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ARTICLES

Central Questions in the Domestication of Plants and Animals MELINDA A. ZEDER

DEFINING DOMESTICATION

All approaches to defining domesti- cation in both plants and animals rec- ognize that domestication involves a relationship between humans and tar- get plant or animal populations. There are, however, distinct and often dis- cordant perspectives taken regarding the balance of power in this relation- ship and its central defining features

(Fig. 1). Many approaches to defining domestication, especially those focus- ing on animals, emphasize the domi- nant role humans play in assuming “mastery” over all aspects of the re- production, movement, distribution, nourishment, and protection of do- mesticates.2–5 Integral to definitions that place humans in control of the process is the notion of intentionality, that humans with foresight and delib- erate intent intervened in the life cycle of target plant and animal popula- tions and assumed responsibility for their care to meet specific and well- defined objectives serving human needs. Also often associated with this emphasis on the human dimension is the notion that domestication in- volves a fundamental change in socio- economic organization in which suc- cessive generations of domesticates become integrated into human societ- ies as objects of ownership.3,6

Other researchers object to “anthro- pocentric” approaches to defining do- mestication that portray domesticates as passive pawns in the process, point- ing out that domesticates also reap benefits through vastly enhanced re- productive fitness and expanded rang- es.7 Those operating within an evolu- tionary biology perspective, in

particular, maintain that the relation- ship between humans and domesti- cates is no different from other mutu- alistic relationships in the “natural world” that bring together species like ants and aphids in partnerships of in- creasing co-dependency.8 Moreover, as one moves further along the spec- trum, from a relatively balanced mu- tualistic perspective to ones that focus on the domesticate, the role of delib- erate human intent declines. The more extreme positions at this end of the spectrum tip the balance in favor of the domesticate, which is seen as manipulating its human partners for its own evolutionary advantage, en- snaring humans in a relationship that may have actually reduced human fit- ness.9

Another axis of variation in defini- tional approaches to domestication is the relative primary given to genetic and associated morphological change. An emphasis on genetic change and its phenotypic expression is particu- larly common among researchers fo- cusing on plant domestication, espe- cially the domestication of large- seeded annuals, where human intervention results in fairly rapid ge- netic changes with easily observed phenotypic expressions.9,10 Some re- searchers focusing on animals also see genetic isolation and subsequent quick-onset morphological change as essential attributes of domestica- tion.11

The requirement that domesticates show evidence of morphological or even genetic change, however, is not universally accepted. Nor is the basic premise underlying this requirement: that the process of domestication is contingent on reproductive isolation and resultant genetically driven mor-

Melinda A. Zeder is Director of the Ar- chaeobiology Program of the Smithso- nian Institution’s National Museum of Nat- ural History. Her research focuses on questions of domestication, origins of ag- riculture, and the environmental and so- cial impacts of early agricultural econo- mies in the ancient Near East. She is the lead editor of the volume, Documenting Domestication: New Genetic and Archae- ological Paradigms, with co-editors Eve Emshwiller, Daniel G. Bradley, and Bruce D. Smith, published in spring of 2006 by the University of California Press.

Key words: domestication; origins of agriculture; plants; animals; climate change; demographic pressure; social forces; Southwest Asia

Along with symbolic communication, tool use, and bipedalism, the domestica- tion of plants and animals, together with the associated emergence of agriculture, stands as one of the pivotal thresholds in human evolution. For more than a hundred years researchers have wrestled with the questions of what domestica- tion is, how it is detected, and why it happened. The past decade in particular has witnessed a remarkable acceleration of interest in domestication, thanks to ad- vances in our ability to detect the context, timing, and process of domestication in a wide array of different plant and animal species around the world.1 This review focuses on overarching issues of defining, documenting, and explaining the do- mestication of plants and animals, tracing a path through often discordant view- points to offer some new perspectives.

Evolutionary Anthropology 15:105–117 (2006)

phological change. This is particularly true for animals, where morphologi- cal change, when it occurs at all, is often both delayed and difficult to tie directly to domestication.12 As a re- sult, many researchers define animal domestication not in terms of ob- served genetic or morphological change, but in terms of causal human behavior. According to this view, do- mestication falls along a continuum of increasing human intervention ranging from predation to genetic en- gineering13 in which there are varying degrees of investment in altering an animal’s natural behavior (its move- ment, breeding schedule, or popula- tion structure) to suit human needs.6,14 A similar view is becoming increasingly common in consider- ations of plant domestication espe- cially perennial plants such as root crops propagated through vegetative cloning or very long-lived tree crops in which genetic and morphological change may be less automatic and more subtle than in annual seed crops.15 Smith,16 for example, main- tains that for both plants and animals the central defining feature of domes- tication and the creation of domesti- cates is the nature of the “ongoing re- lationship of intervention initiated and sustained by humans.” This em-

phasis on the evolving relationship be- tween humans and plant or animal populations turns attention away from a range of secondary conse- quences of domestication, such as ge- netic and morphological change or so- cial notions of property, and properly returns it to a consideration of the new partnership that humans create with target populations.

Domestication does indeed have many features in common with other mutualistic relationships among plants and animals. Both partners in the relationship of domestication clearly derive benefits. Plant and ani- mal partners benefit in increased re- productive fitness and range expan- sion. Human partners gain increased security and predictability in their ac- cess to resources of interest. Both partners respond to this relationship in ways that enhance respective pay- offs and further deepen their mutual investment in its continuation. But the mutualism that lies at the heart of the domestication process involving human societies and target plant and animal populations varies in signifi- cant ways from other similar relation- ships found in nature.

