The Origins of Agriculture as a Natural Experiment in Cultural Evolution

Peter J. Richerson, Department of Environmental Science and Policy, University of California—Davis, Davis, CA 95616, pjricherson@ucdavis.edu

Robert Boyd, Anthropology Department, University of California—Los Angeles, Los Angeles, CA 90024, rboyd@anthro.ucla.edu

Robert L. Bettinger, Anthropology Department, University of California—Davis, Davis, CA 95616, rlbettinger@ucdavis.edu

  Abstract: Several independent trajectories of subsistence intensification, often leading to agriculture, began during the Holocene. No plant rich intensifications are known from the Pleistocene, even from the late Pleistocene when human populations were otherwise quite sophisticated. Recent data from ice core climate proxies show that last glacial climates were extremely hostile to agriculture—dry, low in atmospheric CO₂, and extremely variable on quite short time scales. We argue that agriculture was impossible under last-glacial conditions. The quite abrupt final amelioration of the climate was followed immediately by the beginnings of plant intensive resource use strategies in some areas, although the turn to plants was much later elsewhere. Almost all trajectories of subsistence intensification in the Holocene are progressive and eventually agriculture became the dominant strategy in all but marginal environments. In the Holocene, agriculture is, in the long run, compulsory. We use a mathematical analysis to show that the rate limiting process for intensification trajectories must almost always be the rate of innovation of subsistence technology or subsistence related social organization. At the observed rates of innovation, population growth will always be rapid enough to sustain a high level of population pressure. Several processes appear to retard rates of cultural evolution below the maxima we observe in the most favorable cases.

   Resumen: Varias trayectorias independenties de la intensificación del sustento , muchas de las cuales conducieron a la agricultura, empezaron durante el Holoceno. No conocemos ninguna intensificació que usara recursos vegetales durante el Pleistoceno, inclusive el Pleistoceno último, cuando las poblaciones humanas fueron muy sofisticadas en otros ámbitos. Datos recientes de cilindros de hielo sacados de Groenlandia muestran que la última glaciación fué extremadamente hostil a la agricultura, ya que fué—seca, baja en CO₂, y extremadamente variable en el corto plazo. Proponemos que la agricultura fué imposible en estas condiciones de la última glaciación. La súbita mejora del clima al final de la glaciación fué seguida inmediatamente por la iniciación de usos intensivos de los recursos vegetales en algunos lugares, aunque mucho mas tarde en otras partes. Casi todo las trajectores de intensificación en el Holoceno eran occurrieron sin retroceso. Finalmente, la agricultura se convirtió en el modo principal de sustento en todas partes, excepto por zonas muy frías o muy secas. En el Holoceno, la agricultura se vuelve, a final de cuentas, obligatoria. Usamos un análisis matemático para mostrar que los procesos limitante de la taza de intensificación debe ser casi siempre la taza de la inovación tecnológica en las estrategias de sustento, o la taza de inovación en las formas de organazación social en relación al sustento. Con las tazas de inovación que se observan, el crecimiento de la población siempre es suficientemente rapido como para crear alto nivel de presión poblacional. Al parecer , varios processos normalmente retardan la velocidad de la evolución cultural abajo de las tazas máximas que observamos en el modelo.
 

While observing the barbarous inhabitants of Tierra del Fuego, it struck me that the possession of some property, a fixed abode, and the union of many families under a chief, were the indispensable requisites for civilization. Such habits almost necessitate the cultivation of the ground; and the first steps would probably result from some such accident as the seeds of a fruit tree falling on a heap of refuse, and producing some unusually fine variety. The problem, however, of the first advance of savages toward civilization is at present much too difficult to be solved.

                    Charles Darwin Descent of Man 1874

 

 Evolutionary thinkers have long been fascinated by the origin of agriculture. While Darwin declined to speculate on agricultural origins, Twentieth Century scholars were bolder. The Soviet agronomist Nikolai Vavilov, American geographer Carl O. Sauer, and British archaeologist V. Gordon Childe wrote influential books and papers on the origins of agriculture in the 1920s and 30s (see MacNeish 1991: 4-19, for a discussion of the intellectual history of the origins of agriculture question). These explorations were necessarily speculative and vague, but they stimulated interest. Vavilov and Sauer argued that agriculture originated in locations where the wild ancestors of later crop species attracted the attention of hunters and gatherers, leading eventually to domestication. Childe argued that climate change accompanying the end of the Pleistocene was responsible for the initial steps that led to domesticated crops.

