While the archaeology of the Northern Frontier of Mesoamerica is poorly understood, Pedro Armillas' hypothesis that climatically induced environmental change was the limiting factor for cultural change has become the ruling theory. In order to test this hypothesis original lacustrine pollen profiles were compared with a detailed inspection of the known archaeological record and the previously published paleoecological record. The archaeological evidence suggests that there was a dense human occupation in the northern reaches of Mesoamerica between about AD 600-900. The first indications of human settlements are related to the Chupicuaro culture that reached its apogee about 2000 years ago located along the Rio Lerma. A rustic variant of the Chupicuaro culture spread north and is associated with scattered hamlets. About AD 600 the sedentary population expanded considerably in conjunction with the development of regional centers and the Coyotlatelco red-on-buff ceramic tradition. This expansion can be seen all along the Northern Frontier of Mesoamerica from Alta Vista south to Tula. Between AD 900-1000 there was a dramatic change in settlement patterns and by about AD 1000 most of the northern reaches of Mesoamerica were once again under the control of semi-nomadic hunters and gatherers. A suite of four cores was collected in a transect that crosses the Northern Frontier of Mesoamerica. The goal of selecting sites that minimized human impact was not altogether successful since these cores identify the impact of agriculture. Within the limits of the dating and material available, the pollen profiles from these cores suggest an environmental change between AD 1000 and 1500. From the data available it is not clear if this change, or changes, was the result of changes in human settlement patterns or climatic change. As such Armillas' hypothesis remains unproven.