Abstract
The Medieval Warm Period or, alternately, the Medieval Climatic Anomaly (MCA) has been extensively cited in archaeological literature as an explanation in whole or in part for observations of change in human health and behavior. The MCA was a long-term, low-frequency global warming pattern that is known through paleoclimatic data to have occurred during approximately A.D. 800-1350 in central California. A synthesis of bioarchaeological data on 15,255 skeletons reported from excavations conducted at 298 archaeological sites in northern California between 1867 and 2009 is used to assess whether prehistoric populations in the region were negatively impacted by droughts during the MCA. Population-level health and behavioral indices tabulated as proxy measures of stress before, during, and after the MCA included age, sex, enamel hypoplasia, anemia, dental caries, disease, fractures, violence, and mass inhumations. Results of the study indicate that the health of indigenous populations significantly deteriorated during the MCA relative to temporal phases before and after this event in central California. In addition, a fine-grained temporal analysis demonstrates a diachronic pattern of increased health stress from AD 735 to AD 1390. The prevalence rates of dental caries, enamel hypoplasia, anemia, disease, fractures, and interpersonal violence all exhibit significant increases during the middle (AD 1010-1210) and late (AD 1210-1390) phases of the MCA. The prevalence rates of dental caries, enamel hypoplasia, disease, fractures, and interpersonal violence all exhibit significant decreases following this climatic event (AD 1390-1700). Anemia is the only index that continues to increase following the MCA. Mass burials, while exhibiting significant temporal variability in prevalence at the sub-regional level, proved to be a poor indicator of health stress at this level of analysis for the larger central California region. Sub-regions within central California exhibit diachronic and geographic variability in index prevalence rates throughout the temporal sequence established for the region. This variability was not unexpected given the diverse ecological and cultural buffers that could mitigate particular health stress index prevalence rates in particular areas. However, during the MCA, the Sacramento Valley and San Francisco Bay (the only two regions with adequate sample size for comparative analysis at the micro-scale) demonstrate significant continuity with the high prevalence rates observed among indices of the larger region. These findings are significant, as during no other phase in the temporal sequence can one observe diverse environmental, cultural, and dietary resource zones exhibiting such similarity in health stress marker types and prevalence. The linear pulse of index frequency observed on both the sub-regional and regional level from the Early MCA (AD 735-1010) through the Late MCA (AD 1210-1390) cannot be dismissed as the result of regular patterned increases of the sort expected from population increase and resource intensification during this time. This study has demonstrated that there was a significant increase in health stress during the MCA that abruptly ended following the termination of this event. These increases in health stress cannot be adequately explained through resource intensification, socio-cultural political organization, or territorial constriction, as these processes were in place well before and after this event. It is concluded that these negative impacts result from the effects of the MCA on populations in central California.