In a recent overview of the homolo- gies between human agriculturists and fungus-growing ants, Schultz and

coworkers17 highlight the many paral- lels between these convergent forms of mutualism while also underscoring key qualitative differences between at- tine and human agriculture. The co- dependent relationships between farmer ants and domesticated fungi are the result of a gradual co-evolu- tionary process based on mutation-in- duced behavioral and morphological change in both partners. Humans, on the other hand, are capable of modi- fying their behavioral repertories through “trial and error, observation, and imitation.”17 That ability enables humans to rapidly develop behavioral strategies aimed at meeting con- sciously recognized needs. The highly developed human ability for cultural transmission of learned behavior, Shultz and coworkers argue, greatly accelerated the adaptive modification of human behavior, shifting the bal- ance of power in the emergent mutu- alism. Humans quickly assume a dominant role because they are free to choose among genetic variants in the partner population, to manipulate the behavior and life history of symbionts (even to their own detriment), or to terminate the relationship with one partner symbiont and choose another.

This is where intentionality comes into the picture. It is true that humans

Figure 1. Definitions of domestication tend to fall somewhere along three axes of variation. Definitions that award the balance of power in the domestic relationship to humans tend to stress human intentionality and the social and economic impacts of domestication. Definitions that tip the balance of power in favor of the domesticate tend to discount the role of human intentionality in the process and stress its biological impacts on the domesticate.

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could not have foreseen the adaptive responses by plant and animal part- ners to the new selective factors brought into play by the relationship of domestication. Nor did humans likely appreciate the long-term bene- fits (or the negative consequences) that might accrue from domestication and the subsequent development of agricultural economies. However, rul- ing out this kind of prescience on the part of humans does not take inten- tionality out of the picture. While they might not have understood the princi- ples of genetic engineering, humans could appreciate the fact that tending, nurturing, and intervening in the life cycle of certain plants and animals yielded various immediate benefits. On the basis of these returns they could then consciously and deliber- ately decide to continue to engage in these behaviors, and to elaborate on them, instead of engaging in other strategies.

Intentionality, then, becomes the key factor that distinguishes domesti- cation from other similar mutualistic relationships in nature. The deliberate role humans take in actively pursuing the domestic partnership also distin- guishes it from other biological rela- tionships between humans and plants and animal species, such as commen- sal relationships with mice, sparrows, or weeds that take advantage of new niches created by human habitation. Definitions that try to pigeon-hole do- mestication as either a cultural or bi- ological process are bound to come up short. Clearly, domestication has a bi- ological component as a mutualistic relationship between humans and plant or animal symbionts. Just as clearly, however, human intentional- ity sets domestication apart from other forms of mutualism. The uniqueness of the relationship comes from its cultural component and the dominant role humans play in con- sciously and deliberately perpetuating it to their own advantage.

If the process of domestication is best viewed as a form of mutualism that is asymmetrically enhanced by the human ability to culturally trans- mit learned behavior, then at what point along this developmental trajec- tory does the plant or animal partner become a domesticate? Is there a

threshold that, once crossed, sepa- rates the “wild” from the “domestic”? If so, what does this threshold look like? To some extent, it remains a matter of personal preference to de- cide just when a domestic subsection of a plant or animal species has been created. Threshold criteria that re- quire total genetic isolation and emer- gent speciation or complete depen- dence on humans for survival set a very high bar that many, if not most, widely accepted domesticates would fail to clear. Even somewhat looser standards that involve a lesser degree of genetic modification in the target plant or animal population, or a cer- tain level of human investment in propagating, nurturing, or owning the resource, run the risk of constructing artificial boundaries along what was really a more seamless incremental process.

Ducking the issue by adopting the term “proto-domesticate” also does not help much. This term implies that, if just given enough time and perhaps a little more investment by either part- ner, full domestic status would be achieved. The actual trajectory of do- mestication, however, is highly con- tingent on a wide range of factors, including the ability of the plant or animal to take advantage of the rela- tionship, the strategies and accompa- nying technologies humans develop to manage the resource, and its chang- ing value vis-à-vis available alternative resources. In some plant and animal species, genetic modification and more focused human investment in the resource may quickly follow. In others there may be a long and very stable relationship involving fairly minimal commitment by either part- ner. Further, it appears that budding domestic relationships sometimes fail altogether, never moving beyond an initial courtship phase.

It is best to step back and not focus too closely or obsessively on defining the exact demarcation between do- mestic and wild, and to turn, instead, to a consideration of the full span of the evolving nature of domestic rela- tionships. Different stages in the evo- lution of this relationship might be characterized by the degree of invest- ment by both partners (Fig. 2). For the plant or animal, this would involve the

extent of genetic modification made in response to new selective pressures, the degree of its genetic isolation from populations not involved in the part- nership, the nature of subsequent morphological or behavioral change, and its increasing co-dependency on humans. For humans, this might be the level of investment in the produc- tion of the resource; that is, in tilling, watering, burning, and land clear- ance, sowing, and transplanting plants, or in taming, protecting, herd- ing, culling, and selectively breeding animals. It might also include the de- gree of incorporation of domesticates within the socio-economic organiza- tion of the human groups investing in its production.

