Multi-isotopic and morphometric evidence for the migration of farmers leading up to the Inka conquest of the southern Andes

Human migration has been a significant factor in shaping societies throughout history, impacting cultural change and socio-economic development across various levels of complexity. While the role of migration in driving cultural change was heavily debated in the early 20th century, recent advancements in isotopic and genomic analyses offer new tools to study past migrations with greater accuracy. This study focuses on the Uspallata Valley in Argentina, a region representing the southern frontier of Andean agropastoral economies. This area displays a latitudinal gradient of decreasing agricultural importance and a rich mortuary record spanning the Late Intermediate Period (LIP) and the Inka expansion. The research question centers on identifying and characterizing migration patterns within this region during the LIP, and assessing the role of migration in influencing the socio-economic landscape that the Inka encountered. Understanding the dynamics of pre-Inka migration in this southern frontier is crucial for a nuanced understanding of the broader processes of economic and demographic change in the Andes, moving beyond simplistic models emphasizing only in-situ adaptation and internal processes.

Existing literature on Andean migrations during the LIP is varied. Some studies have focused on the role of internal adaptive responses to climate change in shaping economic strategies, particularly in relation to maize cultivation. Other research highlights the significance of regional developments and the emergence of distinct cultural identities during this period. However, a systematic, multi-scalar study of migration in the southern Andean periphery has been lacking. This research aims to address this gap by integrating isotopic and morphometric analyses to provide a more comprehensive understanding of migration's role in shaping the social and economic landscape during the period leading up to the Inka conquest.

This interdisciplinary study combined several analytical methods to investigate human paleomobility and migration in the Uspallata Valley. The researchers analyzed human remains from seven archaeological sites, spanning AD 800-1500. **Bioavailable Strontium Isotopes:** A strontium isoscape was developed across the Andes using modern and archaeological rodent samples to establish a baseline for comparing human strontium isotope ratios (⁸⁷Sr/⁸⁶Sr). This allowed the researchers to distinguish between local residents and migrants based on their strontium signatures. Thirty individuals were analyzed, representing approximately 15% of the human remains from the study sites. **Craniofacial Geometric Morphometrics:** Three-dimensional geometric morphometric analysis was conducted on the crania of 21 individuals to assess phenotypic variation and potential differences in ancestral lineages between groups. Landmarks were recorded using a Microscribe G2X, and analyses were performed on the entire skull, cranial base, and cranial vault separately to identify potential biocultural signals. This approach allows for a more detailed understanding of cranial shape differences possibly related to biological and/or cultural factors. **Stable Isotopes (Carbon and Nitrogen):** Stable isotope analysis (δ¹³Ccollagen, δ¹³Capatite, and δ¹⁵N) was performed on bone samples from 39 individuals to reconstruct paleodiets and assess the importance of maize agriculture in the diets of both local and migrant populations. This data reveals information on the proportion of C3 and C4 plants in the diet, providing insights into agricultural practices. **Chronology:** Radiocarbon dating of human remains and Bayesian modeling were used to establish the temporal context of the migrations and relate them to regional trends in occupational intensity. Statistical analyses including Mann-Whitney tests, Procrustes ANOVA, Principal Component Analysis (PCA), and Bayesian estimates of Standard Ellipse Areas (SEA) were used to analyze the data.

The results revealed a bimodal distribution of strontium isotope ratios in the human remains, clearly separating two distinct groups: locals and migrants. The 27 individuals assigned as locals (⁸⁷Sr/⁸⁶Sr mean = 0.7090) showed strontium values consistent with residence in or near the Uspallata Valley. The 11 individuals classified as migrants (⁸⁷Sr/⁸⁶Sr mean = 0.7073) had isotope ratios that did not overlap with the local baseline, indicating non-local origins. Importantly, the migrant individuals showed a strong temporal clustering between approximately AD 1280 and 1420, coinciding with a broader regional occupational peak in Mendoza Province, suggesting a potential link between migration and population increase. Craniofacial analysis confirmed the bimodal grouping observed in the strontium isotope data. Significant differences were observed in cranial base shape between local and migrant groups, suggesting distinct ancestral lineages, while the cranial vault analysis revealed differences in cultural modification patterns, particularly in the types and extent of artificial cranial deformation. These variations suggest not only biological but also cultural distinctions. Stable isotope analysis showed marked differences in paleodiet between the two groups. Migrants exhibited highly enriched δ¹³C values (both in collagen and apatite), strongly indicating a high consumption of C4 plants, primarily maize, with a very restricted isotopic niche. This contrasts with the greater dietary diversity observed in the local population, who incorporated C3 plants and camelids into their diet in varying proportions. These results strongly suggest that the migrants were primarily maize farmers. The combined evidence from strontium isotopes, craniofacial morphology, and stable isotopes supports the conclusion of a significant migration event of maize-farming groups into the Uspallata Valley between AD 1280 and 1420, a period shortly before Inka arrival. The migrants appear to have settled permanently, as indicated by their stable strontium isotope ratios reflecting long-term residence in the Uspallata Valley.

This study provides strong evidence for the pre-Inka migration of maize-farming groups into the southern Andean Uspallata Valley, a region on the periphery of the Inka Empire. This migration event coincided with a period of significant demographic growth in the broader region, suggesting complex interactions between environmental change, population pressure, and human movement. The arrival of these new populations, distinct from the pre-existing local communities in terms of biological ancestry, cultural practices, and dietary habits, created a multicultural landscape at the time of Inka arrival. The findings challenge traditional models focusing solely on internal adaptation to climate change to explain economic and demographic changes in the region. The results highlight the importance of incorporating migration as a key factor in understanding the complex dynamics of economic transition and population growth in the Andes during the LIP. The coexistence of distinct groups with varying cultural affiliations and economic practices in the Uspallata Valley adds considerable depth to our understanding of the interaction dynamics between the Inka Empire and the local societies it encountered.

This research presents compelling multi-isotopic and morphometric evidence for a significant migration of maize-farming groups into the Uspallata Valley shortly before the Inka conquest. The findings highlight the complexity of pre-Inka social and economic dynamics in the southern Andes, showing the importance of integrating migration into broader models of Andean cultural and economic change. Future research could focus on determining the precise geographic origins of the migrants and investigating the social and political dynamics between the local and migrant populations, including a closer examination of possible conflict or disease patterns. Further paleogenomic studies could also contribute to clarifying the ancestry of these populations and tracing their relationships.

While the study provides compelling evidence for migration, the precise origin of the migrant groups remains uncertain. Although the study rules out some potential source areas, further research is needed to fully pinpoint their place of origin. The sample size, while significant, could be expanded to further strengthen the statistical power of the analyses. The possibility of diagenetic strontium uptake, although considered unlikely given the study's findings, cannot be completely ruled out. Finally, the analysis of mortuary contexts is limited. While providing some cultural insights, the absence of grave goods in some cemeteries leaves open questions about the social context of migration and integration into Uspallata.