    Immediately after World War II, the American archaeologist Robert Braidwood (Braidwood, et al. 1983) pioneered the systematic study of agricultural origins. From the known antiquity of village sites in the Near East and from the presence of wild ancestor species of many crops and animal domesticates in the same region, Braidwood inferred that this area was likely a locus of early domestication. He settled on a region in the foothills of the southern Zagros Mountains in Iraqi Kurdistan as a likely place for domestication to have begun. He then embarked on an ambitious program of excavation using a multi-disciplinary team of archaeologists, botanists, zoologists, and earth scientists to extract the maximum useful information from the excavations. The availability of ¹⁴C dating gave his team a powerful tool for determining the ages of the sites.

    The Braidwood team’s main effort focussed on the Jarmo site, but excavation at a site nearby uncovered an earlier settlement that had been occupied seasonally by hunter-gatherers who were collecting wild seeds, probably the ancestors of wheat and barley, and hunting the wild ancestors of goats and sheep about 11,000 years ago. (Ages are given here as calendar dates before present (B.P.), where present is taken to be 1950, estimated from ¹⁴C dates according to Stuiver, et al.’s (1998) calibration curves.) Near Eastern sites older than about 15,000 B.P. excavated by Braidwood (Braidwood and Howe 1960) and others were occupied by hunter-gatherers who put much more emphasis on hunting and much less on collecting and processing seeds (Henry 1989; Goring-Morris and Belfer-Cohen 1998). At Jarmo itself, the team excavated an early farming village dating to about 9,000 B. P. Using much the same seed processing technology as their immediate gathering predecessors, the Jarmo people no longer moved their residences with the seasons. Analyses of plant and animal remains suggested that the process of domestication was underway. This early agricultural village is at the base of an archaeological record of larger and increasingly sophisticated agrarian settlements that characterize the Near Eastern archaeological record leading to the first state level societies in Mesopotamia about 5,500 BP.

    Since the pioneering Jarmo excavations, a few dozen multi-disciplinary archaeological teams, have studied likely sites of agricultural transitions in the Near East, North and South America, the Far East, and Sub-Saharan Africa. At least six regions (Table 1) were autonomous centers of plant and/or animal domestication, and incomplete, controversial evidence is taken by some scholars to imply at least two more. Ehret (1998) argues on grounds of “linguistic archaeology” that Saharan and Sub-Saharan Africa had three independent centers of domestication rather than the one more commonly accepted. Historical linguists have reconstructed three sets of agricultural vocabulary uncontaminated by loan-words, implying independent, rather early, episodes of domestication. These investigations have discovered no region in which agriculture developed earlier or faster than in the Middle East, though a North Chinese center of domestication of millet may prove almost as early (see discussion below). Indeed other centers seem to have developed later, or more slowly, or with a different sequence of stages, or all three. The spread of agriculture from centers of origin to more remote areas is well documented for Europe and North America. Ethnography also gives us cases where hunters and gathers persisted to recent times in areas seemingly highly suitable for agriculture, most notably much of western North America and Australia. Attempts to account for this rather complex pattern are a major focus of archaeology.

Origin of agriculture is a macroevolutionary “experiment”

Cultural evolution is best studied using Darwinian methods

Macroevolution is Interesting and Important

Macroevolutionary hypotheses come in two flavors

Climate Change Started the Experiment

Agriculture was impossible in the Pleistocene

    Holocene weather extremes have significantly affected agricultural production (Lamb 1977). It is hard to imagine the impact of the qualitatively greater variation that characterized of most if not all of the Pleistocene. Devastating floods, droughts, windstorms, and the like, which we experience once a century, might have occurred once a decade. Tropical organisms did not escape the impact of this climate variation; temperature and especially rainfall were highly variable at low latitudes (Broecker 1996). Plant and animal populations responded to climatic change by dramatically shifting their ranges, but climate change was significant on the time scales shorter than those necessary for range shifts to occur. As a result, natural communities must have always been in the process chaotic reorganization as the climate varied more rapidly that they could reach equilibrium. The pollen record from the Mediterranean illustrates how much more dynamic plant communities were during the last glacial (Allen, et al. 1999).