By expanding the scope of inquiry to encompass the vast “middle ground” between foraging and farm- ing, hunting and herding,16 we can approach a deeper, more comprehen- sive, and ultimately more informative appreciation of the range of possibili- ties open to humans and their plant and animal partners. This expanded territory of investigation includes the stable, long-lived systems of low-level food production involving a mix of both morphologically altered and nonaltered domesticates, as well as “wild” resources, featured in recent books on indigenous resource man- agement in California and the North- west Coast.18,19 At the other end of the spectrum are highly structured agri- cultural economies with complete de- pendence on domesticates and total investment in their production. Try- ing to understand the full richness of the various ways in which the domes- tic partnership may manifest itself in different contexts, we, in turn, stand a much better chance of being able to document and explain domestication.

DOCUMENTING DOMESTICATION

In both plants and animals, this ef- fort requires identifying clear-cut markers that can be explicitly linked to a specific aspect or stage of the un- folding domestication process.1,20 Dif- ferent markers may be more effective in detecting different stages of this process. Markers will also vary de- pending on the biology of the domes-

ARTICLES Domestication of Plants and Animals 107

ticate and its relationship with hu- mans. There are, in particular, fundamental differences in the selec- tive pressures on plants and animals undergoing domestication, and, as a result, in the corresponding markers used to document plant and animal domestication.

Selective pressures on plants, espe- cially annuals, tend to operate directly on morphological traits that can, in turn, be used as unambiguous mark- ers of domestication.15 Morphological impacts of the domestication of annu- als may come about as largely auto- matic responses to human planting and harvesting that result in such changes as increased seed size, thin- ner seed coats, reconfiguration of seed head architecture, or the development of indehiscent seed pods.15,21,22 Inten- tional selection for specific morpho- logical attributes in annual plants, such as larger fruit size, appear to happen later in the developing rela- tionship of domestication.15

Perennial plants sustained by trans- planting root fragments, on the other hand, are not subjected to the same seed-bed pressure and human har- vesting selective pressures that result in the morphological markers used to document domestication in annual seed plants.15 At the same time, how- ever, because there may be more of an opportunity for humans selectively to replant root fragments with desired traits, these plants may respond fairly quickly to deliberate human selection in the development of larger fruits, the loss of chemical defenses against her- bivory, or changes in sugars and starches.23 While many of these crops were grown in tropical areas with poor preservation of plant macro-fos- sils, the development of breakthrough techniques for the recovery and iden- tification of plant micro-fossils (that is, phytoliths and starch grains) has made it possible to detect these do- mestication-induced morphological

changes in root and other crop plants (Fig. 3).24,25

Recent years have seen an increase the use of nonmorphological markers of the intensification of human-plant interactions that may precede clear- cut evidence of morphological change in plants. Evidence of land clearance, modification of natural drainage sys- tems, intentional burning, and changes in the composition of weedy plants in archeological assemblages have all been effectively used to track human modification of landscapes and plant communities as part of the domestication process.26 –28 The oc- currence of plant macro- or micro fos- sils in areas thought to be far outside their natural range has also been in- terpreted as evidence of human trans- port and tending of plants.26,29,30

There are special challenges to find- ing markers of animal domestication. This is because the leading-edge pres- sures on animals undergoing domes- tication are likely to focus on behav-

Figure 2. Domestication is best viewed as an evolving of mutualism between humans and populations of plants or animals. The relationship can be characterized along various scales of investment by either the human or the plant or animal partners. All of these scales usually are involved in the process of domestication, though they often operate independently of one another. The degree of change along each scale is contingent on the biology of the species involved, as well as the ecological and cultural circumstances of the human partners. Attempting to distinguish just where and along what scale domestication occurs is not only difficult, but may not be very useful.

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ioral attributes rather than on morphological traits.31 There are a va- riety of behaviors that probably made certain animal species better candi- dates for domestication; among them tolerance of penning, a social struc- ture based on dominance hierarchies, sexual precocity, weak alarm systems and, above all, reduced wariness and aggression.32 Behavioral responses to domestication in animals elaborated on these initial preselection qualifying attributes and include a general re- duction in responsiveness to environ- mental stimuli, reduced activity lev- els, increased social compatibility, and intensified sexual behavior.4,33

Many morphological traits com- monly seen in domestic animals are thought to be linked to these behav- ioral changes. These attributes in- clude piebald coats, lop ears and, of special importance here, reduced brain size and an overall juveniliza- tion of cranial form.4,33 This latter fea- ture may result in a shortened muzzle, tooth crowding, and reduction in tooth size, traits frequently seen as leading-edge markers of domestica- tion in dogs and pigs.34,35 Selection for these behavioral traits and their associated morphological effects, however, may not be uniquely re- stricted to domestication. Similar be- havioral traits, such as reduced wari- ness and timidity, are also selected for in animals such as rats and sparrows that develop commensal relationships with humans. These animals also show changes in pelage coloration and brain size.4 It is possible, then, that the initial modifications in tooth size and cranial form in pigs and dogs, widely seen as markers of domestica- tion, may in fact be attributable to an early commensal relationship be- tween humans and such omnivore species that began their association with humans as camp-follower scav- engers.31

Other genetically driven morpho- logical changes in animals undergo- ing domestication come about when humans begin deliberately selecting breeding partners. Changes in the size and shape of horns in animals like goats and sheep, for example, are di- rectly tied to the relaxation of selective pressures for and, quite likely, active selection against large horns once hu-

mans assume control over breeding.36

Other morphological changes may en- sue when animals are moved into new territories, either through founder ef- fects, random genetic drift, or di- rected adaptation to new environmen- tal conditions. Later still, and probably much later in animals than in plants, deliberate human selection for attributes that enhance meat, fi- ber, milk yields, or labor potential may result in still other morphological markers that might be used to detect intensification in the human-animal relationship.

