Evaluating the relationships between climate change, population pressure, economic intensification, and childhood stress in the Prehispanic Nasca region of Peru

Climate change can adversely affect population health by limiting resource availability. Climate shocks may cause resource shortfalls directly via declining economic productivity, but favorable climate conditions may also create food shortages indirectly by promoting demographic growth that then pressures a resource base. Current population health and capacities for resilience reflect long-term human-environment interactions, highlighting the need for studies on the relationship between climate and population health through time. In this article, we combine theory from population ecology and life history with structural equation models to evaluate archaeological evidence of changing population health in response to climate change in the Nasca highlands of Peru from the terminal Middle Horizon to Late Intermediate period (1250 - 500 cal BP). Specifically, we examine potential causal chains between local precipitation, population growth, agricultural intensification, and two skeletal indicators of childhood physiological stress—cribra orbitalia (CO) and porotic hyperostosis (PH)—in 149 radio- carbon dated individuals. We find that higher precipitation has a positive indirect impact on PH frequency that is entirely mediated by local population pressure and maize agricultural intensification. Precipitation has a negative direct effect on CO, which is most common during intervals of extreme drought, low dietary maize inputs, and in periods of higher fertility. Additionally, we find that PH and CO negatively co-vary over time. These findings add to a growing body of evidence that, though PH and CO have historically been considered manifestations of the same pathological process, these two skeletal lesions likely result from distinct sources of physiological stress in the Prehispanic central Andes.

For more information see: McCool, Weston C., Amy S. Anderson, Alexis Ja’net Baide, Toni Gonzalez, Brian F. Codding, (2023). “Evaluating the relationships between climate change, population pressure, economic intensification, and childhood stress in the Prehispanic Nasca region of Peru.” Quaternary International. 

Ecological constrains on violence avoidance tactics in the Prehispanic central Andes

Prehispanic agricultural populations in the central Andes exhibit some of the highest rates of lethal and sublethal trauma ever recorded. Explanations for high rates of violence focus on what factors drove proactive conflict. While it is true that incentives for proactive violence must exist for high rates of trauma to ensue, it is far less recognized how ecological conditions may promote economic activities that constrain violence avoidance tactics and thus influence rates of violence. Here, we draw on models from behavioral ecology to generate predictions about how violence co-varies with environmental gradients, related subsistence strategies, and attendant defensive tactics. We hypothesize that rates of violence will be highest in marginal and variable environments where high-mobility subsistence strategies serve to reduce subsistence risk while increasing the costs of violence avoidance. Our results show that high elevation locations with variable topography where high mobility subsistence strategies are common exhibit the highest rates of generalized interpersonal violence. These results suggest that as ecological conditions become marginal and variable, risk-reducing subsistence strategies are emphasized, which result in increased exposure to violence. This study shows that environmental influences on the efficacy of violence avoidance tactics are important for explaining variability in rates of violence.

For more information see: McCool, Weston C., Kurt M. Wilson, Kenneth B. Vernon, (2022). “Ecological constraints on violence avoidance tactics in the Prehispanic central Andes.” Environmental Archaeology doi.org/10.1080/14614103.2022.2137652

Climate change-induced population pressure drives high rates of lethal violence in the Prehispanic central Andes 

Understanding the influence of climate change and population pressure on human conflict remains a critically important topic in the social sciences. Long-term records that evaluate these dynamics across multiple centuries and outside the range of modern climatic variation are especially capable of elucidating the relative effect of, and the interaction between, climate and demography. This is crucial given that climate change may structure population growth and carrying capacity, while both climate and population influence per capita resource availability. This study couples paleoclimatic and demographic data with osteological evaluations of lethal trauma from 149 directly AMS ¹⁴C dated individuals from the Nasca highland region of Peru. Multiple local and supra-regional precipitation proxies are combined with a summed probability distribution of 149 ¹⁴C dates to estimate population dynamics during a 700-year study window. Counter to previous findings, our analysis reveals a precipitous increase in violent deaths associated with a period of productive and stable climate, but volatile population dynamics. We conclude that favorable local climate conditions fostered population growth that put pressure on the marginal and highly circumscribed resource base, resulting in violent resource competition that manifested in over 450 years of internecine warfare. These findings help support a general theory of intergroup violence, indicating that relative resource scarcity -- whether driven by reduced resource abundance, or increased competition -- can lead to violence in subsistence societies when the outcome is lower per capita resource availability. 

For more information, see: McCool, Weston C., Brian F. Codding, Kenneth B. Vernon, Peter M. Yaworsky, Kurt M. Wilson, Norbert Marwan, Douglas J. Kennett (2022). “Climate change induced population pressure drives high rates of lethal violence in the Prehispanic central Andes.” Proceedings of the National Academy of the Sciences 119(17):e2117556119. doi.org/10.1073/pnas.2117556119

Subsistence strategy mediates ecological drivers of human violence

Inter-personal violence (whether intra- or inter-group) is a pervasive yet highly variable human behavior. Evolutionary anthropologists suggest that the abundance and distribution of resources play an important role in influencing differences in rates of violence, with implications for how resource conditions structure adaptive payoffs. Here, we assess whether differences in large-scale ecological conditions explain variability in levels of inter-personal human violence. Model results reveal a significant relationship between resource conditions and violence that is mediated by subsistence economy. Specifically, we find that interpersonal violence is highest: (1) among foragers and mixed forager/farmers (horticulturalists) in productive, homogeneous environments, and (2) among agriculturalists in unproductive, heterogeneous environments. We argue that the trend reversal between foragers and agriculturalists represents differing competitive pathways to enhanced reproductive success. These alternative pathways may be driven by features of subsistence (i.e., surplus, storage, mobility, privatization), in which foragers use violence to directly acquire fitness-linked social payoffs (i.e., status, mating opportunities, alliances), and agriculturalists use violence to acquire material resources that can be transformed into social payoffs. We suggest that as societies transition from immediate return economies (e.g., foragers) to delayed return economies (e.g., agriculturalists) material resources become an increasingly important adaptive payoff for inter-personal, especially inter-group, violence. 