    In addition to fluctuations in temperature and rainfall, the CO₂ content of the atmosphere was about 190 ppm during the last glacial, rising to about 250 ppm at the beginning of the Holocene (Figure 3). Photosynthesis on earth is CO₂ limited over this range of variation (Sage 1995). During the last glacial period, seed yields may have been something like 2/3s of Holocene yields. Nothing appears to be known about the evolutionary responses of plants to lower CO₂ concentrations. If low CO₂ caused plants to allocate more photosynthate to vegetative rather than reproductive tissues like seeds or storage organs like tubers, then the impact on low CO₂ on the attractiveness of plant foods might be underestimated by the data reviewed by Sage.

    During the last glacial, at least, people lived under environmental conditions that almost certainly made heavy dependence on food plants unattractive. On present evidence we cannot determine whether aridity, low CO₂, or climate variability is the main culprit in preventing the evolution of agriculture. Low CO₂ and climate variation would handicap the evolution of dependence on plant foods everywhere. Plant food rich diets take considerable time to develop. Processing technology has to be invented and made efficient. Plant foods are generally low in protein and often high in toxins. Some time is required to work out a balanced diet of plant foods. The direct archaeological evidence suggests that people began to use extensively the technologies that underpinned agriculture only after about 15,000 B.P. (Bettinger in press). As CO₂ levels rose, climate variability decreased, and rainfall increased, human populations in several parts of the world began to turn to the exploitation of locally abundant plant resources, but only during the so-called Bølling-Allerød period of near interglacial warmth and stability. One last siege of glacial climate, the Younger Dryas from 12,900 yrs B.P. until 11,600 yrs B.P., probably reversed these adaptive trends (e.g., Goring-Morris and Belfer-Cohen 1998). The Younger Dryas climate was appreciably more variable than the preceding Allerød-Bølling and the succeeding Holocene (Grafenstein, et al. 1999; Mayewski, et al. 1993). The ten abrupt, short, warm-cold cycles that punctuate the Younger Dryas ice record were probably felt as dramatic climate shifts all around the world. After 11,600, the Holocene period of relatively very warm, wet, stable, CO₂ rich environments began. Subsistence intensification and eventually agriculture followed. Thus, while not a perfectly instantaneous change, the shift from glacial to Holocene climates was a very large change, and took place much more rapidly than cultural evolution could track.

    Might we not expect agriculture to have emerged in the last interglacial 130,000 years ago or even during one of the even older interglacials? No archaeological evidence has come to light suggesting the presence of complex technologies that might be expected to accompany forays into intensive plant collecting or agriculture at this time. The human populations of the last interglacial were still archaic in anatomy and behavior, though the first anatomically (but not yet culturally) modern humans occur in the archaeological record just after 130,000 years ago (Klein 1999: Ch.7). Very likely, then, the humans of the last interglacial were neither cognitively nor culturally preadapted for the evolution of agricultural subsistence. We should point out, however, that an external explanation might also explain the lack of agriculture during previous interglacials. Ice core data from the thick Antarctic ice cap at Vostok show that each of the last four interglacials over the last 420,000 years was characterized by a short, sharp peak of warmth, rather than the 11,500 year long stable plateau of the Holocene (Petit, et al. 1999). Further, the GRIP ice core suggests the last interglacial (130-80,000 B.P.) was more variable than the Holocene although its lack of agreement with a nearby replicate core for this time period makes this interpretation tenuous. On the other hand, the atmospheric concentration of CO₂ was higher than during the Holocene in the three previous interglacials, and was stable at high levels for about 20,000 years following the warm peak during the last interglacial. The highly continental Vostok site unfortunately does not record the same high frequency variation in the climate as most other proxy climate records, even those in the Southern Hemisphere (Steig, et al. 1998). Some Northern Hemisphere marine and terrestrial records suggest that the Last Interglacial was highly variable while other data from suggest a Holocene length period of stable climates ca. 117,000-127,000 B.P. (Frogley, et al. 1999). In sum, better data on the high frequency part of the Pleistocene beyond the reach of the Greenland ice cores is needed to test hypotheses about human macroevolutionary events antedating the latest Pleistocene. That aside, the comparatively primitive nature of human adaptation during the last interglacial seems sufficient by itself to account for the observed lack of agricultural experimentation then.