Domestication has also been sug- gested to have resulted in a marked and rapid reduction in body size, which, until recently, has been widely held to be a definitive marker of initial domestication.37 This proposed mor-

phological response has been vari- ously attributed to a plastic response to nutritional deficiencies, an adaptive advantage of smaller bodies for ani- mals subjected to impoverished con- ditions, or deliberate human selection for more tractable individuals.38 – 40

But body size in animals is also known to be affected by well-docu- mented factors such as sex, environ- ment, climate, and age, which may be entirely unrelated to domestication and may mask or be mistaken for changes in body size induced by do- mestication.12

Given the looser connection be- tween domestication and morpholog- ical change in animals, it is not sur- prising that considerable attention has been devoted to identifying mark- ers of domestication that do not rely on genetically driven morphological change, but that, instead, reflect hu- man actions directed at managing an- imals. Demographic markers aimed at detecting the different harvest strate- gies of hunters and herders were among the first nonmorphological markers used to detect animal domes- tication.41 Largely abandoned in the 1980s and 1990s, when most archeo- zoologists embraced size reduction as a leading-edge marker of animal do- mestication, demographic markers are seeing a resurgence, thanks in part to the development of methods for constructing high-resolution sex-spe- cific harvest profiles.12,42 Applying these methods to archeological as- semblages has shown that what was once interpreted as evidence of do- mestication-induced body size reduc- tion in goats (and likely other live- stock species) is, instead, a reflection of a change in the demographics of the adult portion of managed herds dominated by females (Fig. 4).12 Un- ambiguous changes in morphological traits such as body size or horn form seem to postdate human management of herd animals by hundreds of years and represent later phases in the do- mestication process.12,43

Both the presence of animals out- side their presumed natural habitat and a sudden dramatic increase in a previously little-exploited animal have also been used as markers of animal domestication.41,43,44 But the use of these markers (in both animals and

Figure 3. Starch grains from wild and domes- tic yams, Dioscorea sp. A. Granules from modern domestic yams (D. trifida). B. starch grains from a wild yam (D. cymosula) from Panama. Starch grains from wild yam spe- cies studied thus far from the Neotropics are distinct in morphology as compared with domestic varieties. As a rule, wild forms are also highly variable within a single tuber, whereas domesticated species have a sin- gle morphological type of starch, which may be a result of human selection.

ARTICLES Domestication of Plants and Animals 109

plants) needs to be tempered by ac- knowledgment of our generally poor understanding of the geographic range of biotic communities in the past and of the paleo-environmental conditions that shaped these ranges. A rapid increase in the abundance of a plant or animal resource in an arche- ological assemblage might simply sig- nal the intensification of hunting and gathering strategies, not the begin- ning of food production.

Markers of animal domestication may also be found in plastic, nonge- netically driven responses such as bone and tooth pathologies, evidence of pandemic disease, or chemical changes in the composition of bone and tooth enamel used to track changes in nutrition and the seasonal movement of managed animals.45– 48

The presence of corrals, pens, or other traces of animals, such as manure or hoof prints, in human settlements, changes in human settlement pat- terns, artifacts related to the exploita- tion of domestic animals (bits or milk churns and storage vessels), and even changes in food distribution patterns have been used with varying effect to build cases for animal domestica- tion.49 The application of these later plastic responses and cultural mark- ers needs to be tempered by the real- ization that they may not be mani- fested in all instances of animal domestication or may result from other pressures unrelated to domesti- cation. Application of such markers is most effective when many of them are brought together to build strong cir- cumstantial cases for domestica- tion.49

Advances in methods for extracting and amplifying both modern and an- cient DNA is recent years have pro- vided an exciting new window on the genetic changes associated with the domestication of plants and ani- mals.1,20,50,51 Some of this work has focused on identifying the genes or gene complexes that are specifically selected for or against in the process of domestication, especially of crop plants.52 However, most genetic stud- ies of domestication look to largely neutral noncoding loci and organellar genomes. These procedures have proven useful in tracing the diver- gence of domesticates from their wild

progenitors; identifying the number and geographic location of domestica- tion events, which now appear to have been multiple for most animal domes- ticates and many plant crops; and in tracking the dispersal of domesticates and their human partners out of cen- ters of origin.

