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Testing Models of Early Paleoindian Colonization and Adaptation Using Cladistics

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Testing Models of Early Paleoindian Colonization and Adaptation Using Cladistics Testing Models of Early Paleoindian Colonization I Buchanan & Collard 59 5 Testing Models of Early Paleoindian Colonization and Adaptation Using Cladistics Briggs Buchanan and Mark Collard lovis and related cultures from the Early America provide the basis for a model of CPaleoindian period (ca. 11,500-10,500 radio­ Paleoindians as highly mobile, wide-ranging big­ carbon years B.P. [RCYBPD represent the first game hunters (Haynes 1980; Kelly and Todd well-documented indications of human occupa­ 1988; West 1996). Initial discoveries of Clovis tion in North America. Currently, there is consid­ projectile points, primarily on the High Plains and erable debate regarding the origins of these ·cul­ in the Southwest, were associated with mammoth tures. The traditional model developed to account remains (Cotter 1937, 1938; Figgins 1933; Haury for them has been challenged, and a number of 1953; Haury et al. 1959; Sellards 1938), prompt­ alternative hypotheses have been proposed. Here ing researchers to suggest that Early Paleoindians we report the results of a study in which various specialized in the hunting of big game. The rapid models that potentially account for the origins of spread of Clovis hunters coincided with the Paleoindian groups were tested by applying the extinction of Late Pleistocene megafauna, leading cladistic method of phylogenetic reconstruction to a number of researchers to support the a data set derived from characteristics of Early "blitzkrieg" hypothesis, which causally links the Paleoindian projectile points. spread of Paleoindian hunters with the demise of the megafauna (Alroy 2001; Fiedel and Haynes MODELS OF PALEOINDIAN 2004; Haynes 2002; Martin 1967). COLONIZATION OF NORTH AMERICA Competing theories of the peopling of the The most widely accepted hypothesis for the ori­ Americas propose early inland or coastal migra­ gin of Paleoindians is commonly referred to as tions from northeast Asia (e.g., Bryan 1969, 1978; the "Clovis-First model." It holds that groups Dixon 2001; Fladmark 1979; Gruhn 1988), and migrated to North America via Beringia, the land­ even the long-held notion of a trans-Atlantic voy­ mass between Siberia and Alaska that was age from Europe has been resurrected recently exposed by sea level regression during glacial (Bradley and Stanford 2004; but see Straus 2000; intervals (Hopkins et al. 1982). These groups are Straus et al. 2005). Other hypotheses regard believed to have traveled through Beringia as regional manifestations of Clovis as indications of they pursued large game migrating east on a quest time depth and initial colonization of those for forage. Once in Alaska, people could have regions (Bonnichsen and Schneider 1999; Mason gained entry to the Great Plains via the ice-free 1962; O'Brien and Wood 1998). corridor that is hypothesized to have been open soon after 12,000 RCYBP (Catto 1996; White et CLADISTICS IN al. 1985; but see Arnold 2002; Mandryk 2004; ANTHROPOLOGICAL RESEARCH Mandryk et al. 2001). First presented coherently in the 1950s (Hennig According to the Clovis-First model, after 1950, 1965, 1966), cladistics is now the dominant reaching the Great Plains the Paleoindians spread method of phylogenetic reconstruction used in rapidly across the continents, eventually reaching biology. Based on a null model in which new taxa the tip of South America in little more than a few arise from the bifurcation of existing ones, cladis­ centuries (Fiedel 2000). Apparent similarities tics defmes phylogenetic relationship in terms of among Clovis and contemporaneous sites situated relative recency of common ancestry. Two taxa across the diverse environmental regions of North are deemed to be more closely related to one 60 Cultural Transmission and Archaeology: Issues and Case Studies another than either is to a third taxon if they share Jordan 2000; Holden 2002; O'Brien and Lyman a common ancestor that is not also shared by the 2003a, 2005; O'Brien et al. 2001; O'Brien et al. third taxon. The evidence for exclusive common 2002; Rexova et al. 2003; Robson-Brown 1996; ancestry is evolutionarily novel, or "derived," Shennan and Collard 2005; Tehrani and Collard character states: Two taxa are inferred to share a 2002). These studies suggest that cladistics can be common ancestor to the exclusion of a third taxon a useful tool for tackling certain cultural evolu­ if they exhibit derived character states that are not tionary problems. Most significant, cladistics also exhibited by the third taxon. offers a well-understood model that can be fitted In its simplest form, cladistic analysis involves to linguistic and material culture data sets in a four steps. First, a character state data matrix is straightforward manner. Where the fit between a generated. This shows the states of the characters cultural data set and the tree model is close, we exhibited by each taxon. Second, the direction of can invoke the principle of parsimony and legiti­ evolutionary change among the states of each mately conclude that the