In the Holocene, agriculture is compulsory in the long run

Alternative hypotheses are weak

Population Growth Has Wrong Time Scale

More realistic population dynamics. The logistic equation assumes that an increment to population density has the same effect on population pressure at low densities as at high densities. We know that this assumption is not correct for all species. For example, hunters pursuing herd animals that are easily spooked may generate much population pressure at low human population densities because killing only a small fraction of the herd make the many survivors difficult (but not impossible) to hunt. On the other hand, subsistence farmers spreading into a uniform fertile plain may feel little population pressure until all of plots of farmland of conveniently workable size are taken. If returns to additional labor on shrinking farms then drop steeply, most population pressure will felt at densities near K. To allow for such effects, ecologists often utilize a generalized logistic equation 

Moreover, this calculation seriously over estimates the amount of time that will pass before any segment of an expanding Eurasian population will experience population pressure because populations will approach carrying capacity locally long before the entire continent is filled with people. R. A. Fisher analyzed the following partial differential equation that captures the interaction between population growth and dispersal in space: 

Consider a population of size N in which the per capita income of the population is given by: 

where y_s is the per capita income necessary for subsistence. If per capita incomes are above this value, population increases; if per capita income falls below y_s, population shrinks. If I is fixed, this equation is another generalization of the logistic equation. In an initially empty environment, population initially grows at a rate

, 

but then slows and reaches an equilibrium population size

. 


To allow for intensification we assume that people intensify whenever their per capita income falls below a threshold value y_i. Thus

The steady state per capita income is above the minimum for subsistence but below the threshold at which people experience population pressure and intensify their production. At this steady state population growth continues at a constant rate,

, 

that is proportional to the rate of growth in intensification. Thus, there is an initial phase in which the population grows rapidly until population growth is slowed by population pressure followed by a steady state in which population pressure is constant, and just enough intensification occurs to compensate for population growth. For plausible parameter values, the second phase of population growth steady state is reached in less than a thousand years. Interestingly, increasing the intrinsic rate of intensification, a, or the intensification threshold, y_i, reduces the waiting time until population pressure is important.

    Thus, for parameter values that seem anywhere near reasonable to us, population growth on millennial time scales will be rate limited by rates of improvement in subsistence efficiency not by the potential of populations to grow, just as Malthus argued. Populations can behave in non-Malthusian ways only under extreme assumptions about population dynamics. If the rate of intensification is more rapid than exponential population growth for any significant time period, then per capita incomes can rise under a regime of very rapid population growth, as in the last few centuries. This regime, if it had occurred in the past, should be quite visible in the historical and archaeological record because it so rapidly leads to large populations. Alternatively, population growth may have been limited in past populations by the analog of the modern demographic transition. Thus, hunter-gatherers might have resisted the adoption of plant based intensification because they viewed the life style associated with plant collecting or planting as a decrement to their incomes. However, resisting intensifications that increase human densities makes such groups vulnerable to competitive displacement by the intensifiers unless the greater wealth of the population limiters allows them to successfully defend their resource rich territories. On the evidence of the fairly rapid rate of spread of intensified strategies once invented, such defense is seldom successful (e.g. Ammerman and Cavalli-Sforza 1971; Bettinger and Baumhoff 1982).

    Of course, in a time as variable as the Pleistocene, populations may well have spent considerable time both far above and far below instantaneous carrying capacity. If agricultural technology were quick and easy to develop, the population pressure argument would lead us to expect Pleistocene populations to shift in and out of agriculture and other intensive strategies as they find themselves in subsistence crises due to environmental deterioration or in periods of plenty due to amelioration. Most likely, minor intensifications and de-intensifications were standard operating procedure in Pleistocene times. However, the time needed to progress much toward plant rich strategies was greater than the fluctuating climate allowed, especially given CO₂ limited plant production.