There are critical differences in the relative rates of evolution in the dif- ferent genomes of plants and animals that play a major role in the genetic markers used. The relatively rapid rate of evolution in mitochondrial DNA (mtDNA) in animals makes mtDNA particularly well suited to tracking the relatively shallow time depth of divergence between domesti- cates and their wild progenitors (�10,000 years). This is why most studies of animal domesticates focus on this genome.51 While it is less vari- able than mtDNA and evolves much

more slowly, low-copy Y-chromo- some nuclear DNA provides a window into patrilineal inheritance, which in many animal domesticates is quite different from matrilineal history.53,54

Variation in noncoding nuclear mic- rosatellite DNA, contributed by both parents, has also proven useful in tracking the divergence of different breeds of animals.55,56

While not approaching the rate of evolution of animal mtDNA, loci in the nuculear genome of plants evolve at about the same rate as nDNA loci in mammals. They also evolve about four times faster than loci in the choloroplast genome and twelve times faster than plant mtDNA. Conse- quently, genetic studies of plant do- mestication tend to focus on nDNA, especially on highly polymorphic mi- crosatellites that provide sufficient in- traspecific variation to document the domestication process.50,57–59 The nu- clear genome in plants has proven es- pecially useful in tracking down the various ancestral genomes contribut- ing to the complicated genetic heri- tage of hybrids and polyploid crop plants. These common conditions in plant crops generally are not found among animal domesticates.50,60

Most genetic approaches to docu- menting domestication are based on modern domesticates and likely wild progenitor species. But the window they provide on the origin and early dispersal of domesticates is unavoid- ably clouded by thousands of years of selective breeding, hybridization, and introgression between wild and do- mestic populations. Ancient DNA (aDNA), on the other hand, has the potential to shed more direct light on the process of genetic divergence of domesticates. Due to the greater pres- ervation of DNA encased in animal bone and the suitability of high-copy mtDNA in animals for tracking shal- low time depth divergences, aDNA studies of animal domesticates have been particularly successful, espe- cially those tracing the more recent dispersal of domestic animals through temperate environments.61– 64 Al- though it is more difficult to extract enough low-copy nDNA from un- charred archeological plant remains to provide meaningful results, some stunning results have recently been

Figure 4. Goat (Capra hircus) first phalanges from Ganj Dareh ca. 10,000 cal B.P. The small bone on the left falls within the size range of such bones from female goats, while the larger specimen on the right is likely from a male. The female phalanx is fully fused, indicating the animal was slaughtered after this bone fused, which in goats is about 13 months of age. The male specimen is unfused, indicated that this an- imal was younger than 13 months of age when slaughtered. The selective slaughter of young male animals with prolonged sur- vivorship of female animals is symptomatic of human management. (Photo by Carl Hansen)

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obtained in the use of aDNA to track the origin and dispersal of domestic plants.65,66

In the excitement over the possible contributions of genetic analysis to the documentation of plant and ani- mal domestication, it is important not to lose sight of the fact that there is more to domestication than ge- netic change. The real power of these new tools for tracking the tra- jectory of domestication can be real- ized only when genetic analyses are more fully integrated into broader archeological analyses. Genetic studies represent one, albeit very powerful, line among many parallel and mutually illuminating lines of evidence, which, when considered together, provide a fine-grained view of unfolding domestic partnerships (Box 1).

EXPLAINING DOMESTICATION

Efforts to explain domestication and the origins of agriculture tend to cycle among a relatively limited num- ber of forcing factors championed as primary and often universal levers of change. These forcing factors can be generally grouped under three banners: environmental change, demographi- cally induced resource pressure, and changes in social organization and ide- ology.

Explanations focusing on environ- mental change can be traced to V. Gordon Childe, who credited post- Pleistocene aridification with bring- ing humans together with plants and animals around water sources in po- sitions of “enforced juxtaposition” that promoted “symbiosis between man and beast,” resulting in domesti- cation.67 While climate models were largely out of favor in the 1960s through 1980s, recent advances in paleo-environmental reconstruction have resulted in its rehabilitation as a primary player in agricultural origins. In particular, the now well-docu- mented brief return to Ice Age condi- tions at about 13,000 cal. B.P., known as the Younger Dryas, is increasingly featured as having played a key role in agricultural origins,68 with domestica- tion coming about either during this climatic downturn as a way of coping with environmental degradation69,70

or after it as a response to the follow-

ing climatic amelioration and stabili- zation.71 One recent climate based model of agricultural origins even maintains that agriculture was a “compulsory” development of cli- matic stabilization and rise in ambi- ent CO2 following the final pulse of Ice Age climate in the Younger Dryas.72

Explanatory frameworks founded on notions of population dynamics and resultant resource pressures can be traced back to Binford’s Edge-Zone Hypothesis of the late 1960s.73 Ac- cording to this theory, agricultural or- igins were the result of resource pres- sure in marginal zones caused by emigration from more optimal zones experiencing high rates of population growth. Mark Cohen’s74 subsequent “food-crisis” model held that runaway population growth worldwide, not just in marginal zones, forced people around the planet to abandon more nutritious hunting and gathering strategies and assume the burden of tending to domesticated plants and animals. Binford’s75 recent return to the topic down-plays the role of pop- ulation growth, resource pressure, and emigration, and instead high- lights population packing, in particu- lar a threshold limit of 9.098 people per 100 km2 as the “universal condi- tioner of change in . . . subsistence strategies” and the deus ex machina of agricultural origins.

