The primary hindrance to the rapid dispersal of early Paleoindian populations in the terminal Pleistocene was the influence of habitat variability. Steele et al. present a demographic simulation model in which rates of spatial range expansion are measured with the assistance of the mapped reconstruction of North American vegetation coverage.
The rapidity of the southward expansion is explained archaeologically in two contrasting scenarios. The first model stipulates one or more late glacial dispersals occurred, characterized by far ranging expanses and rapid population growth. This model implies archaeological material represents a sudden appearance characterized by an increase in artifact densities on a continental scale. The second scenario states that multiple, slower, colonization events occurred defined by low rates of population dispersal and slow population growth. Therefore a gradual increase in the archaeological remains observed in the cultural record occurs.
The archaeological data supports the first model, but taphonomic biases and sampling strategies conflict with the identification of the earliest occupation remains, confounding attempts to elucidate both models. Steele et al. suggest the contribution of paleovegetation reconstruction will optimize the prevailing scenario of a late glacial Beringian colonization of the Americas.
Ethnographic observations indicate hunter-gatherer densities vary according to habitat and therefore it becomes imperative that the paleohabitat be the primary variable in the simulation of population densities for Paleoindian colonizers. Evidence for the reconstruction of the Pleistocene environment is derived from numerous lines of evidence. The most direct evidence comes from fossil remains of vegetation, which provide a diagnostic indicator for particular vegetation zones (Ex: boreal forest, grassland, etc…). Additional sources of data include sedimentological indicators; soil types containing trace elements of overbearing vegetation coverage, and animal fossils; indicating which animals exploited a particular niche and its co-occurring vegetation coverage.
Naturally, the greatest uncertainty in the reconstruction of Pleistocene paleoenvironments in North America occurs for the older time frames and the most accurate, developed with the most samples, occur after 11,000 years.
Figure 1. 13,000 radiocarbon years ago.
Following the last glacial maximum, by around 14,000 BP, a significant warming and moistening of the climate occurs. However, throughout much of southern North America, cold and dry conditions appear to have endured. Nevertheless, continual glacial retreat carved an ice-free corridor although not to the extent that its entire length was navigable; glacial lakes inundated much of the interior.
Figure 2. 12,000 radiocarbon years ago.Boreal forests had spread throughout much of the western lowlands indicating the continued effects of warming and moistening. By this point, the receding glacial wall had revealed a much wider corridor but still presumably impassable on account of vast lakes of meltwater.
Induced by the warming climate and moist conditions, short grasses crept over much of the desert conditions of the Great Plains.
Figure 3. 11,000 radiocarbon years ago.
This period coincides with the Younger Dryas cold shift inducing a brief reversion to cold and arid conditions. Rising sea levels severed North America from Eurasia while the ice-free corridor at last opened extending some 500 km in width permitting passage.
Figure 4. 10,000 radiocarbon years ago.The Younger Dryas concluded shortly after ushering in vast expanses of forest throughout much of North America save for the plains and the western basins. In the North coverage was patchy, creating a forest-grassland mosaic.
This paleoenvironmental reconstruction permits observation of the effect of variation in the continental landscape on predicted Paleoindian occupancy rates. This is aided by a map developed by Faught, Anderson and Gisiger (1994) compiling 10,198 incidences of Paleoindian projectile points over the landscape throughout the continent. Inferred from these point locations along the landscape are the regions occupied during the terminal Pleistocene by initial Paleoindian migrants.
Although this projectile point compilation is an incredible asset to Paleoindian mobility research, it is a large assumption to accept that mapped artifact densities reflect actual deposition and not variability in recovery rates. A glimpse reveals the highest concentrations of Paleoindian projectile points are located on the east coast of the United States, the most densely populated region subject to the most modern landscape development. Although this very well may be a valid depiction of Paleoindian dispersal patterns, the possibility that it is the product of an extreme sampling bias cannot be ignored. Any analysis of Paleoindian mobility and dispersal must take this into account.
Based on the mapped deposition of projectile points and the reconstructed paleovegetation coverage of North America, Steele et al. conclude that population mobility and growth were constrained by the vegetation structure in the terminal Pleistocene. However, this model is complicated by how capable of adapting to the available resource structure early colonists were.
Steele et al. make a bold assumption that the Early Paleoindian populations were initially highly adapted to the resource availability of each new habitat zone. Despite this assumption, the trend indicates an extremely rapid adaptation in light of the significant increase in population density as inferred from larger and more sites and denser artifact assemblages. Although the assumption that Paleoindians entered the New World preconditioned to the new lifestyle is unreasonable, their ability to adapt so rapidly should not affect the results of this study, despite the intrinsic assumption.
Steele, James, Jonathan Adams, Tim Sluckin
1998 Modelling Paleoindian dispersals. World Archaeology 30(2): 286-305.
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