Cultural Evolution Has Wrong Time Scale

Strong Similarities and Differences in Different Replicates of the Experiment

Agriculture was independently invented ~ 10 times in the Holocene

The dates in Table 1 reflect considerable recent revision stemming from accelerator mass spectrometry ¹⁴C dating, which permits the use of very small carbon samples and can be applied directly to carbonized seeds and other plant parts showing morphological changes associated with domestication. Isolated seeds tend to work their way deep into archaeological deposits, and dates based on associated large carbon samples (usually charcoal) often gave anomalously early dates.

    The exact sequence of events also varies quite widely. For example, in the Near East, sedentarism preceded agriculture, at least in the Levantine Natufian sequence, but in Mesoamerica crops seem to have been added to a hunting and gathering system that was dispersed and rather mobile (MacNeish 1991: 27-29). For example, squash seems to have been cultivated around by 10,000 B.P. in Mesoamerica, some 4,000 years before corn and bean domestication began to lead to the origin of a fully agricultural subsistence system (Smith 1997). Some mainly hunting and gathering societies seem to have incorporated small amounts of domesticated plant foods into their subsistence system without this leading to full scale agriculture for a very long time. According to MacNeish, the path forward through the whole intensification sequence varied considerably from case to case.

Intensification of Plant Gathering Precedes Agriculture

More Intensive Technologies Tend to Spread

Agriculture a “Natural” Outcome of More Intensive Technologies

What Regulates Tempo and Mode of Cultural Evolution?

External Processes Play a Role

    Since we know that the original centers of agricultural innovation were relatively small compared to the areas to which agriculture later spread, it is clear that complex adaptations arise infrequently, but are relatively easy to acquire by imitation. The most isolated agricultural region in the world, Highland New Guinea, underwent an economic revolution in the last few centuries with the advent of American sweet potatoes, a crop that thrives in the cooler highlands above the malaria belt of lowland New Guinea (Wiessner and Tumu 1998). The agriculture of both the Old and New Worlds was impacted substantially by the Columbian exchanges beginning 500 years ago. Eurasian agriculture found good uses for American maize, potatoes, squash, beans, peppers, and tomatoes, despite the already deep list of ancient Eurasian cultivars. The marginal benefit of increased numbers, hence diversity, of domesticates and inventors seems to have been substantial even on the largest continent on earth.

Coevolutionary Processes Play a Big Role

Humans have to adapt biologically to agricultural environments. While the transition from hunting and gathering to agriculture resulted in no genetic revolution in humans, a number of modest new biological adaptations were likely involved in becoming farmers. Perhaps the best-documented case is the evolution of adult lactose absorption in human populations with long histories of dairying (Durham, 1991). Most human populations, as in mammals more generally, adults lose the ability to digest the milk sugar lactose. However, the frequency of adult absorption reaches near fixation in Western European and African populations with a long tradition of drinking fluid milk. It is present in intermediate frequencies in populations in the circum-Mediterranean and other subtropical regions that more typically consume milk as cheese or fermented products low in lactose. Other cases suggest human genetic coevolution with domestication. For example, agricultural populations metabolize alcohol more rapidly than hunter-gatherers, either to improve the nutritional yield from such products or to avoid toxic effects. To some extent the relatively slow rate of biological adaptation will act as a drag on the rate of cultural innovation.

    Diseases limit population expansions, protect inter-regional diversity. McNeill (1979) and Crosby (1986) draw our attention to the coevolution of people and diseases. The increases in population density that resulted from the intensification of subsistence invited the evolution of epidemic diseases that could not spread at lower population densities. One result of this process was likely to slow population growth and hence slow the rate at which growing populations fuel the competitive ratchet. In more disease prone environments, such as malarial tropical lowlands, this restraint on the growth process may have been quite severe. A suite of hemoglobins have arisen in different parts of the world that confer partial protection against malarial parasitism (Cavalli-Sforza, et al. 1994). Some authors have suggested that land clearance associated with agriculture increased malarial infection rates. Cavalli-Sforza et al. estimate that it would take about 2,000 years for a new mutant hemoglobin variant to reach equilibrium in a population of 50,000 or so individuals. The relatively late and slow rate of the evolution of agricultural systems in Africa might, in part be attributable to the high rates of disease infection of people and livestock slowing the rate of intensification induced population growth to the macroevolutionary time scale (see also Gifford-Gonzales in press).