Although explicitly rejecting univer- sal normative explanations of domes- tication and agricultural origins, those operating under the general ru- bric of human behavioral ecology (HBE) also feature resource pressure as a primary causal component.76 A basic axiom of HBE models is that humans will always opt for strategies that optimize immediate returns from high-ranking resources, with the rela- tive rank of a resource determined by kilocalorie return over pursuit and handling costs.77 Food production vi- olates these central principals. First of all, farming involves a rise to promi- nence of low-rank plant resources, while herding requires deferring the rewards from high-rank animal re- sources.78,79 Moreover, farming and herding are often seen as carrying higher production and processing costs than do hunting and gather- ing.80 A recent review highlights dif-

ferent paradigms within HBE that might be helpful in understanding the transition from foraging to farming, such as concepts of constrained opti- mization, marginal value, opportunity costs, discounting, and risk-sensitivi- ty.76 Diet-breadth models that include avenues for plant resources of lower profitability to elevate their rank through changes in density or extrac- tion return hold particular promise here. However, even allowing for such advances up the resource-rank ladder, HBE models generally predict that plant resources will largely be ignored so long as animal protein, a more highly ranked resources, is sufficiently abundant.78 By necessity, then, strict adherence to the basic axiomatic te- nets of HBE casts the transition to food production in terms of rather bleak cost-benefit trade-offs. Follow- ing HBE principles, agriculture comes about when people, faced with pres- sure on resources, whether caused by fluctuating climates, population growth, or packing, are forced to fo- cus on resources and extraction strat- egies that under other circumstances would be considered far from opti- mal.

Other theorists deny that external factors like climate change, popula- tion growth, or resource pressure have any causative role in this transi- tion. Instead, they look to forces within the human character, espe- cially a supposedly innate compulsion for self-aggrandizement, as primary forcing factors in the domestication of plants and animals and the transition to agriculture.81 Perhaps the best known and most influential of these socially based explanatory ap- proaches is that promulgated by Brian Hayden.82,83 In contrast to a backdrop of scarcity and stress cen- tral to other models, Hayden conjures up a more Eden-like setting for the origin of plant and animal domestica- tion, claiming that agriculture only develops in contexts of plenty, where an abundance of resources ignites a basic human predisposition for acqui- sition and social competition. In such settings certain particularly success- ful “accumulators” were able to mar- shal high-prestige food items and en- hance their own social advantage through such mechanisms as compet-

ARTICLES Domestication of Plants and Animals 111

itive feasting. Domestic resources were especially attractive since they were more amenable to ownership and their supply could be both manip- ulated and appropriated. This model

predicts, then, that domesticates were not, at least initially, ubiquitous di- etary staples, but were, instead, more likely to be rare and desirable limited- access delicacies, of little nutritional

value, used by avaricious accumula- tors to both display and enhance their social power.

Another stress-free theory, this one championed by Jacques Cauvin,84

Box 1. Gourds, Dogs, and the Peopling of the Americas

Location of archeological sites with directly dated bottle gourd fragments studied by Erickson and coworkers.66

The recent integration of archeo- logical and genetic research on both the bottle gourd (Lagenaria siceraria) and the dog (Canis familiaris) has substantially clarified our understand- ing of the initial domestication of these two very early domesticates, as well as their late Pleistocene radiation from Asia across Beringia into the New World.20

The bottle gourd, indigenous to Af- rica and long valued as a container crop rather than as a food source, has consistently been recovered in arche- ological association with the earliest evidence of New World domesticated plants in many regions of the Ameri- cas. The prevalent consensus has been that L. siceraria was carried by ocean currents as a wild plant from Africa to South America. However, a morphological analysis comparing ar- cheological rind fragments from sites in the Americas (Fig. 1) with recently described wild L. siceraria fruits from Zimbabwe showed that the much thicker archeological rinds represent domesticated plants.66 Direct dates of these fragments indicate that do- mestic bottle gourd was present in the Americas by at least 10,000 years ago. Moreover, DNA recovered from nine archeological rind fragments predating the arrival of Europeans were identical to the modern Asian reference group, indicating that do- mesticated bottle gourds were car- ried to the New World during the late Pleistocene from Asia, not Africa. Al- though it is possible that the bottle gourd could have been carried from Asia to the Americas by the north Pa- cific current, it is more likely this early domesticate accompanied Paleoin- dian colonists as they crossed Ber- ingia into the New World.

A parallel genetic study points to an Old World origin of the domestic dog and suggests that at least five

founding dog lineages invaded North America with humans as they colonized the New World.63 The ear- liest archeological evidence of do- mesticated dogs in the Old World, dating to ca. 13,000 –17,000 B.P., comes from widely dispersed sites extending from the Near East across Eastern Europe. Although the earliest domesticated bottle gourd in the Old World dates to 8,000 –9,000 B.P. in China and Ja-

pan, it is reasonable, given its arrival in central Mexico by 10,000 B.P., to estimate that it was initially domes- ticated in the same general time frame as the dog, ca 13,000 B.P. or earlier. Together, these studies indi- cate that Paleoindians entered the New World with the world’s two ear- liest domestic species, dogs and bottle gourds, and that initial do- mesticates served utilitarian func- tions, but not as sources of food.

112 Zeder ARTICLES

also sets the stage for agricultural or- igins in a time of plenty and denies economic necessity a primary cata- lytic role. But Cauvin looks even more inward into the human psyche for the root cause of this transition. Accord- ing to Cauvin, domestication is a di- rect consequence of a conceptual shift in mankind’s mental template from one that saw humans as part of nature to one that cast humans in a dominant position, now free to manipulate and transform nature to their liking. This profound and irreversible transforma- tion in the way that humans saw themselves in relation to nature, cod- ified in religious ideology, found ex- pression in concrete ways: in art, household and community structure, and the domestication of plants and animals. A similar notion can be found in Hodder’s85 emphasis on the role of symbols and metaphors of hu- man dominance over nature made concrete in the form of the house, as the domus of domestication and the crucible of community.85

Increasing sophistication in ap- proaches to defining and document- ing domestication make it ever more difficult to support explanatory frame- works based on any single forcing fac- tor. This is particularly true for the Near East, where we have, arguably, the most complete record of the initial domestication of many plant and ani- mal species.

