The Evolution of Social Behavior in the Prehistoric American Southwest
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The Evolution of Social Behavior George J.Gumerman Santa FeInstitute in the Prehistoric American 1399Hyde Park Road Southwest Santa Fe,NM 87501 AlanC. Swedlund Departmentof Anthropology Universityof Massachusetts– Amherst Jeffrey S.Dean Laboratory of Tree-Ring Abstract Long HouseV alley, located in theBlack Mesa area of northeastern Arizona (USA), was inhabited bythe Kayenta Research Anasazi from circa 1800 B.C. to circa A.D. 1300.These people TheUniversity of Arizona wereprehistoric precursors of themodern Pueblo cultures of JoshuaM. Epstein theColorado Plateau. Arich paleoenvironmental record, Centeron Social and based on alluvial geomorphology, palynology, and dendroclimatology, permitsthe accurate quantitative Economic Dynamics reconstruction of annual uctuations in potentialagricultural TheBrookings Institution production (kgmaize/ hectare).The archaeological record of and Anasazi farming groups from A.D. 200to 1300provides Santa FeInstitute information on amillennium of sociocultural stasis, variability, change, and adaptation. Wereporton a multi-agentcomputational model of this societythat closely reproduces themain featuresof its actual history, including population ebb and ow, changing spatial settlement Keywords patterns,and eventualrapid decline. Theagents in themodel Agent-basedmodeling, Anasazi, prehistory,American Southwest, are monoagriculturalists, who decideboth whereto situate environmentalreconstruction, their elds and whereto locate their settlements. culturalevolution 1Introduction Acentralquestion that anthropologists have asked for generations concerns how cul- turesevolve or transformthemselves from simple tomore complex forms. Traditional studyof human social changeand cultural evolutionhas resulted in many usefulgener- alizations concerning thetrajectory of change through prehistory and classications of typesof organization. Itis increasingly clear, however,that four fundamental problems havehindered the development of a powerful, unied theoryfor understanding change in human social norms and behaviorsover long periodsof time. The rstof theseproblems isthe use of whole societies as the unit of analysis. Group- leveleffects, however, must themselves be explained. Sustainedcooperative behavior withpeople beyond close kin isachieved in mosthuman societies,and increasingly hierarchical political structuresdo emergethrough time in many cases.Successful explanation and thepossibility of developing fundamental theoryfor understanding theseprocesses depend on understandingbehavior at the level of the individual or thefamily [8]. Amongthe advantages of such base-level approaches isthat they allow specic modeling ofpeoples’ behavioral rangesand norms and theiradaptive strategies ascommunity sizeand structurechange. Second, in addition tosubsuming the behavior of individuals within thatof larger social units,traditional analysesintegrate environmental variability overspace. Current c 2003Massachusetts Instituteof Technology Articial Life 9:435– 444 (2003) ° G.J.Gumerman, A.C.Swedlund, J. S.Dean, and J.M.EpsteinPrehistoric American Southwest researchindicates that stable strategies for interpersonal interactions in aheterogeneous, spatially extendedpopulation may bevery different from thosein ahomogeneous population in which spaceis ignored [11]. Most social interactionsand relationships in human societiesbefore the recent advent of rapid transportationand communication werelocal in nature. Third, cultureshave been considered to behomogeneous,tending toward maximiza- tionof tnessfor their members. Little consideration wasgiven to historical processes in shaping evolutionarytrajectories or tononadaptive aspects of cultural practice. Finally, mostdiscussions of cultural evolutionhave failed totake into account the mechanisms ofcultural inheritanceand theeffects of changesin modesof transmission throughtime [2, 3]. Understandingculture as an inheritancesystem is fundamental to understandingculture change through time. TheArti cial Anasazi projectis at the juncture of theory building and experimenta- tion. Weuseagent-based modeling totest the tbetweenactual archaeological and environmentaldata collectedover many yearsand simulations usingvarious rules about how householdsinteract with one another and withtheir natural environment.By sys- tematically alteringdemographic, social, and environmentalconditions, aswell asthe rulesof interaction, weexpectthat a clearerpicture will emergeas to why the Anasazi followed theevolutionary trajectory we recognize from archaeological investigation. Our long rangegoal isto develop agent-based simulations tounderstand the interac- tionof environmentand human behaviorand theirrole in theevolution of culture. 