similarities and differ­ character is established. Several methods have ences among the cultural units are primarily the been developed to facilitate this, including com­ result of branching. Conversely, where there are munality analysis (Eldredge and Cracraft 1980), numerous homoplasies and the fit between a cul­ ontogenetic analysis (Nelson 1978), and strati­ tural data set and the tree model is poor, we can graphic-sequence analysis (Nelson and Platnick justifiably infer that borrowing or convergent evo­ 1981). Currently, tire favored method is outgroup lution played a more important role in generating analysis (Arnold 1981), which entails examining the similarities and differences among the cultural a close relative of the ingroup taxa to determine units. The instances of homoplasy can then be which character states are derived (those found investigated with biological phylogenetic methods only in the ingroup) and which are ancestral that are not based on the bifurcating-tree model (those found in both the ingroup and the out­ (Bryant et al. 2005; Greenhill and Gray 2005; group). Having determined the probable direction Hendy and Penny 1992; Hurles et al. 2003; see of change for the character states,'the third step is also ch. 4). to construct a branching diagram of relationships for each character. This is done by joining the two CLADISTICS AND PROJECTILE POINTS most-derived taxa by two intersecting lines and Cladistic methods have previously been applied then successively connecting the other taxa to Paleoindian projectile points by O'Brien and according to how derived they are. Each group of colleagues (O'Brien and Lyman 2003a; O'Brien taxa defined by a set of intersecting lines corre­ et al. 2001; O'Brien et al. 2002), who focused on sponds to a "clade," and the diagram is referred to points from the southeastern United States. as a cladogram, or "phylogenetic tree." The fourth Because the majority of Paleoindian points from step is to compile an ensemble tree from the char­ this region are surface finds without associated acter trees. Ideally, the distribution of the charac­ dates, temporal relationships among traditional ter states among the taxa will be such that all the point types remain largely unknown, and the character trees imply relationships among the taxa ordering of types in the region has been devel­ that are congruent with one another. Normally, oped largely by borrowing temporal sequences however, a number of the character trees will sug­ from other parts of the country (primarily the gest relationships that are incompatible. This Plains and Southwest). With this in mind, the pri­ problem is overcome by generating an ensemble mary goal of the analyses conducted by O'Brien tree that is consistent with the largest number of and colleagues was to construct phylogenetic characters and therefore requires the smallest hypotheses that shed light on the evolution of number of ad hoc hypotheses of character change, projectile points in the region. I or "homoplasies," to account for the distribution O'Brien et al. (2001) recorded three qualitative of character states among the taxa. and five quantitative characters on a sample of Recently, a number of studies have appeared in 621 projectile points representing a range of tradi­ which cladistic methods have been applied to cul­ tional projectile point types, including the well­ tural data in order to shed light on historical known Clovis, Dalton, and Cumberland points. events (e.g., Collard and Shennan 2000; Foley They then subjected the specimens to paradig­ 1987; Foley and Lahr 1997,2003; Gray and matic classification (Dunnell 1971) in order to Testing Models of Early Paleoindian Colonization I Buchanan & Collard 61 cluster them into classes with unique combinations with models of cultural transmission, routes of of character states. T_his resulted in 491 classes. population dispersion, and adaptation to regional O'Brien et ai. discarded classes with less than four environments. specimens in order to minimize the impact of idio­ syncratic behaviors. This reduced the number of Materials and Methods classes to 17, and it is these that were used as taxa We used assemblages rather than paradigmatic in the subsequent phylogenetic analyses. O'Brien classes as taxa. We did this because the goal of and Lyman (2003a) later extended the study by our study is to shed light on human population increasing and decreasing the number of taxa in an history, and we think that assemblages are better effort to assess the reliability of the clades formed proxies for past populations than classes obtained using 17 taxa. They found that many of the clades through paradigmatic classification. Using assem­ remained intact when taxa were removed, blages as taxa could potentially be problematic if although in the majority of cases several most-par­ some of the assemblages were created thTough simonious trees were found. multiple occupations over long periods of time, Following from the work of O'Brien et aI., we since it would increase the chances of conflating used cladistic methods to infer a model of the evo­ multiple technological lineages within a single lutionary changes~ points recovered from across assemblage.
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