas, terrestrial, coastal, and perhaps even marine, where humans and plants and animals have engaged in sustained, multigenerational interactions of the kind that, in the case of certain species, results in their domestication.

Domestication: Definition and Overview, Fig. 2

Currently recognized areas of domestication of plants and animals with some examples of plants and animals domesticated in each center. (1) Eastern North America: chenopodia, squash, sunflower, and maygrass; (2) Mesoamerica: maize, squash, beans, and turkeys; (3) northern Peru/Ecuador: squash and lima beans; (4) Amazonia: manioc, yams, peanuts, and Muscovy duck; (5) Andes: oca, potato, quinoa, amaranth, llama, alpaca, and guinea pigs; (6) sub-Saharan Africa: pearl millet, sorghum, and African rice; (7) Horn of Africa/Nile Valley: asses, taurine cattle?; (8) the Near East: wheat, barley, lentils, peas, sheep, goats, taurine cattle, and pigs; (9) Central Asia: horses; (10) South Asia: browntop millet, water buffalo, and zebu cattle; (11) North China: foxtail millet; (12) South China/Southeast Asia: rice and chickens; (13) Japan: barnyard millet, mung bean, burdock; and (14) New Guinea: bananas, yams, and taro. Note: list of domesticates and areas identified not exhaustive (Base map courtesy of the National Geographic Society)

Future Directions

Clearly this expansion in the geographical and taxonomic reach of domestication studies signals a bright future for the study of plant and animal domestication. Moreover, the subsequent dispersal of domesticates and the knowledge of how to manage domestic species represents an additional future research frontier, as we try to better understand the rates and vehicles of transmission and the responses of indigenous people to this new technology.

Principle gains in domestication research are most likely to come in the area of documenting domestication as researchers continue to develop analytical methods capable of tracing the unfolding coevolutionary relationship between humans and target plant and animal species that defines domestication. The greatest advances in genetics will most likely come in the ability to extract and amplify DNA from archaeological plant and animal remains. Success in obtaining ancient DNA will vastly clarify the phylogenies of early domesticates, helping further pinpoint the timing, number, and the geographic and cultural contexts of initial domestication. Even more significant will be the ability to trace the evolution of loci responsible for the morphological, physiological, and behavioral responses that plants and animals made to human manipulation as they embarked on their various pathways to domestication.

We can also expect significant advances in the development of archaeobiological and archaeological methods for tracing these pathways. Thus far, much of the initial work in this area has concentrated on developing markers for annual crop plants and a handful of domesticated mammals. Future work will likely expand these efforts to more difficult challenges of discovering markers for the domestication of crop plants which are vegetatively reproduced or to tracking the responses that long-living crops – like tree crops – make to human manipulation. Advances can also be expected in discovering ways to monitor anthropogenic modifications of landscapes as humans try to influence the biotic productivity of local environments. Documenting the domestication of certain animal species that have thus far proven difficult to trace using archaeobiological and archaeological evidence may become possible with an enhanced attention to the likely pathways that these animals and their human partners took into domestication (Zeder 2012b). Expanding the markers of domestication for these species from a limited set of morphological, or even demographic, markers to a wider array of plastic responses and cultural indicators of their care and exploitation may point the way to a clearer understanding of their domestication.

Expanding the geographical scope of archaeological investigations into the origins of plant and animal domestication, coupled with the enhanced tools for documenting domestication, will provide a much needed empirical foundation for the assessment of various explanatory scenarios offered for why humans and their plant and animal partners started down the road to domestication. By being able to construct fine-grained accounts of the trajectory of a wide range of different types of domesticates in different cultural, environmental, and temporal settings, archaeologists will be able look for commonalities and differences in these trajectories, to isolate driving forces that cut across different instances of domestication from more localized factors that shape domestication trajectories in individual cases. This will make it infinitely harder to propose simple, single-lever, one-size-fits-all explanatory frameworks for the origins of plant and animal domestication and will result in a significantly richer, more robust, and more nuanced understanding of this transformative process.

Cross-References

Abu Hureyra: Agriculture and Domestication

Agricultural Practice: Transformation Through Time

Agricultural Practices: A Case Study from Papua New Guinea

Ali Kosh: Agriculture and Domestication

Anthropogenic Environments, Archaeology of

Archaeobotany of Agricultural Intensification

Çatalhöyük Archaeological Site

Çayönü: Agriculture and Domestication

Duck: Domestication

Göbekli Tepe: Agriculture and Domestication

Jericho Archaeological Site

Kuk Swamp: Agriculture and Domestication

Millets: Origins and Development

Near East (Including Anatolia): Geographic Description and General Chronology of the Paleolithic and Neolithic

Near East (Including Anatolia): Origins and Development of Agriculture

Phytolith Studies in Archaeology

Turkey: Domestication