    As both McNeill and Crosby note, regional populations tend to have their own diseases, to which they are more or less immune, while they tend to be susceptible to the diseases of people from other regions. This pattern of susceptibility will tend to act against communication between regions. Travelling strangers who might bring new ideas and stronger competition will tend to fall sick and to bring diseases that cause their hosts to fall sick (at least temporarily reducing competition for land). Disease barriers will tend to isolate regions and hence slow rates of diffusion of ideas. This effect was probably historically most severe for Africa and other Old World tropical areas that were quite unhealthy for people from temperate Eurasia. On the other hand, in cases like the European invasion of the New World and smaller insular areas with low loads of infectious disease, European conquest was greatly aided if not entirely made possible by a disease gradient operating at the expense of native populations. To the extent that populations sustaining higher densities sustain more endemic diseases, they will have a competitive edge against less dense populations without a history of exposure to so many diseases.

Purely Internal Processes Play a Role

    Parallel problems are probably rife in human subsistence systems. The shift to plant-rich diets is complicated because plant foods are typically deficient in essential amino acids, and vitamins, have toxic compounds to protect them from herbivore attack, and are labor intensive to prepare. The diet of Pleistocene hunters and gathers probably focused on high rates of meat intake supplemented by high quality plant foods such as ripe fruit and nuts. High quality plant resources are scarce and the inefficiency of natural herbivore populations means that meat offtake rates are usually quite limited. Intensification requires a focus on seeds low in essential amino acids (maize), tubers with poisonous protection (bitter manioc), and the like. Even the best plant resources like wheat require protein supplementation with animal products or legumes. Skeletal material suggests that early agricultural peoples were often less well nourished than their hunter-gather ancestors (Cohen and Armelagos 1984). Finding a mix of plant and animal foods that provides adequate diet is not a trivial problem. Many different mixes may work more or less well, but which one(s) are globally or nearly globally optimal?

    New World farmers eventually discovered that boiling maize in wood ashes improved its nutritional value. The hot alkaline solution breaks down an otherwise indigestible seed coat protein that contains some lysine, an amino acid that is low in maize relative to human requirements (Katz, et al. 1974). Hominy and masa harina, the corn flour used to make tortillas, are forms of alkali treated maize. The value of this practice could not have been obvious to its inventors or later adopters, yet all American populations that made heavy use of maize employed it. The dates of origin and spread of alkali cooking are not known. It has not been reinvented in Africa even though many African populations have used maize for centuries.

    The prehistory of subsistence intensification consists of a long sequence of inventions that gradually increase the sophistication of food producing systems. Arboriculture, irrigation, animal traction, dairying, wool use, and the like were added to toolkits thousands of years after people were committed to agricultural production. Plausibly, each of these inventions was the product of prolonged experimentation and development in a center of origin, followed by a slow spread to adjacent areas. Once again, the large and size east-west axis of the Eurasian land mass likely accelerated the pace of development relative to other areas. The New World, among other problems, was poor in animal species suitable for domestication due to the terminal Pleistocene megafaunal extinction event, arguably caused by the initial wave of human settlers (Martin and Klein 1984). To make matters worse, the most important New World domesticates, the camelids of the temperate Andes, did not spread across the wet tropical belt into Central and North America. As intensification proceeded, each innovation adopted had to be fitted into an existing system of land use, labor allocation, and diet. In rapidly developing Eurasia, the rate of subsistence intensification could well have been limited partially or even mainly by the technical complexity of the innovation process.

    Some of the difficulties in evolving new subsistence systems may stem from dynamic problems in the economy. Lee (1986) imagines that the rate of innovation is correlated with the rate of profit for invested capital. In a Ricardian world, profits will be low when population density is low because wages will be too high. Conversely, when population density is too high, profits will be squeezed by rents to land. With profits low, entrepreneurs will lack surplus capital to invest in innovation. Entrepreneurs will thus innovate only in a narrow window between these two constraints. If we imagine a population fluctuating due to epidemic disease, famine, and the like, little bursts of innovation will occur as population fluctuates through the key window. Day and Walter (1989) investigate a similar model the exhibits a chaotic growth path.

