Spatiotemporal diversification of projectile point types in western North America over 13,000 years
Introduction
Understanding diversity and diversification lies at the heart of evolutionary approaches to science. Mathematically, diversification is best modeled as an evolutionary branching process (Karlin, 2014; Kimmel and Axelrod, 2016) where ancestral forms diverge over time in response to the interaction of internal dynamics and external stimuli resulting in the origination of descendent forms, be they biological species or human cultural traits (Henrich and McElreath, 2003; Maffi, 2005; Nettle, 1999; Nunn, 2011). As the human species expanded its range out of Africa, human socioeconomies diversified as populations encountered and adapted to new environments (Hiscock, 2013). Coupled with global-scale climate changes at the end of the Pleistocene, regional technological adaptations led to the modification of ecosystems (i.e., niche construction) that created coevolutionary feedbacks between human populations, their technologies and their environments. In several places around the planet these coevolutionary feedback loops resulted in the increased management and the eventual domestication of plants and animals (Bellwood, 2005). As a result, human cultures, socioeconomies, and social complexity diversified worldwide over time as local adaptations, and their diffusion (Bailey et al., 2012; Bellwood and Renfrew, 2002; Diamond and Bellwood, 2003), led to novel technological, economic, and cultural innovations creating spatiotemporal patterning in the archaeological record.
Social scientists have long been interested in quantifying human cultural diversity. Since the mid-1990s anthropologists have sought to understand the global biogeographic structure of linguistic diversity (Axelsen and Manrubia, 2014; Cashdan, 2001; Collard and Foley, 2002; Currie and Mace, 2012; Gavin et al., 2013; Mace and Pagel, 1995; Maffi, 2005; Nettle, 1999). Similar research has explored ethnic (Ahlerup and Olsson, 2012; Burnside et al., 2012; Cashdan, 2001; Michalopoulos, 2012; Pagel and Mace, 2004), economic (Kummu and Varis, 2011), sociopolitical (Currie and Mace, 2009; Turchin et al., 2018), and mythological diversity (Berezkin, 2005, Berezkin, 2009). Phylogenetic approaches are commonly employed to reconstruct the evolutionary diversification of languages (Atkinson, 2011; Bouckaert et al., 2012; Gray and Atkinson, 2003; Greenhill et al., 2010; Grollemund et al., 2015; Pagel et al., 2007), sociopolitical complexity (Currie et al., 2010, Currie et al., 2013; Walker and Hamilton, 2011), mythologies (d'Huy, 2013a, d'Huy, 2013b, d'Huy, 2013c) and folktales (Da Silva and Tehrani, 2016; Pagel, 2016; Tehrani, 2013).
Others have measured the pace of cultural evolution using archaeological data by quantifying rates of change in artifact form over time (Perreault, 2012). Researchers studying the evolutionary diversification of stone tool technologies, for example, often use morphometric cladistic approaches to measure rates of change in continuous measures of shape, as opposed to the discrete traits required by phylogenetics (Buchanan, 2006; Buchanan et al., 2011; Buchanan and Collard, 2010a, Buchanan and Collard, 2010b; Buchanan and Hamilton, 2009; Costa, 2010; Eren and Lycett, 2012; Iovita, 2011; Iovita and McPherron, 2011; Lyman et al., 2008, Lyman et al., 2009; Lyman and O'Brien, 2000; Mesoudi and O'Brien, 2008a, Mesoudi and O'Brien, 2008b; O'Brien et al., 2001, O'Brien et al., 2002; Thulman, 2012). The resulting evolutionary structure gives insight into the processes of innovation, selection, and drift in the diversification process. Approaches that explicitly consider the spatiotemporal diffusion of technologies and populations use geolocated radiocarbon databases. Here, population expansions, or the diffusion of innovations, are traced through spatiotemporal gradients in the radiocarbon record (Cavalli-Sforza et al., 1993; Collard et al., 2010; Fort, 2012; Fort et al., 2004; Hamilton and Buchanan, 2007, Hamilton and Buchanan, 2010; Pinhasi et al., 2005).
In this paper, we focus on the diversification of stone tool technologies in North America from the initial colonization by humans in the late Pleistocene through the Holocene. Specifically, we examine the spatiotemporal diversification of projectile point types in western North America over a period of about 13,000 years. While the specific timing and nature of the initial colonization of the Americas is an area of active debate (Braje et al., 2017, Braje et al., 2018; Potter et al., 2017, Potter et al., 2018), most experts agree that late Paleolithic hunter-gatherer populations from northeast Asia entered North America via the Bering Land Bridge, first appearing in western Alaska sometime during the late Pleistocene (Hamilton and Buchanan, 2010; Madsen, 2004; Meltzer, 2009). A small founding population later entered the North American continent south of the ice sheets and rapidly expanded across the continent (Hamilton and Buchanan, 2007). Both the genetic and archaeological records show that this small founding population exhibited low levels of diversity. However, recent research shows that early Paleoindian populations had already diversified into distinct, spatially discrete regional variants across North America by ~12,000 cal BP (Buchanan et al., 2016a). Moreover, this diversification suggests an adaptive radiation as regional variation in stone tool technologies and projectile point shapes correlate with regional variation in the body size spectrum of major mammalian prey species (Buchanan et al., 2011).
We chose western North America as our study region as there are various data sets available with which to study diversification over time. First, we summarize the available data. Second, we analyze projectile point type diversification over time. Third, we analyze projectile point type diversification over space. Fourth, we then consider the spatiotemporal dynamics in relation to population growth and regional climate changes over the study period. We also address potential sampling bias in the identification of projectile point types over different time periods.
Section snippets
Projectile point types
We extracted data from Justice's projectile point typologies of the US Southwest (Justice, 2002a), and California and the Great Basin (Justice, 2002b). For each projectile point type, we recorded maximum date (origination) and minimum date (extinction) (Fig. 1), and calibrated them using the calpal Intcal 13 calibration curve (Danzeglocke, 2018). We then digitized the projectile point distribution maps for each type (where available) built shape files of their spatial distribution and measured
Discussion
Our results describe a complex diversification process where, following the initial human colonization of North America and expansion throughout western North America over the late Pleistocene, as temperatures increased over the Holocene, populations grew, innovated, and subdivided into increasingly localized subpopulations. Projectile point diversity consequently increased in proportion to population size, as did the replacement rates of projectile point types.
Cultural diversification of any
Acknowledgements
We thank James Hartley for help digitizing the projectile point spatial extent data and to the anonymous reviewers of the paper whose comments greatly improved the content of the paper.
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