Climate clearly played its part in ag- ricultural origins here. Increases in rainfall and temperature following the Last Glacial Maximum at about 15,000 cal. B.P. undoubtedly contrib- uted to the adoption of increasingly less mobile, more territorially focused strategies centered on intensive ex- ploitation of plant resources rebound- ing out of glacial refugia,70 where they had been used by humans since the Upper Paleolithic.86,87 A subsequent pulse of cold, arid Ice Age conditions during the Younger Dryas, ca. 13,000 to 11,600 cal. B.P., was met with more mobile strategies in the Southern Le- vant88,89 and, quite possibly, the initial domestication of cereal crops and pulses in well-watered oasis localities in the Northern Levant.70 Ameliorat- ing climates following the Younger Dryas saw the domestication of other crop plants in the Southern Levant.86

Animal domestication seems to have come about sometime later on in the Central and Eastern Fertile Cres- cent.12 A brief warming and drying climatic pulse at about 9,000 cal. B.P. coincided with the collapse of early agricultural communities in more arid parts of the Southern Levant and their proliferation throughout the rest of the Near East.88 So while climate change played a role in domestication and agricultural origins in this region, it did not do so in the simple stimulus- response way implied by many mod- els that award environment prime- mover status. Instead, climate change alternately helped push and pull peo- ple along a pathway toward domesti- cation and agriculture, providing both opportunities and challenges that people across the region met in vari- ous ways depending on highly local- ized circumstances.

The transition from foraging to farming in the Near East clearly saw significant changes in mobility, popu- lation growth, and nucleation. But the record from the region does not sup- port the exponential population growth and attendant impoverish- ment of natural resources called for in Cohen’s90 doomsday model. In the ab- sence of settlement-pattern data ro- bust enough to demonstrate that his population-packing Rubicon (9.098 people per 100 km2) had been crossed, Binford pointed to the clear reduction in mobility, intensification in plant re- source use and development of stor- age technology during the postglacial era in the Near East as proxy evidence of packing.75 This troubling circular- ity of using the proposed results of packing as evidence of packing is found in many such demographic models.91 While population clearly in- creased, the admittedly incomplete settlement-pattern data do not sup- port the level of population packing that Binford grants exclusive causality for increasing sedentism, intensifica- tion of resource use and, ultimately, domestication. It is also hard to make a case that the initial focus on re- bounding populations of cereals, pulses, and nut trees after the Glacial maximum was caused by widespread depletion in higher ranking animal re- sources, as is required by most HBE models.89

Rather than being forced to settle down and focus on less desirable re- sources, it seems more likely that peo- ple took advantage of newly abundant high-yield plant resources and associ- ated herbivores in ways that enabled them to increase the size and duration of community nucleation beyond that possible under Ice Age conditions. It is also possible that when people were faced with localized pressures on re- sources resulting from more seden- tary ways of living, an interest in pre- serving the bonds of community provided an important incentive for the development of strategies that helped promote the yield and predict- ability of these resources. Moreover, these same social considerations also probably helped guide the subsequent responses to region-wide pressures caused by the climatic squeeze of the Younger Dryas and the stabilization of climate that followed.

Yet while it is possible to award so- cial factors a more active role in the origins of agriculture in the Near East, the record from the region clearly does not support Brian Hayden’s82,83

competitive feasting model. Even those with the loosest tether to reality would have a hard time seeing cereals, the earliest and most important do- mesticates in the region, as anything but widely available staple resources. And while there is some evidence for feasting on large numbers of ani- mals,92 no credible case can be made for Hayden’s83 blanket assertion that meat was consumed only within con- trolled ritual contexts. Moreover, all of the indicators of status differentia- tion and unequal appropriation of so- cial and economic prestige that Hay- den sees as material manifestations of his greedy accumulators have since been more convincingly cast by Kuijt93 and others as evidence of mechanisms for maintaining an equalitarian status quo in the face of mounting social tensions incurred when larger groups stay together for longer periods of time.93

Kuijt maintains, however, that do- mestication played little role in these social developments, either as a cause or a consequence.94 He bases this con- clusion on the fact that morphologi- cally altered domesticates appear in the archeological record of the Levant

ARTICLES Domestication of Plants and Animals 113

several thousand years after the first signs of leveling mechanisms for pro- moting social cohesion in the Natu- fian and at least a thousand years be- fore their ultimate collapse and the emergence of social inequality in the Late Pre-Pottery Neolithic. Like oth- ers who use the appearance of mor- phological domesticates as a thresh- old moment in their causal scenarios,84 Kuijt makes the common mistake of conflating morphological change with domestication. Once the artificial boundary crossing of mor- phological change is removed and fo- cus is more properly placed on the evolving relationship between hu- mans and plants and animals, then a more complex and ultimately more satisfying picture emerges of the syn- chronous and mutual reinforcing so- cial and economic forces that shaped the Neolithic Near East.

