The construction of large and elaborate monumental architecture, such as mounds, pyramids, tombs, and statues, were important factors in the emergence of socially complex societies, as they are seen as archaeological evidence for the rise of organized religion, social stratification, competition between social groups, and large-scale cooperation. How these processes relate to environmental and ecological factors, however, is poorly understood. This represents a significant limitation in our understanding of the human past and its various trajectories. This project was designed to further our understanding of the socio-ecological factors underlying the emergence of monument construction through archaeological and environmental investigations on the island of Rapa Nui (Easter Island), famous for its elaborate religious monumental architecture. Given Rapa Nui’s small size, isolation, and plethora of religious monuments, it offers an ideal model system to investigate the evolutionary ecology of monument construction.

This project investigated these issues by testing predictions of an evolutionary ecology model known as costly-signaling theory for the origins of monument construction using Rapa Nui as a case study. The central hypothesis was that, in addition to their well-known religious roles, monuments functioned within Rapa Nui society as conspicuous displays (i.e., costly signals) of communities’ competitive ability to control and defend their limited critical resources, which resulted in limited violent conflict or interaction between groups. This hypothesis was tested through archaeological, historical, and environmental investigations of: 1) the energetic costs of building monuments; 2) their visibility on the landscape; 3) relationships between the costs of building monuments, their visibility, and contested subsistence resources; and 4) the form and degree of interaction between prehistoric communities.

These objectives were achieved through field research and laboratory analyses on Rapa Nui and at the University of Oregon between May 2019 and May 2020. Using drone-based remote sensing and photogrammetry, we documented and create high-resolution 3D models for nearly 100 monument sites on Rapa Nui. Because data on Rapa Nui’s agricultural and marine resources are relatively well-known, we focused our environmental analyses on the island’s freshwater sources, which prior to this had been poorly understood. We documented coastal freshwater locations using a combination of thermal remote-sensing, electromagnetic induction, and geochemical measurements. 3D models of monuments were used to calculate energetic labor investment in building these features, which suggests a high degree of variability of investment in the island’s monuments. Most monuments are relatively small (e.g., ≤ 300 m³ of construction stone) with a smaller portion representing a substantial investment of time and energy (>1000 m³ of construction stone). We then used sophisticated spatial modeling and multiple-hypothesis testing to explore the relationship between the location/size of monument sites, their visibility on the landscape, and contested subsistence resources. The results of this modeling provide compelling support for the costly signaling hypothesis by demonstrating a strong relationship between investment in monuments, locations with abundant and high-quality freshwater sources, and highly visible locations. These results suggest that the religious and ritual activities centered around the construction, maintenance, and use of monuments involved signaling access and control of critical resources, as well as broadcasting an honest signal of the group’s collective action potential to defend their territory from other communities. We also demonstrate that the period of greatest investment in monuments coincides with a prolonged drought period that stressed the island’s freshwater sources, which we argue likely increased signaling competition. We hypothesize that this costly signaling dynamic ultimately led to Rapa Nui’s unique settlement pattern and relatively low degrees of inter-group violence.

The results of this project advance anthropological theory and a broader understanding of societal change by illuminating the social and ecological influences behind why cooperation, competition, and religiosity were so frequently manifested as monumental architecture in the human past. Rapa Nui can provide a model case study for the evolution of religious signaling in the human past, which is currently an important topic in several fields outside of archaeology. Our model-based methodological approach can also serve as a template for similar theoretically-focused studies of monumental architecture elsewhere in world history. The results have broader impacts in terms of heritage management and archaeological conservation. The famous monuments of Rapa Nui, a UNESCO World Heritage Site, are at great risk of being destroyed by erosion, storm surges, and development. This photogrammetry products from this project are being used as digital documentation and preservation tools to help conserve Rapa Nui’s monument sites through an on-going collaborative partnership with Rapanui archaeologists and stakeholders. This project also led to several education, training, and outreach initiatives, including training of students in archaeological survey and geospatial field techniques, training and dissertation completion for Co-PI DiNapoli, and dissemination of results in both academic venues and public media. These initiatives directly contributed to enhancing collaborative relationships, training for underrepresented groups, and public engagement in archaeological science.

Last Modified: 09/28/2020
Modified by: Robert J Dinapoli