Human ecological niches and ranges during the LGM in Europe derived from an application of eco-cultural niche modeling

Abstract

We apply eco-cultural niche modeling (ECNM), an heuristic approach adapted from the biodiversity sciences, to identify habitable portions of the European territory for Upper Paleolithic hunter-gatherers during the Last Glacial Maximum (LGM), circumscribe potential geographic extents of the Solutrean and Epigravettian technocomplexes, evaluate environmental and adaptive factors that influenced their distributions, and discuss this method's potential to illuminate past human–environment interaction. Our ECNM approach employed the Genetic Algorithm for Rule-Set Prediction (GARP) and used as input a combination of archaeological and geographic data, in conjunction with high-resolution paleoclimatic simulations for this time frame. The archaeological data consist of geographic coordinates of sites dated by Accelerator Mass Spectrometry to the LGM and attributed to the Solutrean and Epigravettian technocomplexes. The areas predicted by ECNM consistently outline the northern boundary of human presence at 22,000–20,000 cal BP. This boundary is mainly determined by climatic constraints and corresponds well to known southern limits of periglacial environments and permafrost conditions during the LGM. Differences between predicted ecological niches and known ranges of the Solutrean and Epigravettian technocomplexes are interpreted as Solutrean populations being adapted to colder and more humid environments and as reflecting influences of ecological risk on geographic distributions of cultures.

Introduction

The idea of modeling past human–environment interactions is by no means new. Researchers have used archaeological and environmental data sets, and diverse methods, to interpret prehistoric hunter-gatherer behavior in ecological contexts. Well-known European examples concern prehistoric population distributions during Oxygen Isotope Stages 2 and 3 (Gamble et al., 2004, Van Andel and Davies, 2003), as well as the resettlement of regions following severe climatic episodes (Gamble et al., 2005, Straus et al., 2000). These studies were based on spatial distributions of radiometrically dated sites and generalized climatic reconstructions. Others have used a similar approach to estimate population size and kinetics (Bocquet-Appel and Demars, 2000, Bocquet-Appel et al., 2005). More detailed attempts to examine population distributions and human tolerances with respect to environmental variability also exist (Binford, 1999, Binford, 2001, Davies et al., 2003, d'Errico and Sánchez-Goñi, 2003, d'Errico et al., 2006, Sepulchre et al., 2007). However, no agreement yet exists on how best to evaluate influences of environmental factors on prehistoric human populations and their responses to climatic variability.

One common limitation is the use of coarse-scale climatic data (i.e., simulations with resolutions of 3–5° in latitude and longitude) and imprecise chronological data (i.e., reliance on old conventional ages with large sigmas) that make evaluation of human responses to rapid-scale climatic variability, with adequate resolution, difficult. Another shortfall of previous studies is that they have incorporated environmental data into analyses only passively, such that these data are used as backdrops against which the archaeological record is interpreted. While these studies have obvious value, they are limited in their ability to evaluate prehistoric hunter-gatherer responses to the abrupt climatic and environmental changes of the last glacial period. The need for robust methods with which to evaluate more precisely how past human and animal populations responded to these changes is critical.

An important recent advance in the study of biological diversity has been the development of biocomputational architectures for predictive modeling of complex biodiversity phenomena (Guisan and Zimmermann, 2000, Soberón and Peterson, 2005). Such tools can be used to predict species' range (i.e., ecological niche) expansion or contraction in response to real or simulated climatic changes (Peterson et al., 2002). The ecological niche of a species can be defined as the range of environmental conditions within which it can persist without immigrational subsidy (Grinnell, 1924, Hutchinson, 1957). Such methods have considerable potential for reconstructing niches of past human populations and for illuminating the complex mechanisms that regulated the interactions between past hunter-gatherer populations and their environments, which in turn helped shape cultural, genetic, and linguistic geographies. These methods, and related concepts, recently have been termed eco-cultural niche modeling (ECNM) (Banks et al., 2006) when applied to prehistoric human populations. Our application of ECNM interactively integrates climatic, geographic, and archaeological data via a machine-learning genetic algorithm, described below. Comparable work is being pursued by others to analyze North American Paleoindian (Anderson and Gillam in Banks et al., 2006) and Far Eastern Paleolithic (Gillam and Tabarev, 2006) data and have shown promising results. We argue that ECNM is a powerful approach and, when paired with high-resolution climatic simulations, allows one to overcome many limitations of previous studies and evaluate prehistoric human–environment interactions at regional scales.

Here, we apply ECNM to human populations at the Last Glacial Maximum (LGM) in Europe, a well-studied and dated climatic phase known to have had profound impacts on human populations, with three primary objectives: (1) to determine the limits of the potential human range during the LGM, (2) to define the eco-cultural niches of the two main archeological cultures present in Europe at that time (the Solutrean and Epigravettian technocomplexes), and (3) to identify environmental and cultural factors that shaped their geographic ranges.

The last glacial period was marked by dramatic and rapid climatic variability (Dansgaard et al., 1993, Johnsen et al., 1992), with the LGM representing a unique suite of climatic conditions (Ditlevsen et al., 1996, Peyron et al., 1998). This period, centered on 21 kyr cal BP, is characterized by the maximum volume of the ice sheet over Scandinavia and northern Europe, along with cold and generally arid conditions in northern and Western Europe. The LGM archaeological record is characterized by a relatively small number of sites and large gaps in the archaeological record for many regions (cf. Soffer and Gamble, 1990, Straus, 2005, Street and Terberger, 1999). Such a pattern has been interpreted to be the result of the human abandonment of northern Europe and a contraction of the human range to southern regions that served as refugia. Such contraction and consequent demographic reduction is known to have produced a bottleneck in human genetic diversity (Barbujani et al., 1998: p. 490; Torroni et al., 2001, Torroni et al., 1998).

In Western Europe, between ca. 22 kyr and 20 kyr cal BP, human groups responded to LGM environmental conditions by developing a suite of new technologies characterized by a variety of diagnostic projectile points and knives produced by bifacial retouch (Fig. 1A), which define the Solutrean (Mortillet, 1873, Smith, 1966). Straus (2005) proposed that Solutrean populations employed more specialized subsistence systems, relative to earlier Upper Paleolithic technocomplexes, to exploit regions rich in game but under harsh climatic conditions.

In the regions of southeastern Europe, hunter-gatherers of the LGM produced a different lithic technology, termed the early Epigravettian (Laplace, 1964, Mussi, 2001), characterized by shouldered and backed projectile points produced by unifacial retouch (Fig. 1B). Leaf-shaped points are rare and have been recovered from only a few sites in northern Italy (Palma di Cesnola, 1990). Contrary to the Solutrean, which appears as a novel technology, the Epigravettian toolkit is interpreted as being derived from the preceding Gravettian technocomplex (Otte, 1990, Palma di Cesnola, 2001).