The partnership between humans and resurgent plant and animal re- sources that began the domestication process after the Last Glacial Maxi- mum provided the measure of re- source security and predictability needed to establish nucleated seden- tary communities bound together by an ethos of balanced reciprocity and

increasingly elaborate rules of social order and altered world views. By the same token, the goal of maintaining community was probably a factor contributing to the subsequent inten- sification of evolving domestication relationships, with their attendant im- pacts on both the human and the plant and animal partners. The do- mestication process, in turn, created resources that were more amenable to ownership and restricted access, ulti- mately contributing to the overthrow of the egalitarian communities they initially helped foster and maintain.

Thus, rather than a single forcing mechanism, it seems more likely that the trajectory of plant and ani- mal domestication in the Near East and the emergence of agriculture was shaped by various broad-scale factors, such as climate change, eco- nomic goals, and social opportuni- ties and constraints, interacting with highly local, contingent factors, such as the density and diversity of avail- able resources, the history of human occupation, and the agency of indi- viduals coping with their environ- ment, each other, and their universe. While the entire region was engaged in this process, the pace and the di-

rection it took varied depending on the distinctive mix of these factors in the Southern and Northern Levant, the Central and Eastern Fertile Cres- cent.

A similar range of factors operat- ing at the same time in other places on the planet took very different courses (Fig. 5). In Mexico, for ex- ample, squash, corn, and beans, ap- parently domesticated in different places and at different times, were minor components of mobile forag- ing strategies for millennia before the adoption of sedentism and the development of agricultural econo- mies.95 In both eastern North Amer- ica and Japan, small-seeded locally domesticated annuals were blended into the diverse economic round of stable, sedentary low-level food pro- ducers for thousands of years before the introduction of domestic crop plants, maize in eastern North Amer- ica and rice in Japan. Previously used in small numbers, these plants formed the foundation for the emer- gence of more fully agricultural economies and increasingly strati- fied societies.95,96

Thus, the story of domestication and agricultural origins consists of a

Figure 5. Boxes indicate the general location of centers of independent plant or animal domestication. Currently, at least ten such centers are recognized around the world, making research on the origins of agriculture a truly world-wide enterprise. Future research will, no doubt, discover others.

114 Zeder ARTICLES

series of complex regional puzzles shaped in unique ways by a dynamic multi-scalar range of macro- and mi- cro-forces. Attempts at explanation that champion any one of these fac- tors and deny the importance of oth- ers will not, in the long run, contrib- ute to understanding agricultural origins either as a general process or as it played out in particular in- stances.

CONCLUSIONS

As this review demonstrates, central questions about the definition, docu- mentation, and explanation of domes- tication are not easily answered. Do- mestication cannot be simply defined as either a biological or a cultural phe- nomenon, but rather needs to be seen as a form of biological mutualism transformed by the highly developed human capacity to effect behavioral change through learning and cultural transmission. Definitional approaches to domestication are most effective, then, when they focus on the evolving relationship between humans and tar- get plant or animal populations as a nexus between biology and culture, not on the manifestations or conse- quences of such relationships. Ge- netic and related morphological changes in domesticates are not defin- ing features of domestication, but are instead artifacts of evolving relation- ships that vary in their intensity and pace of development. Notions of own- ership and restructuring of social re- lations are similarly best viewed as possible results of domestication, not as central to its definition. Nor are the clear-cut thresholds that define when wild resources become domesticated ones. Rather than looking for defini- tive either-or boundary conditions in defining domesticates, it is much more profitable, if more challenging, to look at the whole span of evolving domestic relationships as they operate over various scales of investment on the part of both human and plant or animal partners.

There are also no easy, universally applicable ways to document domes- tication. Instead, documenting do- mestication requires a clear under- standing of the species-specific linkage between a proposed marker of domestication and the stage of the un-

folding domestication process it is held to mark. It also requires recog- nizing that markers vary depending on the biology of the species involved and the cultural context of human populations engaged in the domesti- cation process. Above all, effective documentation means not letting the availability of new scientific tech- niques lead the search for new mark- ers without first thinking about how the process of domestication might manifest itself in whatever these tech- niques are designed to measure.

So, too, causal scenarios that nar- rowly focus on single, universally ap- plicable prime-mover levers of change will never provide satisfying answers to the critical “why” questions about the origins of domestication and sub- sequent agricultural emergence. It is easy when dealing with complex and extended processes to draw artificial thresholds that help make the case for the primacy of whatever variable one is sponsoring as the cause of events that follow, whether it be climate change, population increase, or social and ideological transformations. Yet we have gained too sophisticated an understanding of the process of do- mestication and the means of detect- ing it to support the kind of drive-by theorizing that selectively chooses ac- commodating bits of information from individual regional scenarios to support a favored epistemology de jour. Advances in answering “why” questions about domestication and agricultural origins can only be gained through close-grained under- standing of complex multi-scalar re- gional puzzles and assessment of the commonality and the differences in the way the pieces of these different puzzles fit together.16,97

There are, then, no easy answers to central questions about domestica- tion and agricultural origins. It is no wonder that for more than 100 years this area of inquiry has held the atten- tion of archeologists working world- wide and representing all of archeolo- gy’s many and rapidly increasing subdisciplines. It is a research domain that carries broad currency with scholars based in biological and phys- ical sciences, social sciences, and hu- manities. It is a topic that captures the imagination of a public interested in

how the familiar world around them came to be. It is a problem that truly matters. With an enhanced under- standing of the nature of the problem and an expanding array of powerful tools for studying it, there has never been a time of greater promise for pursuing challenging questions about the origin and diffusion of domesti- cates and agricultural economies in virtually all areas of the globe.

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