    New social institutions evolve with difficulty. As anthropologists and sociologists such as Julian Steward (1955) have long emphasized, human economies are social economies. Even in the simplest human societies, hunting and gathering is never a solitary occupation. At the minimum, such societies have division of labor between men and women. Hunting is typically a cooperative venture. The unpredictable nature of hunting returns typically favors risk sharing at the level of bands composed of a few cooperating families because most hunters are successful only every week or so (Winterhalder 1986). Portions of kills are distributed widely, sometimes exactly equally, among band members.

    The deployment of new technology requires changes in social institutions to make best use of innovations. The increasing scale of social institutions associated with rising population densities during the Holocene have dramatically reshaped human social life. Even the first steps of intensification required significant social changes. Gathering is generally the province of women and hunting of men. Male prestige systems are generally tied up with hunting success. A shift to plant resources requires scheduling activities around women’s work rather than men’s. Using more plants will conflict with men’s preferences as driven by a desire for hunting success; it will require a certain degree of women’s liberation to intensify subsistence. Since men generally dominate women in group decision-making, male chauvinism will tend to limit intensification. Bettinger and Baumhoff (1982) argue that the spread of Numic speakers across the Great Basin a few hundred years ago was the result of the development of a plant-intensive subsistence system in the Owens Valley. Once developed, the spread eastward to the Rockies because the Numics could outcompete the previous inhabitants who neglected plant resources in favor of the hunt. The evidence suggests a demic expansion rather than a cultural diffusion. Apparently, the groups specialized in the hunt would not or could not shift to the more productive economy to defend themselves, perhaps because males clung to the outmoded, plant poor, subsistence. Those with a vested interest in current social institutions are naturally conservative.

    Classic hunting and gathering societies are relatively small in scale, and are quite egalitarian (Woodburn 1980). Hence they have very informal leadership. The evolution of formal coercive leadership, hierarchical command and control systems, and the rise of marked social inequality are resented and resisted, most likely because they violate our social “instincts,” for example our sense of what is and is not fair (Richerson and Boyd 1998, 1999). As people began to collect in villages, ethnographic analogies suggest that formal political leaders likely became important. As the agricultural toolkit expanded, larger markets for sophisticated products came to favor an ever-finer division of labor. Various sorts of institutions arose to manage the division of labor—caste systems, command economies, market systems, international trade systems and the like. The creation of storable and portable wealth exacerbates defense and security problems, leading to the rise of armies and police forces.

    The evolution of complex social institutions is arguably as or more difficult than the evolution of technological innovations. If so, social institutions will tend to regulate technological progress. For example, North and Thomas (1973) argue that new and better systems of property rights set off the modern industrial revolution, much as Bettinger and Baumhoff argue that new social institutions were required to favor innovations in plant collection strategies at the inception of the intensification trajectory. A major revolution in property rights is likely also necessary for intensive hunting and gathering and agriculture to occur (Bettinger 1999). The sharing ethic that characterizes many simple hunter-gatherer groups discourages individual efforts to intensify production since freeloaders enjoy the extra resources generated by hard working individuals. Thus, accumulating resource surpluses to store for seasons of shortfall is heavily contingent on an ideology in which collected resources are regarded as private property. The shift from resources as public goods to resources as private goods, however, is likely to be difficult since any form of innovation along these lines will, at least initially, be regarded as antisocial. The comparative history of the social institutions of intensifying societies exhibits many examples of societies getting far ahead of others in one dimension or another. For example, the Chinese merit based bureaucratic system of government was established at the expense of the landed aristocracy, beginning in the Han dynasty (2,200 B.P.) and completed in the Tang (1,400 B.P.) (Fairbank 1992). In the West similar institutions arose only in the last few centuries.

    Progress aside, functionally analogous institutions are notoriously different in different culture areas. The Indian caste system is a unique, or at least uniquely hypertrophied, method of organizing a complex division of labor. The Turkish system of raising armies (Janissaries) and even ruling classes (Mamlukes) by enslaving Christian boys is similarly unique. Social institutions seem generally to diffuse much less readily than technology. In the late mediaeval and early modern period, the West acquired a considerable number of important technical innovations from China, but certainly not the idea of a Confucian bureaucracy. Social institutions violate four of the conditions that tend to facilitate diffusion (Rogers 1983). Foreign social institutions are often (i) not compatible with existing institutions, (ii) complex, (iii) difficult to observe, and (iv) difficult to try out on a small scale. Technical innovations like the compass and the cannon are thus much more easily diffused than social institutions. Hence historical differences in social organization, many of them less nearly optimal than those already traditional elsewhere, are quite persistent.