2The Study Area Thetest area forexploring the use of agent-based modeling forunderstanding social evolutionis the prehistoric American Southwestfrom about A.D. 200to 1450 using a culturearchaeologists refer to as the Anasazi and alocality called Long HouseV alley. TheAnasazi arethe ancestors of the present day Pueblo peoples,such as the Hopi, the Zuni, theAcoma, and thegroups along theRio Grandein NewMexico. Acommonly heldview is that technological, social, and linguistic complexitycoevolve. Anasazi cul- tural developmentunderscores the interdependence of these aspects of culture. The Anasazi werea technologically simple agricultural societywhose major foodsource wasmaize supplementedby beans, squash, wild plants, and game. Inthe A.D. 200 to 1450period theonly major technological changesthat are archaeologically veriable areagricultural intensication (terracingand ditch irrigation) and theintroduction ofa more efcient system for grinding maize. Duringthis time, however,there is evidence ofgreatly increased social complexity. ContemporaryPueblo peoplehave complicated social systemsmade up ofsodalities(distinct social associations) including clans, moi- eties(division ofthe village intotwo units), feastgroups, religious societiesand cults(68 differentceremonial groupshave been recorded), war societies,healing groups, winter and summer governments,and village governments.Details of the groups come from historical documentsand contemporaryethnographies. The economic, religious, and social realms ofPueblo societyare so tightlyintegrated it is dif cult tounderstand them asseparate elements of the society. Long HouseV alley, a180km 2 landform in northeasternArizona, providesa re- alistic archaeological testof the agent-based modeling ofsettlement and economic behavioramong subsistence-levelagricultural societiesin marginal habitats. This area iswell suitedfor such a testfor a number ofreasons. First, it is a topographically bounded, self-containedlandscape thatcan berealistically reproducedon acomputer. Second, arich paleoenvironmental record, basedon alluvial geomorphology,palynol- ogy,and dendroclimatology, permitsthe accurate quantitativereconstruction of annual 436 Articial Life Volume 9,Number 4 G.J.Gumerman, A.C.Swedlund, J. S.Dean, and J.M.EpsteinPrehistoric American Southwest uctuationsin potentialagricultural production in kilograms ofmaize perhectare [6]. Combined, thesefactors permit thecomputerized creation ofadynamic resourceland- scapethat accurately replicates actual conditions in thevalley from A.D. 200 to the present.The agents of the simulation interactwith one another and withtheir environ- menton thislandscape. Third, tree-ringchronology provides annual calendric dating. Fourth,intensive archaeological research,involving a 100%survey of the area supple- mentedby limited excavations,creates a databaseon human behaviorduring thelast 2,000years that constitutes the real-world targetfor the modeling [7]. Finally, histori- cal and ethnographic reportsof contemporary Pueblo groupsprovide anthropological analogs forprehistoric human behavior. Betweenroughly 7000 and 1800 B.C.,thevalley was sparsely occupied bypeople who dependedon huntingand gathering.The introduction ofmaize around 1800 B.C. beganthe transition to afood-producing economyand thebeginning of the Anasazi cul- tural tradition, which persisteduntil theabandonment ofthe region around A.D. 1300. Long HouseV alley providesarchaeological data on economic, settlement,social, and religious conditions among alocalized Anasazi population. Thesearchaeological data provideevidence of stasis, variability, and changeagainst which theagent-based sim- ulation ofhuman behavioron thedynamic, articial Long HouseV alley landscape can be judged. Wehavetested a largenumber ofhypothesesabout theLong HouseV alley Anasazi [6, 1], butwe focus on onlytwo issues here: (1) therole of environment in explain- ing thepopulation dynamics ofsettlement placement, thelarge population increase after A.D. 1000,and thecomplete abandonment ofthe region around A.D. 1300;and (2) thesize of simulated and actual settlementsthat were selected and abandoned under variousenvironmental, demographic, and social conditions in differentyears. 3 Methods TheArti cial Anasazi Projectis an agent-basedmodeling studybased on theSugarscape model createdby Joshua M.Epsteinand RobertAxtell [10]. The project was created to providean empirical, real-world evaluation ofthe principles and proceduresembodied in theSugarscape model and toexplore the ways in which bottom-up, agent-based computersimulations can illuminate human behaviorin areal world setting.The land- scape(analogous toSugarscape) iscreated from reconstructedenvironmental variables and ispopulated byarti cial agents—in thiscase households, the basic social unitof local Anasazi society.Agent demographic