For more information, see: McCool, Weston C. Kenneth B. Vernon, Peter M. Yaworsky, Brian F. Codding,  “Subsistence strategy mediates ecological drivers of human violence.” PLoS One 17(5): e0268257. doi.org/10.1371/journal.pone.0268257

The Character of Conflict: A bioarchaeological study of violence in the Nasca highlands of Peru during the Late Intermediate Period (950 – 1450 C.E.)

Objectives: This study uses osteological and radiocarbon datasets combined with formal quantitative analyses to test hypotheses concerning the character of conflict in the Nasca highlands during the Late Intermediate Period (LIP, 950 – 1450 C.E.). We develop and test osteological expectations regarding what patterns should be observed if violence was characterized by intra-group violence, ritual conflict, intermittent raiding, or internecine warfare. 

Materials and methods: Crania (n = 267) were examined for ante-mortem and peri-mortem, overkill, and critical trauma. All age groups and both sexes are represented in the sample. 124 crania were AMS dated, allowing a detailed analysis of diachronic patterns in violence among various demographic groups. 

Results:  Thirty-eight percent (102/267) of crania exhibit some form of cranial trauma, a significant increase from the preceding Middle Horizon era. There are distinct trauma frequencies within the three sub-phases of the LIP, but Phase III (1300 – 1450 C.E.) exhibits the highest frequencies of all trauma types. Males exhibit significantly more ante-mortem trauma than females, but both exhibit similar peri-mortem trauma rates.   

Discussion: There was chronic, internecine warfare throughout the Late Intermediate Period with important variations in violence throughout the three temporal phases. Evidence for heterogeneity in violent mortality shows a pattern consistent with social substitutability, whereby any and all members of the Nasca highland population were appropriate targets for lethal and sublethal violence. We argue that by testing hypotheses regarding the targets and types of conflict we are better able to explain the causes and consequences of human conflict. 

For more information, see: McCool, Weston C. Joan Brenner-Coltrain, Aldo Accinelli, Douglas J. Kennett. “The Character of Conflict: A bioarchaeological study of violence in the Nasca highlands of Peru during the Late Intermediate Period (950 – 1450 C.E.).” American Journal of Biological Anthropology, 174(4), pp.614-630.

Using a multimethod life history approach to navigate the osteological paradox: A case study from the Nasca highlands

Objectives: We leverage recent bioarchaeological approaches and life history theory to address the implications of the osteological paradox in a study population. The goal of this paper is to evaluate morbidity and mortality patterns as well as variability in the risk of disease and death during the Late Intermediate period (LIP; 950 – 1450 C.E.) in the Nasca highlands of Peru. We demonstrate how the concurrent use of multiple analytical techniques and life history theory can engage the osteological paradox and provide salient insights into the study of stress, frailty, and resilience in past populations.

Materials and methods: Crania from LIP burial contexts in the Nasca highlands were examined for cribra orbitalia (n = 325) and porotic hyperostosis (n = 270). All age groups and both sexes are represented in the sample. Survivor/non-survivor analysis assessed demographic differences in lesion frequency and severity. Hazard models were generated to assess differences in survivorship. The relationship between dietary diversity and heterogeneity in morbidity was assessed using stable δ¹⁵N and δ¹³C isotope values for bone collagen and carbonate. 124 crania were directly AMS radiocarbon dated, allowing for a diachronic analysis of morbidity and mortality. 

Results: The frequency and expression of both orbital and vault lesions increases significantly during the LIP. Survivor/non-survivor analysis indicates cranial lesions co-vary with frailty rather than robusticity or longevity. Hazard models show 1) decreasing survivorship with the transition into the LIP, 2) significantly lower adult life expectancy for females compared to males, 3) individuals with cranial lesions have lower survivorship across the life course. Stable isotope results show very little dietary diversity. Mortality risk and frequency of pathological skeletal lesions were highest during Phase III (1300 – 1450 C.E.) of the LIP. 

Conclusion: Results provide compelling evidence of increasing physiological stress and mortality in the Nasca highlands during the LIP, but also reveal substantial heterogeneity in frailty and the risk of death. Certain members of society experienced a heavier disease burden and higher mortality compared to their contemporaries. Elevated levels of disease and lethal trauma among females account for some of the sex differences in survivorship but cannot explain the large degree of female-biased mortality. We hypothesize that parental investment in males or increased female fertility rates may explain these differences.  

For more information, see: McCool, Weston C., Amy S. Anderson, Douglas J. Kennett. “Using a multimethod life history approach to navigate the osteological paradox: A case study from the Nasca highlands”. American Journal of Biological Anthropology175(4), pp.816-833.