    Ideology may play a role. Forces other than strictly utilitarian ones like natural selection govern the evolution of fads, fashion, and belief systems. They are susceptible feedback and runaway dynamics that defy common sense (Boyd and Richerson 1985: Chap. 8). The arbitrariness of the runaway dynamic produces path dependent historical change, often in defiance of economic or fitness rationality. The links between belief systems and subsistence are nevertheless incontestably strong. To build a cathedral requires an economy that produces surpluses that can be devoted to grand gestures on the part of the faithful. The moral precepts inculcated by the clergy in the cathedral underpin the institutions that in turn regulate the economy. Arguably, ideological innovations often drive economic change. Recall Max Weber’s classical argument about the role of Calvinism in the rise of capitalism. The Twentieth Century’s largely failed experiments with nationalist and socialist command economies is an example of an ideology driven exploration in the space of all possible social formations, but not one that was very well controlled to make progress.    Contemporary Russia illustrates the great difficulty of making the transition from a failed set of institutions to a new set. So, arguably, does the inability of the United States to take risks with economic growth in pursuit of policies that may be necessary to prevent global warming and similar threats to economic sustainability. Contemporary consumerism is arguably the result of institutionalizing the pathological tendency of status competition to escalate without limit (Frank and Cook 1995, Easterlin 1995).

We Have a Shadowy Outline of the Tempo and Mode of Cultural Evolution

The End of Agriculture?

REFERENCES CITED

Fiedel, S.J.

 
 

TABLE 2. World Distribution of Large-Seeded Grass Species (From Diamond, 1997, after Blumler, 1992)
Area	Number of species
West Asia, Europe, North Africa	33
Mediterranean zone		32
England		1
East Asia	6
Sub-Saharan Africa	4
Americas	11
North America		4
Mesoamerica		5
South America		2
Northern Austalia	2
	Total	56

 

Figure 1. Profiles of a temperature index, d¹⁸O, and a index of dust content, Ca²⁺, from the GRIP Greenland ice core. 200 year means are plotted. Histograms show how much noisier the last glacial and perhaps last interglacial were compared to the Holocene. The parts of the GRIP profile representing the last interglacial (MIS 5e) may have been affected by ice flow so their interpretation is uncertain. (GRIP, 1993, copyright © 1993 Macmillan Magazines Limited.)
 

Figure 2. High resolution analysis of the GRIP ice core d¹⁸O data. The low pass filtered data shows that the Holocene is much less variable than the Pleistocene on time scales of 150 years and longer. The high pass filtered data shows that the Pleistocene was also much more variable on time scales less that 150 years. Since diffusion increasingly affects deeper parts of the core by averaging variation on the smallest scales, the high pass variance is reduced in the older parts of the core. In spite of this effect, the Pleistocene/Holocene transition is very strongly marked. (Ditlevsen, et al., 1996, copyright © 1993 Macmillan Magazines Limited.)
 
 

Figure 3. Panel a shows the curve of atmospheric CO₂ as estimated from gas bubbles trapped in Antarctic glacial ice. Data from Barnola et al. (1987). Panel b summarizes responses of several plant species to experimental atmospheres containing various levels of CO_2.  Based on data summarized by Sage (1995).
 
 

Figure 4. A numerical simulation of Fisher’s equation showing that after an initial period, population spreads at a constant rate so that at any point in space population pressure increases to its maximum in less than 500 years for reasonable parameter values.(Redrawn from Ammerman and Cavalli-Sforza, 1984).
 
 

Figure 5 plots the logarithm of population size as a function of time for the model described in the text. Initially, when there is little population pressure, population grows at a high rate. As the population grows, per capita income decreases, and people intensify. Eventually the population growth rate approaches a constant value at which the growth of intensification balances growth in population. For reasonable parameters (a = 0.005, r = 0.02, y_m = 1, y_s = 0.1, y_i = 0.2, initial population size 1% of initial carrying capacity), it takes less than 500 years to shift from the initial low population pressure mode of growth to the final high population pressure mode of growth.